[PATCH] coroutine: cap per-thread local pool size

[Stefan Hajnoczi]

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 <srao@redhat.com>
Reported-by: Boaz Ben Shabat <bbenshab@redhat.com>
Reported-by: Joe Mario <jmario@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
This patch obsoletes "[PATCH v2] virtio-blk: iothread-vq-mapping
coroutine pool sizing" because the pool size is now global instead of
per thread:
20240312151204.412624-1-stefanha@redhat.com/">https://lore.kernel.org/qemu-devel/20240312151204.412624-1-stefanha@redhat.com/

Please don't apply "[PATCH v2] virtio-blk: iothread-vq-mapping coroutine
pool sizing".

 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