Re: [PATCH BUGFIX RFC 0/2] reverting two commits causing freezes

From: Jens Axboe
Date: Fri Jan 18 2019 - 11:29:20 EST

On 1/18/19 6:35 AM, Jens Axboe wrote:
> On 1/18/19 4:52 AM, Paolo Valente wrote:
>> Hi Jens,
>> a user reported a warning, followed by freezes, in case he increases
>> nr_requests to more than 64 [1]. After reproducing the issues, I
>> reverted the commit f0635b8a416e ("bfq: calculate shallow depths at
>> init time"), plus the related commit bd7d4ef6a4c9 ("bfq-iosched:
>> remove unused variable"). The problem went away.
> For reverts, please put the justification into the actual revert
> commit. With this series, if applied as-is, we'd have two patches
> in the tree that just says "revert X" without any hint as to why
> that was done.
>> Maybe the assumption in commit f0635b8a416e ("bfq: calculate shallow
>> depths at init time") does not hold true?
> It apparently doesn't! But let's try and figure this out instead of
> blindly reverting it. OK, I think I see it. For the sched_tags
> case, when we grow the requests, we allocate a new set. Hence any
> old cache would be stale at that point.
> How about something like this? It still keeps the code of having
> to update this out of the hot IO path, and only calls it when we
> actually change the depths.
> Totally untested...

Now tested, and it seems to work for me. Note that haven't tried to
reproduce the issue, I just verified that the patch functionally
does what it should - when depths are updated, the hook is invoked
and updates the internal BFQ depth map.

Jens Axboe