Re: [PATCH 1/6] block: add blk_io_plug_call() API

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, May 19, 2023 at 10:45:57AM +0200, Stefano Garzarella wrote:
> On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
> > Introduce a new API for thread-local blk_io_plug() that does not
> > traverse the block graph. The goal is to make blk_io_plug() multi-queue
> > friendly.
> > 
> > Instead of having block drivers track whether or not we're in a plugged
> > section, provide an API that allows them to defer a function call until
> > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
> > called multiple times with the same fn/opaque pair, then fn() is only
> > called once at the end of the function - resulting in batching.
> > 
> > This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
> > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
> > because the plug state is now thread-local.
> > 
> > Later patches convert block drivers to blk_io_plug_call() and then we
> > can finally remove .bdrv_co_io_plug() once all block drivers have been
> > converted.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > MAINTAINERS                       |   1 +
> > include/sysemu/block-backend-io.h |  13 +--
> > block/block-backend.c             |  22 -----
> > block/plug.c                      | 159 ++++++++++++++++++++++++++++++
> > hw/block/dataplane/xen-block.c    |   8 +-
> > hw/block/virtio-blk.c             |   4 +-
> > hw/scsi/virtio-scsi.c             |   6 +-
> > block/meson.build                 |   1 +
> > 8 files changed, 173 insertions(+), 41 deletions(-)
> > create mode 100644 block/plug.c
> > 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 50585117a0..574202295c 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -2644,6 +2644,7 @@ F: util/aio-*.c
> > F: util/aio-*.h
> > F: util/fdmon-*.c
> > F: block/io.c
> > +F: block/plug.c
> > F: migration/block*
> > F: include/block/aio.h
> > F: include/block/aio-wait.h
> > diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
> > index d62a7ee773..be4dcef59d 100644
> > --- a/include/sysemu/block-backend-io.h
> > +++ b/include/sysemu/block-backend-io.h
> > @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
> > int blk_get_max_iov(BlockBackend *blk);
> > int blk_get_max_hw_iov(BlockBackend *blk);
> > 
> > -/*
> > - * blk_io_plug/unplug are thread-local operations. This means that multiple
> > - * IOThreads can simultaneously call plug/unplug, but the caller must ensure
> > - * that each unplug() is called in the same IOThread of the matching plug().
> > - */
> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk);
> > -void co_wrapper blk_io_plug(BlockBackend *blk);
> > -
> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk);
> > -void co_wrapper blk_io_unplug(BlockBackend *blk);
> > +void blk_io_plug(void);
> > +void blk_io_unplug(void);
> > +void blk_io_plug_call(void (*fn)(void *), void *opaque);
> > 
> > AioContext *blk_get_aio_context(BlockBackend *blk);
> > BlockAcctStats *blk_get_stats(BlockBackend *blk);
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index ca537cd0ad..1f1d226ba6 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
> >     notifier_list_add(&blk->insert_bs_notifiers, notify);
> > }
> > 
> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk)
> > -{
> > -    BlockDriverState *bs = blk_bs(blk);
> > -    IO_CODE();
> > -    GRAPH_RDLOCK_GUARD();
> > -
> > -    if (bs) {
> > -        bdrv_co_io_plug(bs);
> > -    }
> > -}
> > -
> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk)
> > -{
> > -    BlockDriverState *bs = blk_bs(blk);
> > -    IO_CODE();
> > -    GRAPH_RDLOCK_GUARD();
> > -
> > -    if (bs) {
> > -        bdrv_co_io_unplug(bs);
> > -    }
> > -}
> > -
> > BlockAcctStats *blk_get_stats(BlockBackend *blk)
> > {
> >     IO_CODE();
> > diff --git a/block/plug.c b/block/plug.c
> > new file mode 100644
> > index 0000000000..6738a568ba
> > --- /dev/null
> > +++ b/block/plug.c
> > @@ -0,0 +1,159 @@
> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
> > +/*
> > + * Block I/O plugging
> > + *
> > + * Copyright Red Hat.
> > + *
> > + * This API defers a function call within a blk_io_plug()/blk_io_unplug()
> > + * section, allowing multiple calls to batch up. This is a performance
> > + * optimization that is used in the block layer to submit several I/O requests
> > + * at once instead of individually:
> > + *
> > + *   blk_io_plug(); <-- start of plugged region
> > + *   ...
> > + *   blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
> > + *   blk_io_plug_call(my_func, my_obj); <-- another
> > + *   blk_io_plug_call(my_func, my_obj); <-- another
> > + *   ...
> > + *   blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
> > + *
> > + * This code is actually generic and not tied to the block layer. If another
> > + * subsystem needs this functionality, it could be renamed.
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "qemu/coroutine-tls.h"
> > +#include "qemu/notify.h"
> > +#include "qemu/thread.h"
> > +#include "sysemu/block-backend.h"
> > +
> > +/* A function call that has been deferred until unplug() */
> > +typedef struct {
> > +    void (*fn)(void *);
> > +    void *opaque;
> > +} UnplugFn;
> > +
> > +/* Per-thread state */
> > +typedef struct {
> > +    unsigned count;       /* how many times has plug() been called? */
> > +    GArray *unplug_fns;   /* functions to call at unplug time */
> > +} Plug;
> > +
> > +/* Use get_ptr_plug() to fetch this thread-local value */
> > +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
> > +
> > +/* Called at thread cleanup time */
> > +static void blk_io_plug_atexit(Notifier *n, void *value)
> > +{
> > +    Plug *plug = get_ptr_plug();
> > +    g_array_free(plug->unplug_fns, TRUE);
> > +}
> > +
> > +/* This won't involve coroutines, so use __thread */
> > +static __thread Notifier blk_io_plug_atexit_notifier;
> > +
> > +/**
> > + * blk_io_plug_call:
> > + * @fn: a function pointer to be invoked
> > + * @opaque: a user-defined argument to @fn()
> > + *
> > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
> > + * section.
> 
> Just to understand better, what if two BlockDrivers share the same
> iothread but one calls blk_io_plug()/blk_io_unplug(), while the other
> calls this function not in a blk_io_plug()/blk_io_unplug() section?
> 
> If the call is in the middle of the other BlockDriver's section, it is
> deferred, right?

Yes, the call is deferred until blk_io_unplug().

> Is this situation possible?

One scenario I can think of is when aio_poll() is called between
plug/unplug. In that case, some I/O associated with device B might
happen while device A is with plug/unplug.

> Or should we prevent blk_io_plug_call() from being called out of a
> blk_io_plug()/blk_io_unplug() section?

blk_io_plug_call() is called outside blk_io_plug()/blk_io_unplug() when
device emulation doesn't use plug/unplug. For example, IDE doesn't use
it but still calls down into the same Linux AIO or io_uring code that
invokes blk_io_plug_call(). This is why blk_io_plug_call() calls fn()
immediately when invoked outside plug/unplug.

Stefan

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