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From 1a6556bc1317af4669d058e6df70bc1c036d37a5 Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Fri, 14 Sep 2018 10:55:04 +0200
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Subject: [PATCH 13/49] block: Avoid unnecessary aio_poll() in AIO_WAIT_WHILE()
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20180914105540.18077-7-kwolf@redhat.com>
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Patchwork-id: 82158
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O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 06/42] block: Avoid unnecessary aio_poll() in AIO_WAIT_WHILE()
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Bugzilla: 1601212
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RH-Acked-by: John Snow <jsnow@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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Commit 91af091f923 added an additional aio_poll() to BDRV_POLL_WHILE()
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in order to make sure that all pending BHs are executed on drain. This
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was the wrong place to make the fix, as it is useless overhead for all
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other users of the macro and unnecessarily complicates the mechanism.
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This patch effectively reverts said commit (the context has changed a
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bit and the code has moved to AIO_WAIT_WHILE()) and instead polls in the
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loop condition for drain.
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The effect is probably hard to measure in any real-world use case
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because actual I/O will dominate, but if I run only the initialisation
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part of 'qemu-img convert' where it calls bdrv_block_status() for the
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whole image to find out how much data there is copy, this phase actually
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needs only roughly half the time after this patch.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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(cherry picked from commit 1cc8e54ada97f7ac479554e15ca9e426c895b158)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/io.c | 11 ++++++++++-
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include/block/aio-wait.h | 22 ++++++++--------------
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2 files changed, 18 insertions(+), 15 deletions(-)
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diff --git a/block/io.c b/block/io.c
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index e5fc42c..4d332c3 100644
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--- a/block/io.c
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+++ b/block/io.c
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@@ -181,13 +181,22 @@ static void bdrv_drain_invoke(BlockDriverState *bs, bool begin)
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BDRV_POLL_WHILE(bs, !data.done);
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}
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+/* Returns true if BDRV_POLL_WHILE() should go into a blocking aio_poll() */
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+static bool bdrv_drain_poll(BlockDriverState *bs)
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+{
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+ /* Execute pending BHs first and check everything else only after the BHs
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+ * have executed. */
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+ while (aio_poll(bs->aio_context, false));
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+ return atomic_read(&bs->in_flight);
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+}
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+
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static bool bdrv_drain_recurse(BlockDriverState *bs)
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{
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BdrvChild *child, *tmp;
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bool waited;
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/* Wait for drained requests to finish */
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- waited = BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0);
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+ waited = BDRV_POLL_WHILE(bs, bdrv_drain_poll(bs));
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QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) {
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BlockDriverState *bs = child->bs;
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diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
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index 8c90a2e..783d367 100644
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--- a/include/block/aio-wait.h
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+++ b/include/block/aio-wait.h
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@@ -73,29 +73,23 @@ typedef struct {
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*/
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#define AIO_WAIT_WHILE(wait, ctx, cond) ({ \
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bool waited_ = false; \
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- bool busy_ = true; \
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AioWait *wait_ = (wait); \
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AioContext *ctx_ = (ctx); \
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if (in_aio_context_home_thread(ctx_)) { \
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- while ((cond) || busy_) { \
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- busy_ = aio_poll(ctx_, (cond)); \
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- waited_ |= !!(cond) | busy_; \
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+ while ((cond)) { \
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+ aio_poll(ctx_, true); \
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+ waited_ = true; \
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} \
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} else { \
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assert(qemu_get_current_aio_context() == \
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qemu_get_aio_context()); \
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/* Increment wait_->num_waiters before evaluating cond. */ \
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atomic_inc(&wait_->num_waiters); \
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- while (busy_) { \
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- if ((cond)) { \
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- waited_ = busy_ = true; \
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- aio_context_release(ctx_); \
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- aio_poll(qemu_get_aio_context(), true); \
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- aio_context_acquire(ctx_); \
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- } else { \
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- busy_ = aio_poll(ctx_, false); \
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- waited_ |= busy_; \
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- } \
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+ while ((cond)) { \
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+ aio_context_release(ctx_); \
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+ aio_poll(qemu_get_aio_context(), true); \
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+ aio_context_acquire(ctx_); \
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+ waited_ = true; \
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} \
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atomic_dec(&wait_->num_waiters); \
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} \
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--
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1.8.3.1
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