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Re: [Qemu-block] [PATCH for-2.9-rc5 v4 2/2] block: Drain BH in bdrv_drai
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From: |
Fam Zheng |
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Subject: |
Re: [Qemu-block] [PATCH for-2.9-rc5 v4 2/2] block: Drain BH in bdrv_drained_begin |
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Date: |
Wed, 7 Feb 2018 09:48:37 +0800 |
On Tue, Feb 6, 2018 at 11:32 PM, Kevin Wolf <address@hidden> wrote:
> 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 <address@hidden>
>> Signed-off-by: Fam Zheng <address@hidden>
>
> Fam, do you remember whether this was really only about drain? Because
> in that case...
Yes I believe so.
>
>> 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).
The convert_iteration_sectors loop looks wasteful. Why cannot we
report progress simply based on (offset/size) * 100% so we don't need
to do this estimation?
>
> 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.
Sounds fine to me. Maybe we could add a boolean parameter to BDRV_POLL_WHILE?
Fam