[PATCH 2/2] coroutine: take exactly one batch from global pool at a time

[wanghonghao <wanghonghao@bytedance.com>]

This patch replace the global coroutine queue with a lock-free stack of which
the elements are coroutine queues. Threads can put coroutine queues into the
stack or take queues from it and each coroutine queue has exactly
POOL_BATCH_SIZE coroutines. Note that the stack is not strictly LIFO, but it's
enough for buffer pool.

Coroutines will be put into thread-local pools first while release. Now the
fast pathes of both allocation and release are atomic-free, and there won't
be too many coroutines remain in a single thread since POOL_BATCH_SIZE has been
reduced to 16.

In practice, I've run a VM with two block devices binding to two different
iothreads, and run fio with iodepth 128 on each device. It maintains around
400 coroutines and has about 1% chance of calling to `qemu_coroutine_new`
without this patch. And with this patch, it maintains no more than 273
coroutines and doesn't call `qemu_coroutine_new` after initial allocations.

Signed-off-by: wanghonghao <wanghonghao@bytedance.com>
---
 util/qemu-coroutine.c | 63 ++++++++++++++++++++++++++++---------------
 1 file changed, 42 insertions(+), 21 deletions(-)

diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
index c3caa6c770..070d492edc 100644
--- a/util/qemu-coroutine.c
+++ b/util/qemu-coroutine.c
@@ -21,13 +21,14 @@
 #include "block/aio.h"
 
 enum {
-    POOL_BATCH_SIZE = 64,
+    POOL_BATCH_SIZE = 16,
+    POOL_MAX_BATCHES = 32,
 };
 
-/** Free list to speed up creation */
-static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);
-static unsigned int release_pool_size;
-static __thread QSLIST_HEAD(, Coroutine) alloc_pool = QSLIST_HEAD_INITIALIZER(pool);
+/** Free stack to speed up creation */
+static QSLIST_HEAD(, Coroutine) pool[POOL_MAX_BATCHES];
+static int pool_top;
+static __thread QSLIST_HEAD(, Coroutine) alloc_pool;
 static __thread unsigned int alloc_pool_size;
 static __thread Notifier coroutine_pool_cleanup_notifier;
 
@@ -49,20 +50,26 @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque)
     if (CONFIG_COROUTINE_POOL) {
         co = QSLIST_FIRST(&alloc_pool);
         if (!co) {
-            if (release_pool_size > POOL_BATCH_SIZE) {
-                /* Slow path; a good place to register the destructor, too.  */
-                if (!coroutine_pool_cleanup_notifier.notify) {
-                    coroutine_pool_cleanup_notifier.notify = coroutine_pool_cleanup;
-                    qemu_thread_atexit_add(&coroutine_pool_cleanup_notifier);
+            int top;
+
+            /* Slow path; a good place to register the destructor, too.  */
+            if (!coroutine_pool_cleanup_notifier.notify) {
+                coroutine_pool_cleanup_notifier.notify = coroutine_pool_cleanup;
+                qemu_thread_atexit_add(&coroutine_pool_cleanup_notifier);
+            }
+
+            while ((top = atomic_read(&pool_top)) > 0) {
+                if (atomic_cmpxchg(&pool_top, top, top - 1) != top) {
+                    continue;
                 }
 
-                /* 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.
-                 */
-                alloc_pool_size = atomic_xchg(&release_pool_size, 0);
-                QSLIST_MOVE_ATOMIC(&alloc_pool, &release_pool);
+                QSLIST_MOVE_ATOMIC(&alloc_pool, &pool[top - 1]);
                 co = QSLIST_FIRST(&alloc_pool);
+
+                if (co) {
+                    alloc_pool_size = POOL_BATCH_SIZE;
+                    break;
+                }
             }
         }
         if (co) {
@@ -86,16 +93,30 @@ static void coroutine_delete(Coroutine *co)
     co->caller = NULL;
 
     if (CONFIG_COROUTINE_POOL) {
-        if (release_pool_size < POOL_BATCH_SIZE * 2) {
-            QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);
-            atomic_inc(&release_pool_size);
-            return;
-        }
+        int top, value, old;
+
         if (alloc_pool_size < POOL_BATCH_SIZE) {
             QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);
             alloc_pool_size++;
             return;
         }
+
+        for (top = atomic_read(&pool_top); top < POOL_MAX_BATCHES; top++) {
+            QSLIST_REPLACE_ATOMIC(&pool[top], &alloc_pool);
+            if (!QSLIST_EMPTY(&alloc_pool)) {
+                continue;
+            }
+
+            value = top + 1;
+
+            do {
+                old = atomic_cmpxchg(&pool_top, top, value);
+            } while (old != top && (top = old) < value);
+
+            QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);
+            alloc_pool_size = 1;
+            return;
+        }
     }
 
     qemu_coroutine_delete(co);
-- 
2.24.3 (Apple Git-128)