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

[Jens Axboe <axboe@kernel.dk>]

On 1/18/19 10:24 AM, Paolo Valente wrote:
> 
> 
>> Il giorno 18 gen 2019, alle ore 14:35, Jens Axboe <axboe@kernel.dk> ha scritto:
>>
>> 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.
>>
> 
> I forget to say explicitly that these patches were meant only to give
> you and anybody else something concrete to test and check.
> 
> With me you're as safe as houses, in terms of amount of comments in
> final patches :)

It's almost an example of the classic case of "if you want a real
solution to a problem, post a knowingly bad and half assed solution".
That always gets people out of the woodwork :-)

>>> 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.
> 
> Totally agree.
> 
>> 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.
>>
> 
> ok
> 
>> 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.
>>
> 
> Looks rather clean and efficient.
> 
>> Totally untested...
>>
> 
> It seems to work here too.

OK good, I've posted it "officially" now.
 
-- 
Jens Axboe