source: libcfa/src/concurrency/ready_queue.cfa

//
// 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__

// #define __CFA_DEBUG_PRINT_READY_QUEUE__


#define USE_AWARE_STEALING

#include "bits/defs.hfa"
#include "device/cpu.hfa"
#include "kernel/cluster.hfa"
#include "kernel/private.hfa"

// #include <errno.h>
// #include <unistd.h>

#include "ready_subqueue.hfa"

static const size_t cache_line_size = 64;

#if !defined(__CFA_NO_STATISTICS__)
        #define __STATS(...) __VA_ARGS__
#else
        #define __STATS(...)
#endif

static inline struct thread$ * try_pop(struct cluster * cltr, unsigned w __STATS(, __stats_readyQ_pop_t & stats));
static inline struct thread$ * try_pop(struct cluster * cltr, unsigned i, unsigned j __STATS(, __stats_readyQ_pop_t & stats));
static inline struct thread$ * search(struct cluster * cltr);

//=======================================================================
// Cforall Ready Queue used for scheduling
//=======================================================================
// void ?{}(__ready_queue_t & this) with (this) {
//      lanes.data   = 0p;
//      lanes.tscs   = 0p;
//      lanes.caches = 0p;
//      lanes.count  = 0;
// }

// void ^?{}(__ready_queue_t & this) with (this) {
//      free(lanes.data);
//      free(lanes.tscs);
//      free(lanes.caches);
// }

//-----------------------------------------------------------------------
__attribute__((hot)) void push(struct cluster * cltr, struct thread$ * thrd, unpark_hint hint) with (cltr->sched) {
        struct processor * const proc = kernelTLS().this_processor;
        const bool external = (!proc) || (cltr != proc->cltr);
        const bool remote   = hint == UNPARK_REMOTE;
        const size_t lanes_count = readyQ.count;

        /* paranoid */ verify( __shard_factor.readyq > 0 );
        /* paranoid */ verify( lanes_count > 0 );

        unsigned i;
        if( external || remote ) {
                // Figure out where thread was last time and make sure it's valid
                /* paranoid */ verify(thrd->preferred >= 0);
                unsigned start = thrd->preferred * __shard_factor.readyq;
                if(start < lanes_count) {
                        do {
                                unsigned r = __tls_rand();
                                i = start + (r % __shard_factor.readyq);
                                /* paranoid */ verify( i < lanes_count );
                                // If we can't lock it retry
                        } while( !__atomic_try_acquire( &readyQ.data[i].l.lock ) );
                } else {
                        do {
                                i = __tls_rand() % lanes_count;
                        } while( !__atomic_try_acquire( &readyQ.data[i].l.lock ) );
                }
        } else {
                do {
                        unsigned r = proc->rdq.its++;
                        i = proc->rdq.id + (r % __shard_factor.readyq);
                        /* paranoid */ verify( i < lanes_count );
                        // If we can't lock it retry
                } while( !__atomic_try_acquire( &readyQ.data[i].l.lock ) );
        }

        // Actually push it
        push(readyQ.data[i], thrd);

        // Unlock and return
        __atomic_unlock( &readyQ.data[i].l.lock );

        #if !defined(__CFA_NO_STATISTICS__)
                if(unlikely(external || remote)) __atomic_fetch_add(&cltr->stats->ready.push.extrn.success, 1, __ATOMIC_RELAXED);
                else __tls_stats()->ready.push.local.success++;
        #endif
}

__attribute__((hot)) struct thread$ * pop_fast(struct cluster * cltr) with (cltr->sched) {
        const size_t lanes_count = readyQ.count;

        /* paranoid */ verify( __shard_factor.readyq > 0 );
        /* paranoid */ verify( lanes_count > 0 );
        /* paranoid */ verify( kernelTLS().this_processor );
        /* paranoid */ verify( kernelTLS().this_processor->rdq.id < lanes_count );

        struct processor * const proc = kernelTLS().this_processor;
        unsigned this = proc->rdq.id;
        /* paranoid */ verify( this < lanes_count );
        __cfadbg_print_safe(ready_queue, "Kernel : pop from %u\n", this);

        // Figure out the current cache is
        const unsigned this_cache = cache_id(cltr, this / __shard_factor.readyq);
        const unsigned long long ctsc = rdtscl();

        if(proc->rdq.target == UINT_MAX) {
                uint64_t chaos = __tls_rand();
                unsigned ext = chaos & 0xff;
                unsigned other  = (chaos >> 8) % (lanes_count);

                if(ext < 3 || __atomic_load_n(&caches[other / __shard_factor.readyq].id, __ATOMIC_RELAXED) == this_cache) {
                        proc->rdq.target = other;
                }
        }
        else {
                const unsigned target = proc->rdq.target;
                __cfadbg_print_safe(ready_queue, "Kernel : %u considering helping %u, tcsc %llu\n", this, target, readyQ.tscs[target].t.tv);
                /* paranoid */ verify( readyQ.tscs[target].t.tv != ULLONG_MAX );
                if(target < lanes_count) {
                        const __readyQ_avg_t cutoff = calc_cutoff(ctsc, proc->rdq.id, lanes_count, cltr->sched.readyQ.data, cltr->sched.readyQ.tscs, __shard_factor.readyq, true);
                        const __readyQ_avg_t age = moving_average(ctsc, readyQ.tscs[target].t.tv, readyQ.tscs[target].t.ma, false);
                        __cfadbg_print_safe(ready_queue, "Kernel : Help attempt on %u from %u, age %'llu vs cutoff %'llu, %s\n", target, this, age, cutoff, age > cutoff ? "yes" : "no");
                        if(age > cutoff) {
                                thread$ * t = try_pop(cltr, target __STATS(, __tls_stats()->ready.pop.help));
                                if(t) return t;
                        }
                }
                proc->rdq.target = UINT_MAX;
        }

        for(__shard_factor.readyq) {
                unsigned i = this + (proc->rdq.itr++ % __shard_factor.readyq);
                if(thread$ * t = try_pop(cltr, i __STATS(, __tls_stats()->ready.pop.local))) return t;
        }

        // All lanes where empty return 0p
        return 0p;

}
__attribute__((hot)) struct thread$ * pop_slow(struct cluster * cltr) {
        unsigned i = __tls_rand() % (cltr->sched.readyQ.count);
        return try_pop(cltr, i __STATS(, __tls_stats()->ready.pop.steal));
}
__attribute__((hot)) struct thread$ * pop_search(struct cluster * cltr) {
        return search(cltr);
}

//=======================================================================
// Various Ready Queue utilities
//=======================================================================
// these function work the same or almost the same
// whether they are using work-stealing or relaxed fifo scheduling

//-----------------------------------------------------------------------
// try to pop from a lane given by index w
static inline struct thread$ * try_pop(struct cluster * cltr, unsigned w __STATS(, __stats_readyQ_pop_t & stats)) with (cltr->sched) {
        /* paranoid */ verify( w < readyQ.count );
        __STATS( stats.attempt++; )

        // Get relevant elements locally
        __intrusive_lane_t & lane = readyQ.data[w];

        // If list looks empty retry
        if( is_empty(lane) ) {
                return 0p;
        }

        // If we can't get the lock retry
        if( !__atomic_try_acquire(&lane.l.lock) ) {
                return 0p;
        }

        // If list is empty, unlock and retry
        if( is_empty(lane) ) {
                __atomic_unlock(&lane.l.lock);
                return 0p;
        }

        // Actually pop the list
        struct thread$ * thrd;
        unsigned long long ts_prev = ts(lane);
        unsigned long long ts_next;
        [thrd, ts_next] = pop(lane);

        /* paranoid */ verify(thrd);
        /* paranoid */ verify(ts_next);
        /* paranoid */ verify(lane.l.lock);

        // Unlock and return
        __atomic_unlock(&lane.l.lock);

        // Update statistics
        __STATS( stats.success++; )

        touch_tsc(readyQ.tscs, w, ts_prev, ts_next, true);

        thrd->preferred = w / __shard_factor.readyq;

        // return the popped thread
        return thrd;
}

//-----------------------------------------------------------------------
// try to pop from any lanes making sure you don't miss any threads push
// before the start of the function
static inline struct thread$ * search(struct cluster * cltr) {
        const size_t lanes_count = cltr->sched.readyQ.count;
        /* paranoid */ verify( lanes_count > 0 );
        unsigned count = __atomic_load_n( &lanes_count, __ATOMIC_RELAXED );
        unsigned offset = __tls_rand();
        for(i; count) {
                unsigned idx = (offset + i) % count;
                struct thread$ * thrd = try_pop(cltr, idx __STATS(, __tls_stats()->ready.pop.search));
                if(thrd) {
                        return thrd;
                }
        }

        // All lanes where empty return 0p
        return 0p;
}

//-----------------------------------------------------------------------
// get preferred ready for new thread
unsigned ready_queue_new_preferred() {
        unsigned pref = UINT_MAX;
        if(struct thread$ * thrd = publicTLS_get( this_thread )) {
                pref = thrd->preferred;
        }

        return pref;
}

//-----------------------------------------------------------------------
// Given 2 indexes, pick the list with the oldest push an try to pop from it
static inline struct thread$ * try_pop(struct cluster * cltr, unsigned i, unsigned j __STATS(, __stats_readyQ_pop_t & stats)) with (cltr->sched) {
        // Pick the bet list
        int w = i;
        if( __builtin_expect(!is_empty(readyQ.data[j]), true) ) {
                w = (ts(readyQ.data[i]) < ts(readyQ.data[j])) ? i : j;
        }

        return try_pop(cltr, w __STATS(, stats));
}