Sending this PULL feels little rushed, as I still have un-answered questions on the inital patch posting just a few hours ago....
On Tue, Mar 19, 2024 at 11:09:38AM -0400, Stefan Hajnoczi wrote: > The coroutine pool implementation can hit the Linux vm.max_map_count > limit, causing QEMU to abort with "failed to allocate memory for stack" > or "failed to set up stack guard page" during coroutine creation. > > This happens because per-thread pools can grow to tens of thousands of > coroutines. Each coroutine causes 2 virtual memory areas to be created. > Eventually vm.max_map_count is reached and memory-related syscalls fail. > The per-thread pool sizes are non-uniform and depend on past coroutine > usage in each thread, so it's possible for one thread to have a large > pool while another thread's pool is empty. > > Switch to a new coroutine pool implementation with a global pool that > grows to a maximum number of coroutines and per-thread local pools that > are capped at hardcoded small number of coroutines. > > This approach does not leave large numbers of coroutines pooled in a > thread that may not use them again. In order to perform well it > amortizes the cost of global pool accesses by working in batches of > coroutines instead of individual coroutines. > > The global pool is a list. Threads donate batches of coroutines to when > they have too many and take batches from when they have too few: > > .-----------------------------------. > | Batch 1 | Batch 2 | Batch 3 | ... | global_pool > `-----------------------------------' > > Each thread has up to 2 batches of coroutines: > > .-------------------. > | Batch 1 | Batch 2 | per-thread local_pool (maximum 2 batches) > `-------------------' > > The goal of this change is to reduce the excessive number of pooled > coroutines that cause QEMU to abort when vm.max_map_count is reached > without losing the performance of an adequately sized coroutine pool. > > Here are virtio-blk disk I/O benchmark results: > > RW BLKSIZE IODEPTH OLD NEW CHANGE > randread 4k 1 113725 117451 +3.3% > randread 4k 8 192968 198510 +2.9% > randread 4k 16 207138 209429 +1.1% > randread 4k 32 212399 215145 +1.3% > randread 4k 64 218319 221277 +1.4% > randread 128k 1 17587 17535 -0.3% > randread 128k 8 17614 17616 +0.0% > randread 128k 16 17608 17609 +0.0% > randread 128k 32 17552 17553 +0.0% > randread 128k 64 17484 17484 +0.0% > > See files/{fio.sh,test.xml.j2} for the benchmark configuration: > https://gitlab.com/stefanha/virt-playbooks/-/tree/coroutine-pool-fix-sizing > > Buglink: https://issues.redhat.com/browse/RHEL-28947 > Reported-by: Sanjay Rao <s...@redhat.com> > Reported-by: Boaz Ben Shabat <bbens...@redhat.com> > Reported-by: Joe Mario <jma...@redhat.com> > Reviewed-by: Kevin Wolf <kw...@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > Message-ID: <20240318183429.1039340-1-stefa...@redhat.com> > --- > util/qemu-coroutine.c | 282 +++++++++++++++++++++++++++++++++--------- > 1 file changed, 223 insertions(+), 59 deletions(-) > > diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c > index 5fd2dbaf8b..2790959eaf 100644 > --- a/util/qemu-coroutine.c > +++ b/util/qemu-coroutine.c > @@ -18,39 +18,200 @@ > #include "qemu/atomic.h" > #include "qemu/coroutine_int.h" > #include "qemu/coroutine-tls.h" > +#include "qemu/cutils.h" > #include "block/aio.h" > > -/** > - * The minimal batch size is always 64, coroutines from the release_pool are > - * reused as soon as there are 64 coroutines in it. The maximum pool size > starts > - * with 64 and is increased on demand so that coroutines are not deleted > even if > - * they are not immediately reused. > - */ > enum { > - POOL_MIN_BATCH_SIZE = 64, > - POOL_INITIAL_MAX_SIZE = 64, > + COROUTINE_POOL_BATCH_MAX_SIZE = 128, > }; > > -/** Free list to speed up creation */ > -static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool); > -static unsigned int pool_max_size = POOL_INITIAL_MAX_SIZE; > -static unsigned int release_pool_size; > +/* > + * Coroutine creation and deletion is expensive so a pool of unused > coroutines > + * is kept as a cache. When the pool has coroutines available, they are > + * recycled instead of creating new ones from scratch. Coroutines are added > to > + * the pool upon termination. > + * > + * The pool is global but each thread maintains a small local pool to avoid > + * global pool contention. Threads fetch and return batches of coroutines > from > + * the global pool to maintain their local pool. The local pool holds up to > two > + * batches whereas the maximum size of the global pool is controlled by the > + * qemu_coroutine_inc_pool_size() API. > + * > + * .-----------------------------------. > + * | Batch 1 | Batch 2 | Batch 3 | ... | global_pool > + * `-----------------------------------' > + * > + * .-------------------. > + * | Batch 1 | Batch 2 | per-thread local_pool (maximum 2 batches) > + * `-------------------' > + */ > +typedef struct CoroutinePoolBatch { > + /* Batches are kept in a list */ > + QSLIST_ENTRY(CoroutinePoolBatch) next; > > -typedef QSLIST_HEAD(, Coroutine) CoroutineQSList; > -QEMU_DEFINE_STATIC_CO_TLS(CoroutineQSList, alloc_pool); > -QEMU_DEFINE_STATIC_CO_TLS(unsigned int, alloc_pool_size); > -QEMU_DEFINE_STATIC_CO_TLS(Notifier, coroutine_pool_cleanup_notifier); > + /* This batch holds up to @COROUTINE_POOL_BATCH_MAX_SIZE coroutines */ > + QSLIST_HEAD(, Coroutine) list; > + unsigned int size; > +} CoroutinePoolBatch; > > -static void coroutine_pool_cleanup(Notifier *n, void *value) > +typedef QSLIST_HEAD(, CoroutinePoolBatch) CoroutinePool; > + > +/* Host operating system limit on number of pooled coroutines */ > +static unsigned int global_pool_hard_max_size; > + > +static QemuMutex global_pool_lock; /* protects the following variables */ > +static CoroutinePool global_pool = QSLIST_HEAD_INITIALIZER(global_pool); > +static unsigned int global_pool_size; > +static unsigned int global_pool_max_size = COROUTINE_POOL_BATCH_MAX_SIZE; > + > +QEMU_DEFINE_STATIC_CO_TLS(CoroutinePool, local_pool); > +QEMU_DEFINE_STATIC_CO_TLS(Notifier, local_pool_cleanup_notifier); > + > +static CoroutinePoolBatch *coroutine_pool_batch_new(void) > +{ > + CoroutinePoolBatch *batch = g_new(CoroutinePoolBatch, 1); > + > + QSLIST_INIT(&batch->list); > + batch->size = 0; > + return batch; > +} > + > +static void coroutine_pool_batch_delete(CoroutinePoolBatch *batch) > { > Coroutine *co; > Coroutine *tmp; > - CoroutineQSList *alloc_pool = get_ptr_alloc_pool(); > > - QSLIST_FOREACH_SAFE(co, alloc_pool, pool_next, tmp) { > - QSLIST_REMOVE_HEAD(alloc_pool, pool_next); > + QSLIST_FOREACH_SAFE(co, &batch->list, pool_next, tmp) { > + QSLIST_REMOVE_HEAD(&batch->list, pool_next); > qemu_coroutine_delete(co); > } > + g_free(batch); > +} > + > +static void local_pool_cleanup(Notifier *n, void *value) > +{ > + CoroutinePool *local_pool = get_ptr_local_pool(); > + CoroutinePoolBatch *batch; > + CoroutinePoolBatch *tmp; > + > + QSLIST_FOREACH_SAFE(batch, local_pool, next, tmp) { > + QSLIST_REMOVE_HEAD(local_pool, next); > + coroutine_pool_batch_delete(batch); > + } > +} > + > +/* Ensure the atexit notifier is registered */ > +static void local_pool_cleanup_init_once(void) > +{ > + Notifier *notifier = get_ptr_local_pool_cleanup_notifier(); > + if (!notifier->notify) { > + notifier->notify = local_pool_cleanup; > + qemu_thread_atexit_add(notifier); > + } > +} > + > +/* Helper to get the next unused coroutine from the local pool */ > +static Coroutine *coroutine_pool_get_local(void) > +{ > + CoroutinePool *local_pool = get_ptr_local_pool(); > + CoroutinePoolBatch *batch = QSLIST_FIRST(local_pool); > + Coroutine *co; > + > + if (unlikely(!batch)) { > + return NULL; > + } > + > + co = QSLIST_FIRST(&batch->list); > + QSLIST_REMOVE_HEAD(&batch->list, pool_next); > + batch->size--; > + > + if (batch->size == 0) { > + QSLIST_REMOVE_HEAD(local_pool, next); > + coroutine_pool_batch_delete(batch); > + } > + return co; > +} > + > +/* Get the next batch from the global pool */ > +static void coroutine_pool_refill_local(void) > +{ > + CoroutinePool *local_pool = get_ptr_local_pool(); > + CoroutinePoolBatch *batch; > + > + WITH_QEMU_LOCK_GUARD(&global_pool_lock) { > + batch = QSLIST_FIRST(&global_pool); > + > + if (batch) { > + QSLIST_REMOVE_HEAD(&global_pool, next); > + global_pool_size -= batch->size; > + } > + } > + > + if (batch) { > + QSLIST_INSERT_HEAD(local_pool, batch, next); > + local_pool_cleanup_init_once(); > + } > +} > + > +/* Add a batch of coroutines to the global pool */ > +static void coroutine_pool_put_global(CoroutinePoolBatch *batch) > +{ > + WITH_QEMU_LOCK_GUARD(&global_pool_lock) { > + unsigned int max = MIN(global_pool_max_size, > + global_pool_hard_max_size); > + > + if (global_pool_size < max) { > + QSLIST_INSERT_HEAD(&global_pool, batch, next); > + > + /* Overshooting the max pool size is allowed */ > + global_pool_size += batch->size; > + return; > + } > + } > + > + /* The global pool was full, so throw away this batch */ > + coroutine_pool_batch_delete(batch); > +} > + > +/* Get the next unused coroutine from the pool or return NULL */ > +static Coroutine *coroutine_pool_get(void) > +{ > + Coroutine *co; > + > + co = coroutine_pool_get_local(); > + if (!co) { > + coroutine_pool_refill_local(); > + co = coroutine_pool_get_local(); > + } > + return co; > +} > + > +static void coroutine_pool_put(Coroutine *co) > +{ > + CoroutinePool *local_pool = get_ptr_local_pool(); > + CoroutinePoolBatch *batch = QSLIST_FIRST(local_pool); > + > + if (unlikely(!batch)) { > + batch = coroutine_pool_batch_new(); > + QSLIST_INSERT_HEAD(local_pool, batch, next); > + local_pool_cleanup_init_once(); > + } > + > + if (unlikely(batch->size >= COROUTINE_POOL_BATCH_MAX_SIZE)) { > + CoroutinePoolBatch *next = QSLIST_NEXT(batch, next); > + > + /* Is the local pool full? */ > + if (next) { > + QSLIST_REMOVE_HEAD(local_pool, next); > + coroutine_pool_put_global(batch); > + } > + > + batch = coroutine_pool_batch_new(); > + QSLIST_INSERT_HEAD(local_pool, batch, next); > + } > + > + QSLIST_INSERT_HEAD(&batch->list, co, pool_next); > + batch->size++; > } > > Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque) > @@ -58,31 +219,7 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry, > void *opaque) > Coroutine *co = NULL; > > if (IS_ENABLED(CONFIG_COROUTINE_POOL)) { > - CoroutineQSList *alloc_pool = get_ptr_alloc_pool(); > - > - co = QSLIST_FIRST(alloc_pool); > - if (!co) { > - if (release_pool_size > POOL_MIN_BATCH_SIZE) { > - /* Slow path; a good place to register the destructor, too. > */ > - Notifier *notifier = > get_ptr_coroutine_pool_cleanup_notifier(); > - if (!notifier->notify) { > - notifier->notify = coroutine_pool_cleanup; > - qemu_thread_atexit_add(notifier); > - } > - > - /* This is not exact; there could be a little skew between > - * release_pool_size and the actual size of release_pool. > But > - * it is just a heuristic, it does not need to be perfect. > - */ > - set_alloc_pool_size(qatomic_xchg(&release_pool_size, 0)); > - QSLIST_MOVE_ATOMIC(alloc_pool, &release_pool); > - co = QSLIST_FIRST(alloc_pool); > - } > - } > - if (co) { > - QSLIST_REMOVE_HEAD(alloc_pool, pool_next); > - set_alloc_pool_size(get_alloc_pool_size() - 1); > - } > + co = coroutine_pool_get(); > } > > if (!co) { > @@ -100,19 +237,10 @@ static void coroutine_delete(Coroutine *co) > co->caller = NULL; > > if (IS_ENABLED(CONFIG_COROUTINE_POOL)) { > - if (release_pool_size < qatomic_read(&pool_max_size) * 2) { > - QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next); > - qatomic_inc(&release_pool_size); > - return; > - } > - if (get_alloc_pool_size() < qatomic_read(&pool_max_size)) { > - QSLIST_INSERT_HEAD(get_ptr_alloc_pool(), co, pool_next); > - set_alloc_pool_size(get_alloc_pool_size() + 1); > - return; > - } > + coroutine_pool_put(co); > + } else { > + qemu_coroutine_delete(co); > } > - > - qemu_coroutine_delete(co); > } > > void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co) > @@ -223,10 +351,46 @@ AioContext *qemu_coroutine_get_aio_context(Coroutine > *co) > > void qemu_coroutine_inc_pool_size(unsigned int additional_pool_size) > { > - qatomic_add(&pool_max_size, additional_pool_size); > + QEMU_LOCK_GUARD(&global_pool_lock); > + global_pool_max_size += additional_pool_size; > } > > void qemu_coroutine_dec_pool_size(unsigned int removing_pool_size) > { > - qatomic_sub(&pool_max_size, removing_pool_size); > + QEMU_LOCK_GUARD(&global_pool_lock); > + global_pool_max_size -= removing_pool_size; > +} > + > +static unsigned int get_global_pool_hard_max_size(void) > +{ > +#ifdef __linux__ > + g_autofree char *contents = NULL; > + int max_map_count; > + > + /* > + * Linux processes can have up to max_map_count virtual memory areas > + * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond this limit. > We > + * must limit the coroutine pool to a safe size to avoid running out of > + * VMAs. > + */ > + if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, NULL, > + NULL) && > + qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) { > + /* > + * This is a conservative upper bound that avoids exceeding > + * max_map_count. Leave half for non-coroutine users like library > + * dependencies, vhost-user, etc. Each coroutine takes up 2 VMAs so > + * halve the amount again. > + */ > + return max_map_count / 4; > + } > +#endif > + > + return UINT_MAX; > +} > + > +static void __attribute__((constructor)) qemu_coroutine_init(void) > +{ > + qemu_mutex_init(&global_pool_lock); > + global_pool_hard_max_size = get_global_pool_hard_max_size(); > } > -- > 2.44.0 > > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|