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From: "Holger Hoffstätte" <holger@applied-asynchrony.com>
To: Paolo Valente <paolo.valente@linaro.org>
Cc: Luca Mariotti <mariottiluca1@hotmail.it>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block <linux-block@vger.kernel.org>,
	linux-kernel@vger.kernel.org,
	Pietro Pedroni <pedroni.pietro.96@gmail.com>
Subject: Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check
Date: Mon, 24 May 2021 19:13:59 +0200	[thread overview]
Message-ID: <c6a3d259-a118-0c7a-0faf-1ab48f9cd2ff@applied-asynchrony.com> (raw)
In-Reply-To: <E9D95C99-D07B-4A63-8C0E-67356887DE23@linaro.org>

On 2021-05-24 18:57, Paolo Valente wrote:
> 
> 
>> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte <holger@applied-asynchrony.com> ha scritto:
>>
>> On 2021-05-18 12:43, Luca Mariotti wrote:
>>> When attempting to schedule a merge of a given bfq_queue with the currently
>>> in-service bfq_queue or with a cooperating bfq_queue among the scheduled
>>> bfq_queues, delayed stable merge is checked for rotational or non-queueing
>>> devs. For this stable merge to be performed, some conditions must be met.
>>> If the current bfq_queue underwent some split from some merged bfq_queue,
>>> one of these conditions is that two hundred milliseconds must elapse from
>>> split, otherwise this condition is always met.
>>> Unfortunately, by mistake, time_is_after_jiffies() was written instead of
>>> time_is_before_jiffies() for this check, verifying that less than two
>>> hundred milliseconds have elapsed instead of verifying that at least two
>>> hundred milliseconds have elapsed.
>>> Fix this issue by replacing time_is_after_jiffies() with
>>> time_is_before_jiffies().
>>> Signed-off-by: Luca Mariotti <mariottiluca1@hotmail.it>
>>> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
>>> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@gmail.com>
>>> ---
>>>   block/bfq-iosched.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>> index acd1f881273e..2adb1e69c9d2 100644
>>> --- a/block/bfq-iosched.c
>>> +++ b/block/bfq-iosched.c
>>> @@ -2697,7 +2697,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>>>   	if (unlikely(!bfqd->nonrot_with_queueing)) {
>>>   		if (bic->stable_merge_bfqq &&
>>>   		    !bfq_bfqq_just_created(bfqq) &&
>>> -		    time_is_after_jiffies(bfqq->split_time +
>>> +		    time_is_before_jiffies(bfqq->split_time +
>>>   					  msecs_to_jiffies(200))) {
>>>   			struct bfq_queue *stable_merge_bfqq =
>>>   				bic->stable_merge_bfqq;
>>
>> Not sure why but with this patch I quickly got a division-by-zero in BFQ and
>> complete system halt. Unfortunately I couldn't capture the exact stack trace,
>> but it read something like bfq_calc_weight() or something ike that.
>> I looked through the code and found bfq_delta(), so maybe weight got
>> reduced to 0?
>>
> 
> Hi Holger,
> is this (easily) reproducible for you?  If so, I'd like to propose you
> a candidate fix.

Yes, it's easily reproducible (should be reproducible on 5.13-rc as well).
Simple read/write I/O on a cold FS (rotational disk obviously) will crash
pretty much immediately; without it everything works fine, likely because the
bug (in the recent queue merging patches?) is never triggered due to the
accidentally-wrong time calculation.
Will gladly test your patch! :)

cheers
Holger

  reply	other threads:[~2021-05-24 17:14 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-18 10:43 [PATCH BUGFIX] block, bfq: fix delayed stable merge check Luca Mariotti
2021-05-20  7:15 ` Holger Hoffstätte
2021-05-20 16:39   ` Holger Hoffstätte
2021-05-24 16:57   ` Paolo Valente
2021-05-24 17:13     ` Holger Hoffstätte [this message]
2021-05-24 17:41       ` Paolo Valente
2021-05-24 18:45         ` Holger Hoffstätte
2021-05-25 10:40           ` Paolo Valente

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