linux-kernel - Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check

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Date:   Mon, 24 May 2021 18:57:23 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc:     Luca Mariotti <mariottiluca1@...mail.it>,
        Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Pietro Pedroni <pedroni.pietro.96@...il.com>
Subject: Re: [PATCH BUGFIX] block, bfq: fix delayed stable merge check



> Il giorno 20 mag 2021, alle ore 09:15, Holger Hoffstätte <holger@...lied-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@...mail.it>
>> Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
>> Signed-off-by: Pietro Pedroni <pedroni.pietro.96@...il.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.

Thanks,
Paolo

> -h