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Re: [PATCH v2 5/6] block/linux-aio: convert to blk_io_plug_call() API


From: Stefano Garzarella
Subject: Re: [PATCH v2 5/6] block/linux-aio: convert to blk_io_plug_call() API
Date: Wed, 24 May 2023 10:52:03 +0200

On Tue, May 23, 2023 at 01:12:59PM -0400, Stefan Hajnoczi wrote:
Stop using the .bdrv_co_io_plug() API because it is not multi-queue
block layer friendly. Use the new blk_io_plug_call() API to batch I/O
submission instead.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
include/block/raw-aio.h |  7 -------
block/file-posix.c      | 28 ----------------------------
block/linux-aio.c       | 41 +++++++++++------------------------------
3 files changed, 11 insertions(+), 65 deletions(-)

diff --git a/include/block/raw-aio.h b/include/block/raw-aio.h
index da60ca13ef..0f63c2800c 100644
--- a/include/block/raw-aio.h
+++ b/include/block/raw-aio.h
@@ -62,13 +62,6 @@ int coroutine_fn laio_co_submit(int fd, uint64_t offset, 
QEMUIOVector *qiov,

void laio_detach_aio_context(LinuxAioState *s, AioContext *old_context);
void laio_attach_aio_context(LinuxAioState *s, AioContext *new_context);
-
-/*
- * laio_io_plug/unplug work in the thread's current AioContext, therefore the
- * caller must ensure that they are paired in the same IOThread.
- */
-void laio_io_plug(void);
-void laio_io_unplug(uint64_t dev_max_batch);
#endif
/* io_uring.c - Linux io_uring implementation */
#ifdef CONFIG_LINUX_IO_URING
diff --git a/block/file-posix.c b/block/file-posix.c
index 7baa8491dd..ac1ed54811 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2550,26 +2550,6 @@ static int coroutine_fn raw_co_pwritev(BlockDriverState 
*bs, int64_t offset,
    return raw_co_prw(bs, offset, bytes, qiov, QEMU_AIO_WRITE);
}

-static void coroutine_fn raw_co_io_plug(BlockDriverState *bs)
-{
-    BDRVRawState __attribute__((unused)) *s = bs->opaque;
-#ifdef CONFIG_LINUX_AIO
-    if (s->use_linux_aio) {
-        laio_io_plug();
-    }
-#endif
-}
-
-static void coroutine_fn raw_co_io_unplug(BlockDriverState *bs)
-{
-    BDRVRawState __attribute__((unused)) *s = bs->opaque;
-#ifdef CONFIG_LINUX_AIO
-    if (s->use_linux_aio) {
-        laio_io_unplug(s->aio_max_batch);
-    }
-#endif
-}
-
static int coroutine_fn raw_co_flush_to_disk(BlockDriverState *bs)
{
    BDRVRawState *s = bs->opaque;
@@ -3914,8 +3894,6 @@ BlockDriver bdrv_file = {
    .bdrv_co_copy_range_from = raw_co_copy_range_from,
    .bdrv_co_copy_range_to  = raw_co_copy_range_to,
    .bdrv_refresh_limits = raw_refresh_limits,
-    .bdrv_co_io_plug        = raw_co_io_plug,
-    .bdrv_co_io_unplug      = raw_co_io_unplug,
    .bdrv_attach_aio_context = raw_aio_attach_aio_context,

    .bdrv_co_truncate                   = raw_co_truncate,
@@ -4286,8 +4264,6 @@ static BlockDriver bdrv_host_device = {
    .bdrv_co_copy_range_from = raw_co_copy_range_from,
    .bdrv_co_copy_range_to  = raw_co_copy_range_to,
    .bdrv_refresh_limits = raw_refresh_limits,
-    .bdrv_co_io_plug        = raw_co_io_plug,
-    .bdrv_co_io_unplug      = raw_co_io_unplug,
    .bdrv_attach_aio_context = raw_aio_attach_aio_context,

    .bdrv_co_truncate                   = raw_co_truncate,
@@ -4424,8 +4400,6 @@ static BlockDriver bdrv_host_cdrom = {
    .bdrv_co_pwritev        = raw_co_pwritev,
    .bdrv_co_flush_to_disk  = raw_co_flush_to_disk,
    .bdrv_refresh_limits    = cdrom_refresh_limits,
-    .bdrv_co_io_plug        = raw_co_io_plug,
-    .bdrv_co_io_unplug      = raw_co_io_unplug,
    .bdrv_attach_aio_context = raw_aio_attach_aio_context,

    .bdrv_co_truncate                   = raw_co_truncate,
@@ -4552,8 +4526,6 @@ static BlockDriver bdrv_host_cdrom = {
    .bdrv_co_pwritev        = raw_co_pwritev,
    .bdrv_co_flush_to_disk  = raw_co_flush_to_disk,
    .bdrv_refresh_limits    = cdrom_refresh_limits,
-    .bdrv_co_io_plug        = raw_co_io_plug,
-    .bdrv_co_io_unplug      = raw_co_io_unplug,
    .bdrv_attach_aio_context = raw_aio_attach_aio_context,

    .bdrv_co_truncate                   = raw_co_truncate,
diff --git a/block/linux-aio.c b/block/linux-aio.c
index 442c86209b..5021aed68f 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -15,6 +15,7 @@
#include "qemu/event_notifier.h"
#include "qemu/coroutine.h"
#include "qapi/error.h"
+#include "sysemu/block-backend.h"

/* Only used for assertions.  */
#include "qemu/coroutine_int.h"
@@ -46,7 +47,6 @@ struct qemu_laiocb {
};

typedef struct {
-    int plugged;
    unsigned int in_queue;
    unsigned int in_flight;
    bool blocked;
@@ -236,7 +236,7 @@ static void 
qemu_laio_process_completions_and_submit(LinuxAioState *s)
{
    qemu_laio_process_completions(s);

-    if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
+    if (!QSIMPLEQ_EMPTY(&s->io_q.pending)) {
        ioq_submit(s);
    }
}
@@ -277,7 +277,6 @@ static void qemu_laio_poll_ready(EventNotifier *opaque)
static void ioq_init(LaioQueue *io_q)
{
    QSIMPLEQ_INIT(&io_q->pending);
-    io_q->plugged = 0;
    io_q->in_queue = 0;
    io_q->in_flight = 0;
    io_q->blocked = false;
@@ -354,31 +353,11 @@ static uint64_t laio_max_batch(LinuxAioState *s, uint64_t 
dev_max_batch)
    return max_batch;
}

-void laio_io_plug(void)
+static void laio_unplug_fn(void *opaque)
{
-    AioContext *ctx = qemu_get_current_aio_context();
-    LinuxAioState *s = aio_get_linux_aio(ctx);
+    LinuxAioState *s = opaque;

-    s->io_q.plugged++;
-}
-
-void laio_io_unplug(uint64_t dev_max_batch)
-{
-    AioContext *ctx = qemu_get_current_aio_context();
-    LinuxAioState *s = aio_get_linux_aio(ctx);
-
-    assert(s->io_q.plugged);
-    s->io_q.plugged--;
-
-    /*
-     * Why max batch checking is performed here:
-     * Another BDS may have queued requests with a higher dev_max_batch and
-     * therefore in_queue could now exceed our dev_max_batch. Re-check the max
-     * batch so we can honor our device's dev_max_batch.
-     */
-    if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||

Why are we removing this condition?
Could the same situation occur with the new API?

Thanks,
Stefano

-        (!s->io_q.plugged &&
-         !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {
+    if (!s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
        ioq_submit(s);
    }
}
@@ -410,10 +389,12 @@ static int laio_do_submit(int fd, struct qemu_laiocb 
*laiocb, off_t offset,

    QSIMPLEQ_INSERT_TAIL(&s->io_q.pending, laiocb, next);
    s->io_q.in_queue++;
-    if (!s->io_q.blocked &&
-        (!s->io_q.plugged ||
-         s->io_q.in_queue >= laio_max_batch(s, dev_max_batch))) {
-        ioq_submit(s);
+    if (!s->io_q.blocked) {
+        if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch)) {
+            ioq_submit(s);
+        } else {
+            blk_io_plug_call(laio_unplug_fn, s);
+        }
    }

    return 0;
--
2.40.1





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