source: libcfa/src/concurrency/ready_queue.cfa @ 7768b8d

//
// 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;
}

//=======================================================================
// 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
//=======================================================================
static const size_t fields_offset = offsetof( thread_desc, next );

// 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 ) - fields_offset
        );
        /* 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 ) - fields_offset
        );
        /* paranoid */ verify(rtail);
        return rtail;
}

// Ctor
void ?{}( __intrusive_ready_queue_t & this ) {
        this.before.prev = 0p;
        this.before.next = tail(this);

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

        // We add a boat-load of assertions here because the anchor code is very fragile
        /* paranoid */ verify(((uintptr_t)( head(this) ) + fields_offset) == (uintptr_t)(&this.before));
        /* paranoid */ verify(((uintptr_t)( tail(this) ) + fields_offset) == (uintptr_t)(&this.after ));
        /* paranoid */ verify(head(this)->prev == 0p );
        /* paranoid */ verify(head(this)->next == tail(this) );
        /* paranoid */ verify(tail(this)->next == 0p );
        /* paranoid */ verify(tail(this)->prev == head(this) );
        /* paranoid */ verify(&head(this)->prev == &this.before.prev );
        /* paranoid */ verify(&head(this)->next == &this.before.next );
        /* paranoid */ verify(&tail(this)->prev == &this.after .prev );
        /* paranoid */ verify(&tail(this)->next == &this.after .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));
}

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



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


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

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

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

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

[thread_desc *, bool] pop(__intrusive_ready_queue_t & this) {
        verify(this.lock);
        thread_desc * head = head(this);
        thread_desc * tail = tail(this);

        thread_desc * node = head->next;
        thread_desc * next = node->next;
        if(node == tail) return [0p, false];

        /* paranoid */ verify(node);

        head->next = next;
        next->prev = head;

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

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

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