[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/6] block: add blk_io_plug_call() API
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@xxxxxxxxxx> > > --- > > 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 Attachment:
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