Re: [RFC PATCH] sbitmap: fix batching wakeup

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 02, 2023 at 06:05:53PM +0200, Jan Kara wrote:
> On Fri 21-07-23 17:57:15, Ming Lei wrote:
> > From: David Jeffery <djeffery@redhat.com>
> > 
> > Current code supposes that it is enough to provide forward progress by just
> > waking up one wait queue after one completion batch is done.
> > 
> > Unfortunately this way isn't enough, cause waiter can be added to
> > wait queue just after it is woken up.
> > 
> > Follows one example(64 depth, wake_batch is 8)
> > 
> > 1) all 64 tags are active
> > 
> > 2) in each wait queue, there is only one single waiter
> > 
> > 3) each time one completion batch(8 completions) wakes up just one waiter in each wait
> > queue, then immediately one new sleeper is added to this wait queue
> > 
> > 4) after 64 completions, 8 waiters are wakeup, and there are still 8 waiters in each
> > wait queue
> > 
> > 5) after another 8 active tags are completed, only one waiter can be wakeup, and the other 7
> > can't be waken up anymore.
> > 
> > Turns out it isn't easy to fix this problem, so simply wakeup enough waiters for
> > single batch.
> > 
> > Cc: David Jeffery <djeffery@redhat.com>
> > Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> > Cc: Gabriel Krisman Bertazi <krisman@suse.de>
> > Cc: Chengming Zhou <zhouchengming@bytedance.com>
> > Cc: Jan Kara <jack@suse.cz>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> I'm sorry for the delay - I was on vacation. I can see the patch got
> already merged and I'm not strictly against that (although I think Gabriel
> was experimenting with this exact wakeup scheme and as far as I remember
> the more eager waking up was causing performance decrease for some
> configurations). But let me challenge the analysis above a bit. For the
> sleeper to be added to a waitqueue in step 3), blk_mq_mark_tag_wait() must
> fail the blk_mq_get_driver_tag() call. Which means that all tags were used

Here only allocating request by blk_mq_get_tag() is involved, and
getting driver tag isn't involved.

> at that moment. To summarize, anytime we add any new waiter to the
> waitqueue, all tags are used and thus we should eventually receive enough
> wakeups to wake all of them. What am I missing?

When running the final retry(__blk_mq_get_tag) before
sleeping(io_schedule()) in blk_mq_get_tag(), the sleeper has been added to
wait queue.

So when two completion batch comes, the two may wake up same wq because
same ->wake_index can be observed from two completion path, and both two
wake_up_nr() can return > 0 because adding sleeper into wq and wake_up_nr()
can be interleaved, then 16 completions just wakeup 2 sleepers added to
same wq.

If the story happens on one wq with >= 8 sleepers, io hang will be
triggered, if there are another two pending wq.


Thanks, 
Ming