source: libcfa/src/concurrency/ready_queue.cfa@ b798713

//
// Cforall Version 1.0.0 Copyright (C) 2019 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// ready_queue.cfa --
//
// Author           : Thierry Delisle
// Created On       : Mon Nov dd 16:29:18 2019
// Last Modified By :
// Last Modified On :
// Update Count     :
//

#define __cforall_thread__

#include "bits/defs.hfa"
#include "kernel_private.hfa"

#define _GNU_SOURCE
#include "stdlib.hfa"

static const size_t cache_line_size = 64;

static inline unsigned __max_processors_fallback() {
        #ifdef __CFA_MAX_PROCESSORS__
                return __CFA_MAX_PROCESSORS__;
        #else
                // No overriden function, no environment variable, no define
                // fall back to a magic number
                return 128;
        #endif
}

__attribute__((weak)) unsigned __max_processors() {
        const char * max_cores_s = getenv("CFA_MAX_PROCESSORS");
        if(!max_cores_s) {
                __cfaabi_dbg_print_nolock("No CFA_MAX_PROCESSORS in ENV");
                return __max_processors_fallback();
        }

        char * endptr = 0p;
        long int max_cores_l = strtol(max_cores_s, &endptr, 10);
        if(max_cores_l < 1 || max_cores_l > 65535) {
                __cfaabi_dbg_print_nolock("CFA_MAX_PROCESSORS out of range : %ld", max_cores_l);
                return __max_processors_fallback();
        }
        if('\0' != *endptr) {
                __cfaabi_dbg_print_nolock("CFA_MAX_PROCESSORS not a decimal number : %s", max_cores_s);
                return __max_processors_fallback();
        }

        return max_cores_l;
}

static inline unsigned rand_bit(unsigned rnum, size_t mask) {
        verify(sizeof(mask) == 8);
        unsigned bit = mask ? rnum % __builtin_popcountl(mask) : 0;
#if !defined(__BMI2__)
        uint64_t v = mask;   // Input value to find position with rank r.
        unsigned int r = bit + 1;// Input: bit's desired rank [1-64].
        unsigned int s;      // Output: Resulting position of bit with rank r [1-64]
        uint64_t a, b, c, d; // Intermediate temporaries for bit count.
        unsigned int t;      // Bit count temporary.

        // Do a normal parallel bit count for a 64-bit integer,
        // but store all intermediate steps.
        a =  v - ((v >> 1) & ~0UL/3);
        b = (a & ~0UL/5) + ((a >> 2) & ~0UL/5);
        c = (b + (b >> 4)) & ~0UL/0x11;
        d = (c + (c >> 8)) & ~0UL/0x101;


        t = (d >> 32) + (d >> 48);
        // Now do branchless select!
        s  = 64;
        s -= ((t - r) & 256) >> 3; r -= (t & ((t - r) >> 8));
        t  = (d >> (s - 16)) & 0xff;
        s -= ((t - r) & 256) >> 4; r -= (t & ((t - r) >> 8));
        t  = (c >> (s - 8)) & 0xf;
        s -= ((t - r) & 256) >> 5; r -= (t & ((t - r) >> 8));
        t  = (b >> (s - 4)) & 0x7;
        s -= ((t - r) & 256) >> 6; r -= (t & ((t - r) >> 8));
        t  = (a >> (s - 2)) & 0x3;
        s -= ((t - r) & 256) >> 7; r -= (t & ((t - r) >> 8));
        t  = (v >> (s - 1)) & 0x1;
        s -= ((t - r) & 256) >> 8;
        return s - 1;
#else
        uint64_t picked = _pdep_u64(1ul << bit, mask);
        return picked ? __builtin_ctzl(picked) : 0;
#endif
}

static inline __cfa_readyQ_mask_t readyQ_mask_full       () { return (8 * sizeof(__cfa_readyQ_mask_t)) - 1; }
static inline __cfa_readyQ_mask_t readyQ_mask_shit_length() { return (8 * sizeof(__cfa_readyQ_mask_t)) - __builtin_clzl(readyQ_mask_full()); }

static inline [__cfa_readyQ_mask_t, __cfa_readyQ_mask_t] extract(__cfa_readyQ_mask_t idx) {
        __cfa_readyQ_mask_t word = idx >> readyQ_mask_shit_length();
        __cfa_readyQ_mask_t bit  = idx &  readyQ_mask_full();
        return [bit, word];
}

//=======================================================================
// Cluster wide reader-writer lock
//=======================================================================
void  ?{}(__clusterRWLock_t & this) {
        this.max   = __max_processors();
        this.alloc = 0;
        this.ready = 0;
        this.lock  = false;
        this.data  = alloc(this.max);

        /*paranoid*/ verify( 0 == (((uintptr_t)(this.data    )) % 64) );
        /*paranoid*/ verify( 0 == (((uintptr_t)(this.data + 1)) % 64) );
        /*paranoid*/ verify(__atomic_is_lock_free(sizeof(this.alloc), &this.alloc));
        /*paranoid*/ verify(__atomic_is_lock_free(sizeof(this.ready), &this.ready));

}
void ^?{}(__clusterRWLock_t & this) {
        free(this.data);
}

void ?{}( __processor_id & this, struct processor * proc ) {
        this.handle = proc;
        this.lock   = false;
}

//=======================================================================
// Lock-Free registering/unregistering of threads
unsigned doregister( struct cluster * cltr, struct processor * proc ) with(cltr->ready_lock) {
        // Step - 1 : check if there is already space in the data
        uint_fast32_t s = ready;

        // Check among all the ready
        for(uint_fast32_t i = 0; i < s; i++) {
                processor * null = 0p; // Re-write every loop since compare thrashes it
                if( __atomic_load_n(&data[i].handle, (int)__ATOMIC_RELAXED) == null
                        && __atomic_compare_exchange_n( &data[i].handle, &null, proc, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)) {
                        /*paranoid*/ verify(i < ready);
                        /*paranoid*/ verify(__alignof__(data[i]) == cache_line_size);
                        /*paranoid*/ verify((((uintptr_t)&data[i]) % cache_line_size) == 0);
                        return i;
                }
        }

        if(max <= alloc) abort("Trying to create more than %ud processors", cltr->ready_lock.max);

        // Step - 2 : F&A to get a new spot in the array.
        uint_fast32_t n = __atomic_fetch_add(&alloc, 1, __ATOMIC_SEQ_CST);
        if(max <= n) abort("Trying to create more than %ud processors", cltr->ready_lock.max);

        // Step - 3 : Mark space as used and then publish it.
        __processor_id * storage = (__processor_id *)&data[n];
        (*storage){ proc };
        while(true) {
                unsigned copy = n;
                if( __atomic_load_n(&ready, __ATOMIC_RELAXED) == n
                        && __atomic_compare_exchange_n(&ready, &copy, n + 1, true, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
                        break;
                asm volatile("pause");
        }

        // Return new spot.
        /*paranoid*/ verify(n < ready);
        /*paranoid*/ verify(__alignof__(data[n]) == cache_line_size);
        /*paranoid*/ verify((((uintptr_t)&data[n]) % cache_line_size) == 0);
        return n;
}

void unregister( struct cluster * cltr, struct processor * proc ) with(cltr->ready_lock) {
        unsigned id = proc->id;
        /*paranoid*/ verify(id < ready);
        /*paranoid*/ verify(proc == __atomic_load_n(&data[id].handle, __ATOMIC_RELAXED));
        __atomic_store_n(&data[id].handle, 0p, __ATOMIC_RELEASE);
}

//-----------------------------------------------------------------------
// Writer side : acquire when changing the ready queue, e.g. adding more
//  queues or removing them.
uint_fast32_t ready_mutate_lock( struct cluster & cltr ) with(cltr.ready_lock) {
        // Step 1 : lock global lock
        // It is needed to avoid processors that register mid Critical-Section
        //   to simply lock their own lock and enter.
        __atomic_acquire( &lock );

        // Step 2 : lock per-proc lock
        // Processors that are currently being registered aren't counted
        //   but can't be in read_lock or in the critical section.
        // All other processors are counted
        uint_fast32_t s = ready;
        for(uint_fast32_t i = 0; i < s; i++) {
                __atomic_acquire( &data[i].lock );
        }

        return s;
}

void ready_mutate_unlock( struct cluster & cltr, uint_fast32_t last_s ) with(cltr.ready_lock) {
        // Step 1 : release local locks
        // This must be done while the global lock is held to avoid
        //   threads that where created mid critical section
        //   to race to lock their local locks and have the writer
        //   immidiately unlock them
        // Alternative solution : return s in write_lock and pass it to write_unlock
        for(uint_fast32_t i = 0; i < last_s; i++) {
                verify(data[i].lock);
                __atomic_store_n(&data[i].lock, (bool)false, __ATOMIC_RELEASE);
        }

        // Step 2 : release global lock
        /*paranoid*/ assert(true == lock);
        __atomic_store_n(&lock, (bool)false, __ATOMIC_RELEASE);
}

//=======================================================================
// Intrusive Queue used by ready queue
//=======================================================================
// Get the head pointer (one before the first element) from the anchor
static inline thread_desc * head(const __intrusive_ready_queue_t & this) {
        thread_desc * rhead = (thread_desc *)(
                (uintptr_t)( &this.before ) - offsetof( thread_desc, link )
        );
        /* paranoid */ verify(rhead);
        return rhead;
}

// Get the tail pointer (one after the last element) from the anchor
static inline thread_desc * tail(const __intrusive_ready_queue_t & this) {
        thread_desc * rtail = (thread_desc *)(
                (uintptr_t)( &this.after ) - offsetof( thread_desc, link )
        );
        /* paranoid */ verify(rtail);
        return rtail;
}

// Ctor
void ?{}( __intrusive_ready_queue_t & this ) {
        this.lock = false;
        this.last_id = -1u;

        this.before.link.prev = 0p;
        this.before.link.next = tail(this);
        this.before.link.ts   = 0;

        this.after .link.prev = head(this);
        this.after .link.next = 0p;
        this.after .link.ts   = 0;

        #if !defined(__CFA_NO_SCHED_STATS__)
                this.stat.diff = 0;
                this.stat.push = 0;
                this.stat.pop  = 0;
        #endif

        // We add a boat-load of assertions here because the anchor code is very fragile
        /* paranoid */ verify(((uintptr_t)( head(this) ) + offsetof( thread_desc, link )) == (uintptr_t)(&this.before));
        /* paranoid */ verify(((uintptr_t)( tail(this) ) + offsetof( thread_desc, link )) == (uintptr_t)(&this.after ));
        /* paranoid */ verify(head(this)->link.prev == 0p );
        /* paranoid */ verify(head(this)->link.next == tail(this) );
        /* paranoid */ verify(tail(this)->link.next == 0p );
        /* paranoid */ verify(tail(this)->link.prev == head(this) );
        /* paranoid */ verify(&head(this)->link.prev == &this.before.link.prev );
        /* paranoid */ verify(&head(this)->link.next == &this.before.link.next );
        /* paranoid */ verify(&tail(this)->link.prev == &this.after .link.prev );
        /* paranoid */ verify(&tail(this)->link.next == &this.after .link.next );
        /* paranoid */ verify(sizeof(__intrusive_ready_queue_t) == 128);
        /* paranoid */ verify(sizeof(this) == 128);
        /* paranoid */ verify(__alignof__(__intrusive_ready_queue_t) == 128);
        /* paranoid */ verify(__alignof__(this) == 128);
        /* paranoid */ verifyf(((intptr_t)(&this) % 128) == 0, "Expected address to be aligned %p %% 128 == %zd", &this, ((intptr_t)(&this) % 128));

        /* paranoid */ verifyf(readyQ_mask_shit_length() == 6 , "%zu", readyQ_mask_shit_length());
        /* paranoid */ verifyf(readyQ_mask_full()        == 63, "%zu", readyQ_mask_full());
}

// Dtor is trivial
void ^?{}( __intrusive_ready_queue_t & this ) {
        // Make sure the list is empty
        /* paranoid */ verify(head(this)->link.prev == 0p );
        /* paranoid */ verify(head(this)->link.next == tail(this) );
        /* paranoid */ verify(tail(this)->link.next == 0p );
        /* paranoid */ verify(tail(this)->link.prev == head(this) );
}



bool push(__intrusive_ready_queue_t & this, thread_desc * node) {
        verify(this.lock);
        verify(node->link.ts != 0);
        verify(node->link.next == 0p);
        verify(node->link.prev == 0p);

        if(this.before.link.ts == 0l) {
                verify(tail(this)->link.next == 0p);
                verify(tail(this)->link.prev == head(this));
                verify(head(this)->link.next == tail(this));
                verify(head(this)->link.prev == 0p);
        }

        // Get the relevant nodes locally
        thread_desc * tail = tail(this);
        thread_desc * prev = tail->link.prev;

        // Do the push
        node->link.next = tail;
        node->link.prev = prev;
        prev->link.next = node;
        tail->link.prev = node;

        // Update stats
        #ifndef __CFA_NO_SCHED_STATS__
                this.stat.diff++;
                this.stat.push++;
        #endif

        verify(node->link.next == tail(this));

        // Check if the queue used to be empty
        if(this.before.link.ts == 0l) {
                this.before.link.ts = node->link.ts;
                verify(node->link.prev == head(this));
                return true;
        }
        return false;
}

[thread_desc *, bool] pop(__intrusive_ready_queue_t & this) {
        verify(this.lock);
        verify(this.before.link.ts != 0ul);
        thread_desc * head = head(this);
        thread_desc * tail = tail(this);

        thread_desc * node = head->link.next;
        thread_desc * next = node->link.next;
        if(node == tail) {
                verify(false);
                verify(this.before.link.ts == 0ul);
                verify(tail(this)->link.next == 0p);
                verify(tail(this)->link.prev == head(this));
                verify(head(this)->link.next == tail(this));
                verify(head(this)->link.prev == 0p);
                return [0p, false];
        }

        /* paranoid */ verify(node);

        head->link.next = next;
        next->link.prev = head;

        #ifndef __CFA_NO_SCHED_STATS__
                this.stat.diff--;
                this.stat.pop ++;
        #endif

        if(next == tail) {
                this.before.link.ts = 0ul;
                verify(tail(this)->link.next == 0p);
                verify(tail(this)->link.prev == head(this));
                verify(head(this)->link.next == tail(this));
                verify(head(this)->link.prev == 0p);
                node->link.[next, prev] = 0p;
                return [node, true];
        }
        else {
                verify(next->link.ts != 0);
                this.before.link.ts = next->link.ts;
                verify(this.before.link.ts != 0);
                node->link.[next, prev] = 0p;
                return [node, false];
        }
}

static inline unsigned long long ts(__intrusive_ready_queue_t & this) {
        return this.before.link.ts;
}

//=======================================================================
// Cforall Reqdy Queue used by ready queue
//=======================================================================

static __attribute__((aligned(128))) thread_local struct {
        struct {
                struct {
                        size_t attempt;
                        size_t success;
                } push;
                struct {
                        size_t maskrds;
                        size_t attempt;
                        size_t success;
                } pop;
        } pick;
        struct {
                size_t value;
                size_t count;
        } full;
} tls = {
        /* pick */{
                /* push */{ 0, 0 },
                /* pop  */{ 0, 0, 0 },
        },
        /* full */{ 0, 0 }
};

//-----------------------------------------------------------------------

void ?{}(__ready_queue_t & this) with (this) {
        empty.count = 0;
        for( i ; __cfa_readyQ_mask_size ) {
                empty.mask[i] = 0;
        }

        list.data = alloc(4);
        for( i; 4 ) {
                (list.data[i]){};
        }
        list.count = 4;

        #if !defined(__CFA_NO_STATISTICS__)
                global_stats.pick.push.attempt = 0;
                global_stats.pick.push.success = 0;
                global_stats.pick.pop .maskrds = 0;
                global_stats.pick.pop .attempt = 0;
                global_stats.pick.pop .success = 0;

                global_stats.full.value = 0;
                global_stats.full.count = 0;
        #endif
}

void ^?{}(__ready_queue_t & this) with (this) {
        verify( 4  == list .count );
        verify( 0  == empty.count );

        for( i; 4 ) {
                ^(list.data[i]){};
        }
        free(list.data);


        #if defined(__CFA_WITH_VERIFY__)
                for( i ; __cfa_readyQ_mask_size ) {
                        assert( 0 == empty.mask[i] );
                }
        #endif
}

//-----------------------------------------------------------------------

__attribute__((hot)) bool push(struct cluster * cltr, struct thread_desc * thrd) with (cltr->ready_queue) {
        thrd->link.ts = rdtscl();

        while(true) {
                // Pick a random list
                unsigned i = tls_rand() % list.count;

                #if !defined(__CFA_NO_STATISTICS__)
                        tls.pick.push.attempt++;
                #endif

                // If we can't lock it retry
                if( !__atomic_try_acquire( &list.data[i].lock ) ) continue;
                verify(list.data[i].last_id == -1u);
                list.data[i].last_id = kernelTLS.this_processor->id;

                __attribute__((unused)) size_t num = __atomic_load_n( &empty.count, __ATOMIC_RELAXED );
                bool first = false;

                verify( list.data[i].last_id == kernelTLS.this_processor->id );
                verify( list.data[i].lock );
                // Actually push it
                if(push(list.data[i], thrd)) {
                        size_t ret = __atomic_fetch_add( &empty.count, 1z, __ATOMIC_SEQ_CST);
                        first = (ret == 0);

                        __cfa_readyQ_mask_t word;
                        __cfa_readyQ_mask_t bit;
                        [bit, word] = extract(i);
                        verifyf((empty.mask[word] & (1ull << bit)) == 0, "Before set %llu:%llu (%u), %llx & %llx", word, bit, i, empty.mask[word], (1ull << bit));
                        __attribute__((unused)) bool ret = bts(&empty.mask[word], bit);
                        verify(!(bool)ret);
                        verifyf((empty.mask[word] & (1ull << bit)) != 0, "After set %llu:%llu (%u), %llx & %llx", word, bit, i, empty.mask[word], (1ull << bit));
                }
                verify(empty.count <= (int)list.count);
                verify( list.data[i].last_id == kernelTLS.this_processor->id );
                verify( list.data[i].lock );

                // Unlock and return
                list.data[i].last_id = -1u;
                __atomic_unlock( &list.data[i].lock );

                #if !defined(__CFA_NO_STATISTICS__)
                        tls.pick.push.success++;
                        tls.full.value += num;
                        tls.full.count += 1;
                #endif
                return first;
        }
}

//-----------------------------------------------------------------------

static struct thread_desc * try_pop(struct cluster * cltr, unsigned i, unsigned j) with (cltr->ready_queue) {
        #if !defined(__CFA_NO_STATISTICS__)
                tls.pick.pop.attempt++;
        #endif

        // Pick the bet list
        int w = i;
        if( __builtin_expect(ts(list.data[j]) != 0, true) ) {
                w = (ts(list.data[i]) < ts(list.data[j])) ? i : j;
        }

        __intrusive_ready_queue_t & list = list.data[w];
        // If list looks empty retry
        if( ts(list) == 0 ) return 0p;

        // If we can't get the lock retry
        if( !__atomic_try_acquire(&list.lock) ) return 0p;
        verify(list.last_id == -1u);
        list.last_id = kernelTLS.this_processor->id;

        verify(list.last_id == kernelTLS.this_processor->id);

        __attribute__((unused)) int num = __atomic_load_n( &empty.count, __ATOMIC_RELAXED );


        // If list is empty, unlock and retry
        if( ts(list) == 0 ) {
                list.last_id = -1u;
                __atomic_unlock(&list.lock);
                return 0p;
        }
        {
                __cfa_readyQ_mask_t word;
                __cfa_readyQ_mask_t bit;
                [bit, word] = extract(w);
                verify((empty.mask[word] & (1ull << bit)) != 0);
        }

        verify(list.last_id == kernelTLS.this_processor->id);
        verify(list.lock);

        // Actually pop the list
        struct thread_desc * thrd;
        bool emptied;
        [thrd, emptied] = pop(list);
        verify(thrd);

        verify(list.last_id == kernelTLS.this_processor->id);
        verify(list.lock);

        if(emptied) {
                __atomic_fetch_sub( &empty.count, 1z, __ATOMIC_SEQ_CST);

                __cfa_readyQ_mask_t word;
                __cfa_readyQ_mask_t bit;
                [bit, word] = extract(w);
                verify((empty.mask[word] & (1ull << bit)) != 0);
                __attribute__((unused)) bool ret = btr(&empty.mask[word], bit);
                verify(ret);
                verify((empty.mask[word] & (1ull << bit)) == 0);
        }

        verify(list.lock);

        // Unlock and return
        list.last_id = -1u;
        __atomic_unlock(&list.lock);
        verify(empty.count >= 0);

        #if !defined(__CFA_NO_STATISTICS__)
                tls.pick.pop.success++;
                tls.full.value += num;
                tls.full.count += 1;
        #endif

        return thrd;
}

__attribute__((hot)) thread_desc * pop(struct cluster * cltr) with (cltr->ready_queue) {
        verify( list.count > 0 );
        while( __atomic_load_n( &empty.count, __ATOMIC_RELAXED ) != 0) {
                #if !defined(__CFA_READQ_NO_BITMASK__)
                        tls.pick.pop.maskrds++;
                        unsigned i, j;
                        {
                                #if !defined(__CFA_NO_SCHED_STATS__)
                                        tls.pick.pop.maskrds++;
                                #endif

                                // Pick two lists at random
                                unsigned num = ((__atomic_load_n( &list.count, __ATOMIC_RELAXED ) - 1) >> 6) + 1;

                                unsigned ri = tls_rand();
                                unsigned rj = tls_rand();

                                unsigned wdxi = (ri >> 6u) % num;
                                unsigned wdxj = (rj >> 6u) % num;

                                size_t maski = __atomic_load_n( &empty.mask[wdxi], __ATOMIC_RELAXED );
                                size_t maskj = __atomic_load_n( &empty.mask[wdxj], __ATOMIC_RELAXED );

                                if(maski == 0 && maskj == 0) continue;

                                unsigned bi = rand_bit(ri, maski);
                                unsigned bj = rand_bit(rj, maskj);

                                verifyf(bi < 64, "%zu %u", maski, bi);
                                verifyf(bj < 64, "%zu %u", maskj, bj);

                                i = bi | (wdxi << 6);
                                j = bj | (wdxj << 6);

                                verifyf(i < list.count, "%u", wdxi << 6);
                                verifyf(j < list.count, "%u", wdxj << 6);
                        }

                        struct thread_desc * thrd = try_pop(cltr, i, j);
                        if(thrd) return thrd;
                #else
                        // Pick two lists at random
                        int i = tls_rand() % __atomic_load_n( &list.count, __ATOMIC_RELAXED );
                        int j = tls_rand() % __atomic_load_n( &list.count, __ATOMIC_RELAXED );

                        struct thread_desc * thrd = try_pop(cltr, i, j);
                        if(thrd) return thrd;
                #endif
        }

        return 0p;
}

//-----------------------------------------------------------------------

static void check( __ready_queue_t & q ) with (q) {
        #if defined(__CFA_WITH_VERIFY__)
                {
                        int idx = 0;
                        for( w ; __cfa_readyQ_mask_size ) {
                                for( b ; 8 * sizeof(__cfa_readyQ_mask_t) ) {
                                        bool is_empty = idx < list.count ? (ts(list.data[idx]) == 0) : true;
                                        bool should_be_empty = 0 == (empty.mask[w] & (1z << b));
                                        assertf(should_be_empty == is_empty, "Inconsistent list %d, mask expect : %d, actual is got %d", idx, should_be_empty, (bool)is_empty);
                                        assert(__cfa_max_readyQs > idx);
                                        idx++;
                                }
                        }
                }

                {
                        for( idx ; list.count ) {
                                __intrusive_ready_queue_t & sl = list.data[idx];
                                assert(!list.data[idx].lock);

                                assert(head(sl)->link.prev == 0p );
                                assert(head(sl)->link.next->link.prev == head(sl) );
                                assert(tail(sl)->link.next == 0p );
                                assert(tail(sl)->link.prev->link.next == tail(sl) );

                                if(sl.before.link.ts == 0l) {
                                        assert(tail(sl)->link.next == 0p);
                                        assert(tail(sl)->link.prev == head(sl));
                                        assert(head(sl)->link.next == tail(sl));
                                        assert(head(sl)->link.prev == 0p);
                                }
                        }
                }
        #endif
}

// Call this function of the intrusive list was moved using memcpy
// fixes the list so that the pointers back to anchors aren't left
// dangling
static inline void fix(__intrusive_ready_queue_t & ll) {
        // if the list is not empty then follow he pointer
        // and fix its reverse
        if(ll.before.link.ts != 0l) {
                head(ll)->link.next->link.prev = head(ll);
                tail(ll)->link.prev->link.next = tail(ll);
        }
        // Otherwise just reset the list
        else {
                tail(ll)->link.next = 0p;
                tail(ll)->link.prev = head(ll);
                head(ll)->link.next = tail(ll);
                head(ll)->link.prev = 0p;
        }
}

void ready_queue_grow  (struct cluster * cltr) {
        uint_fast32_t last_size = ready_mutate_lock( *cltr );
        check( cltr->ready_queue );

        with( cltr->ready_queue ) {
                size_t ncount = list.count;

                // Check that we have some space left
                if(ncount + 4 >= __cfa_max_readyQs) abort("Program attempted to create more than maximum number of Ready Queues (%zu)", __cfa_max_readyQs);

                ncount += 4;

                // Allocate new array
                list.data = alloc(list.data, ncount);

                // Fix the moved data
                for( idx; (size_t)list.count ) {
                        fix(list.data[idx]);
                }

                // Construct new data
                for( idx; (size_t)list.count ~ ncount) {
                        (list.data[idx]){};
                }

                // Update original
                list.count = ncount;
                // fields in empty don't need to change
        }

        // Make sure that everything is consistent
        check( cltr->ready_queue );
        ready_mutate_unlock( *cltr, last_size );
}

void ready_queue_shrink(struct cluster * cltr) {
        uint_fast32_t last_size = ready_mutate_lock( *cltr );
        with( cltr->ready_queue ) {
                size_t ocount = list.count;
                // Check that we have some space left
                if(ocount < 8) abort("Program attempted to destroy more Ready Queues than were created");

                list.count -= 4;

                // redistribute old data
                verify(ocount > list.count);
                for( idx; (size_t)list.count ~ ocount) {
                        // This is not strictly needed but makes checking invariants much easier
                        bool locked = __atomic_try_acquire(&list.data[idx].lock);
                        verify(locked);
                        while(0 != ts(list.data[idx])) {
                                struct thread_desc * thrd;
                                __attribute__((unused)) bool _;
                                [thrd, _] = pop(list.data[idx]);
                                verify(thrd);
                                push(cltr, thrd);
                        }

                        __atomic_unlock(&list.data[idx].lock);

                        // TODO print the queue statistics here

                        ^(list.data[idx]){};
                }

                // clear the now unused masks
                {
                        __cfa_readyQ_mask_t fword, fbit, lword, lbit;
                        [fbit, fword] = extract(ocount);
                        [lbit, lword] = extract(list.count);

                        // For now assume that all queues where coverd by the same bitmask
                        // This is highly probable as long as grow and shrink use groups of 4
                        // exclusively
                        verify(fword == lword);
                        __cfa_readyQ_mask_t clears = ~0;

                        for( b ; fbit ~ lbit ) {
                                clears ^= 1 << b;
                        }

                        empty.mask[fword] &= clears;
                }

                // Allocate new array
                list.data = alloc(list.data, list.count);

                // Fix the moved data
                for( idx; (size_t)list.count ) {
                        fix(list.data[idx]);
                }
        }

        // Make sure that everything is consistent
        check( cltr->ready_queue );
        ready_mutate_unlock( *cltr, last_size );
}

//-----------------------------------------------------------------------

#if !defined(__CFA_NO_STATISTICS__)
void stats_tls_tally(struct cluster * cltr) with (cltr->ready_queue) {
        __atomic_fetch_add( &global_stats.pick.push.attempt, tls.pick.push.attempt, __ATOMIC_SEQ_CST );
        __atomic_fetch_add( &global_stats.pick.push.success, tls.pick.push.success, __ATOMIC_SEQ_CST );
        __atomic_fetch_add( &global_stats.pick.pop .maskrds, tls.pick.pop .maskrds, __ATOMIC_SEQ_CST );
        __atomic_fetch_add( &global_stats.pick.pop .attempt, tls.pick.pop .attempt, __ATOMIC_SEQ_CST );
        __atomic_fetch_add( &global_stats.pick.pop .success, tls.pick.pop .success, __ATOMIC_SEQ_CST );

        __atomic_fetch_add( &global_stats.full.value, tls.full.value, __ATOMIC_SEQ_CST );
        __atomic_fetch_add( &global_stats.full.count, tls.full.count, __ATOMIC_SEQ_CST );
}
#endif