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

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Message-ID: <f23f8090-4a55-3c16-1bdd-f86634cd6f3b@applied-asynchrony.com>
Date:   Thu, 20 May 2021 09:15:42 +0200
From:   Holger Hoffstätte <holger@...lied-asynchrony.com>
To:     Luca Mariotti <mariottiluca1@...mail.it>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>
Cc:     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

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?

-h