Re: [RFC PATCH] sbitmap: fix batching wakeup

[Gabriel Krisman Bertazi <krisman@suse.de>]

Ming Lei <ming.lei@redhat.com> writes:

> 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.

yes, I think this makes sense. When working on this algorithm I remember
I considered it (thus wake_up_nr being ready), but ended up believing it
wasn't needed.  please take:

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>

I wonder how likely it is to reach it.  Did you get a bug report?

Thanks,

-- 
Gabriel Krisman Bertazi