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Subject: Re: [PATCH BUGFIX RFC 0/2] reverting two commits causing freezes
To:     Paolo Valente <paolo.valente@linaro.org>
Cc:     linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Mark Brown <broonie@kernel.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@googlegroups.com>, oleksandr@natalenko.name,
        hurikhan77+bko@gmail.com
References: <20190118115219.63576-1-paolo.valente@linaro.org>
 <c3b182a2-7f82-2860-436b-c1d59d6661ff@kernel.dk>
 <D212BCED-49BD-47F0-AF7F-0CCDBC82D131@linaro.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <c71255da-c142-8ea1-2dc4-840a6581689b@kernel.dk>
Date:   Fri, 18 Jan 2019 10:36:07 -0700
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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