[Qemu-devel] [PATCH v3 1/2] coroutine: make pool size dynamic

[Stefan Hajnoczi <stefanha@redhat.com>]

Allow coroutine users to adjust the pool size.  For example, if the
guest has multiple emulated disk drives we should keep around more
coroutines.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/block/coroutine.h | 11 +++++++++++
 qemu-coroutine.c          | 26 +++++++++++++++++++++++---
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/include/block/coroutine.h b/include/block/coroutine.h
index a1797ae..07eeb3c 100644
--- a/include/block/coroutine.h
+++ b/include/block/coroutine.h
@@ -223,4 +223,15 @@ void coroutine_fn co_aio_sleep_ns(AioContext *ctx, QEMUClockType type,
  * Note that this function clobbers the handlers for the file descriptor.
  */
 void coroutine_fn yield_until_fd_readable(int fd);
+
+/**
+ * Add or subtract from the coroutine pool size
+ *
+ * The coroutine implementation keeps a pool of coroutines to be reused by
+ * qemu_coroutine_create().  This makes coroutine creation cheap.  Heavy
+ * coroutine users should call this to reserve pool space.  Call it again with
+ * a negative number to release pool space.
+ */
+void qemu_coroutine_adjust_pool_size(int n);
+
 #endif /* QEMU_COROUTINE_H */
diff --git a/qemu-coroutine.c b/qemu-coroutine.c
index 4708521..bd574aa 100644
--- a/qemu-coroutine.c
+++ b/qemu-coroutine.c
@@ -19,14 +19,14 @@
 #include "block/coroutine_int.h"
 
 enum {
-    /* Maximum free pool size prevents holding too many freed coroutines */
-    POOL_MAX_SIZE = 64,
+    POOL_DEFAULT_SIZE = 64,
 };
 
 /** Free list to speed up creation */
 static QemuMutex pool_lock;
 static QSLIST_HEAD(, Coroutine) pool = QSLIST_HEAD_INITIALIZER(pool);
 static unsigned int pool_size;
+static unsigned int pool_max_size = POOL_DEFAULT_SIZE;
 
 Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
 {
@@ -55,7 +55,7 @@ static void coroutine_delete(Coroutine *co)
 {
     if (CONFIG_COROUTINE_POOL) {
         qemu_mutex_lock(&pool_lock);
-        if (pool_size < POOL_MAX_SIZE) {
+        if (pool_size < pool_max_size) {
             QSLIST_INSERT_HEAD(&pool, co, pool_next);
             co->caller = NULL;
             pool_size++;
@@ -137,3 +137,23 @@ void coroutine_fn qemu_coroutine_yield(void)
     self->caller = NULL;
     coroutine_swap(self, to);
 }
+
+void qemu_coroutine_adjust_pool_size(int n)
+{
+    qemu_mutex_lock(&pool_lock);
+
+    pool_max_size += n;
+
+    /* Callers should never take away more than they added */
+    assert(pool_max_size >= POOL_DEFAULT_SIZE);
+
+    /* Trim oversized pool down to new max */
+    while (pool_size > pool_max_size) {
+        Coroutine *co = QSLIST_FIRST(&pool);
+        QSLIST_REMOVE_HEAD(&pool, pool_next);
+        pool_size--;
+        qemu_coroutine_delete(co);
+    }
+
+    qemu_mutex_unlock(&pool_lock);
+}
-- 
1.9.3