From: Stefan Hajnoczi <stefanha@redhat.com>

Date: Mon, 11 May 2020 19:36:30 +0100

Subject: [PATCH] aio-posix: disable fdmon-io_uring when GSource is used

The glib event loop does not call fdmon_io_uring_wait() so fd handlers

waiting to be submitted build up in the list. There is no benefit is

using io_uring when the glib GSource is being used, so disable it

instead of implementing a more complex fix.

This fixes a memory leak where AioHandlers would build up and increasing

amounts of CPU time were spent iterating them in aio_pending(). The

symptom is that guests become slow when QEMU is built with io_uring

support.

Buglink: https://bugs.launchpad.net/qemu/+bug/1877716

Fixes: 73fd282e7b6dd4e4ea1c3bbb3d302c8db51e4ccf ("aio-posix: add io_uring fd monitoring implementation")

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

---

 include/block/aio.h |  3 +++

 util/aio-posix.c    | 12 ++++++++++++

 util/aio-win32.c    |  4 ++++

 util/async.c        |  1 +

 4 files changed, 20 insertions(+)

diff --git a/include/block/aio.h b/include/block/aio.h

index 62ed954344..b2f703fa3f 100644

--- a/include/block/aio.h

+++ b/include/block/aio.h

@@ -701,6 +701,9 @@ void aio_context_setup(AioContext *ctx);

  */

 void aio_context_destroy(AioContext *ctx);

+/* Used internally, do not call outside AioContext code */

+void aio_context_use_g_source(AioContext *ctx);

+

 /**

  * aio_context_set_poll_params:

  * @ctx: the aio context

diff --git a/util/aio-posix.c b/util/aio-posix.c

index 8af334ab19..1b2a3af65b 100644

--- a/util/aio-posix.c

+++ b/util/aio-posix.c

@@ -682,6 +682,18 @@ void aio_context_destroy(AioContext *ctx)

     aio_free_deleted_handlers(ctx);

 }

+void aio_context_use_g_source(AioContext *ctx)

+{

+    /*

+     * Disable io_uring when the glib main loop is used because it doesn't

+     * support mixed glib/aio_poll() usage. It relies on aio_poll() being

+     * called regularly so that changes to the monitored file descriptors are

+     * submitted, otherwise a list of pending fd handlers builds up.

+     */

+    fdmon_io_uring_destroy(ctx);

+    aio_free_deleted_handlers(ctx);

+}

+

 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,

                                  int64_t grow, int64_t shrink, Error **errp)

 {

diff --git a/util/aio-win32.c b/util/aio-win32.c

index 729d533faf..953c56ab48 100644

--- a/util/aio-win32.c

+++ b/util/aio-win32.c

@@ -414,6 +414,10 @@ void aio_context_destroy(AioContext *ctx)

 {

 }

+void aio_context_use_g_source(AioContext *ctx)

+{

+}

+

 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,

                                  int64_t grow, int64_t shrink, Error **errp)

 {

diff --git a/util/async.c b/util/async.c

index 3165a28f2f..1319eee3bc 100644

--- a/util/async.c

+++ b/util/async.c

@@ -362,6 +362,7 @@ static GSourceFuncs aio_source_funcs = {

 GSource *aio_get_g_source(AioContext *ctx)

 {

+    aio_context_use_g_source(ctx);

     g_source_ref(&ctx->source);

     return &ctx->source;

 }