Am 18.04.2017 um 16:30 hat Fam Zheng geschrieben:
> During block job completion, nothing is preventing
> block_job_defer_to_main_loop_bh from being called in a nested
> aio_poll(), which is a trouble, such as in this code path:
>
> qmp_block_commit
> commit_active_start
> bdrv_reopen
> bdrv_reopen_multiple
> bdrv_reopen_prepare
> bdrv_flush
> aio_poll
> aio_bh_poll
> aio_bh_call
> block_job_defer_to_main_loop_bh
> stream_complete
> bdrv_reopen
>
> block_job_defer_to_main_loop_bh is the last step of the stream job,
> which should have been "paused" by the bdrv_drained_begin/end in
> bdrv_reopen_multiple, but it is not done because it's in the form of a
> main loop BH.
>
> Similar to why block jobs should be paused between drained_begin and
> drained_end, BHs they schedule must be excluded as well. To achieve
> this, this patch forces draining the BH in BDRV_POLL_WHILE.
>
> As a side effect this fixes a hang in block_job_detach_aio_context
> during system_reset when a block job is ready:
>
> #0 0x0000555555aa79f3 in bdrv_drain_recurse
> #1 0x0000555555aa825d in bdrv_drained_begin
> #2 0x0000555555aa8449 in bdrv_drain
> #3 0x0000555555a9c356 in blk_drain
> #4 0x0000555555aa3cfd in mirror_drain
> #5 0x0000555555a66e11 in block_job_detach_aio_context
> #6 0x0000555555a62f4d in bdrv_detach_aio_context
> #7 0x0000555555a63116 in bdrv_set_aio_context
> #8 0x0000555555a9d326 in blk_set_aio_context
> #9 0x00005555557e38da in virtio_blk_data_plane_stop
> #10 0x00005555559f9d5f in virtio_bus_stop_ioeventfd
> #11 0x00005555559fa49b in virtio_bus_stop_ioeventfd
> #12 0x00005555559f6a18 in virtio_pci_stop_ioeventfd
> #13 0x00005555559f6a18 in virtio_pci_reset
> #14 0x00005555559139a9 in qdev_reset_one
> #15 0x0000555555916738 in qbus_walk_children
> #16 0x0000555555913318 in qdev_walk_children
> #17 0x0000555555916738 in qbus_walk_children
> #18 0x00005555559168ca in qemu_devices_reset
> #19 0x000055555581fcbb in pc_machine_reset
> #20 0x00005555558a4d96 in qemu_system_reset
> #21 0x000055555577157a in main_loop_should_exit
> #22 0x000055555577157a in main_loop
> #23 0x000055555577157a in main
>
> The rationale is that the loop in block_job_detach_aio_context cannot
> make any progress in pausing/completing the job, because bs->in_flight
> is 0, so bdrv_drain doesn't process the block_job_defer_to_main_loop
> BH. With this patch, it does.
>
> Reported-by: Jeff Cody <jc...@redhat.com>
> Signed-off-by: Fam Zheng <f...@redhat.com>
Fam, do you remember whether this was really only about drain? Because
in that case...
> diff --git a/include/block/block.h b/include/block/block.h
> index 97d4330..5ddc0cf 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -381,12 +381,13 @@ void bdrv_drain_all(void);
>
> #define BDRV_POLL_WHILE(bs, cond) ({ \
> bool waited_ = false; \
> + bool busy_ = true; \
> BlockDriverState *bs_ = (bs); \
> AioContext *ctx_ = bdrv_get_aio_context(bs_); \
> if (aio_context_in_iothread(ctx_)) { \
> - while ((cond)) { \
> - aio_poll(ctx_, true); \
> - waited_ = true; \
> + while ((cond) || busy_) { \
> + busy_ = aio_poll(ctx_, (cond)); \
> + waited_ |= !!(cond) | busy_; \
> } \
> } else { \
> assert(qemu_get_current_aio_context() == \
> @@ -398,11 +399,16 @@ void bdrv_drain_all(void);
> */ \
> assert(!bs_->wakeup); \
> bs_->wakeup = true; \
> - while ((cond)) { \
> - aio_context_release(ctx_); \
> - aio_poll(qemu_get_aio_context(), true); \
> - aio_context_acquire(ctx_); \
> - waited_ = true; \
> + while (busy_) { \
> + if ((cond)) { \
> + waited_ = busy_ = true; \
> + aio_context_release(ctx_); \
> + aio_poll(qemu_get_aio_context(), true); \
> + aio_context_acquire(ctx_); \
> + } else { \
> + busy_ = aio_poll(ctx_, false); \
> + waited_ |= busy_; \
> + } \
> } \
> bs_->wakeup = false; \
> } \
...fixing it in BDRV_POLL_WHILE() rather than the drain functions may
have been a bad idea.
I got a bug assigned where we have a large (200 GB) fragmented qcow2
image, and qemu-img convert takes two hours before it even starts to
copy data. What it does in this time is iterating through the whole
image with bdrv_block_status_above() in order to estimate the work to be
done (and of course it will repeat the same calls while actually copying
data).
One of the things I saw is that between each pair of lseek() calls, we
also have unnecessary poll() calls, and these were introduced by this
patch. If I modify bdrv_common_block_status_above() so that it doesn't
poll if data.done == true already, the time needed to iterate through my
test image is cut in half.
Now, of course, I'm still only seeing a few seconds for a 100 GB image,
so there must be more that is wrong for the reporter, but it suggests to
me that BDRV_POLL_WHILE() shouldn't be polling unconditionally when only
one of its users actually needs this.
Kevin
