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Subject: Re: [PATCH BUGFIX 1/1] block, bfq: honor already-setup queue merges
To:     Paolo Valente <paolo.valente@linaro.org>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        davidezini2@gmail.com
References: <20210802141352.74353-1-paolo.valente@linaro.org>
 <20210802141352.74353-2-paolo.valente@linaro.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <df5c8291-9282-1512-cdbf-28aad60beaec@kernel.dk>
Date:   Thu, 2 Sep 2021 06:37:16 -0600
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On 8/2/21 8:13 AM, Paolo Valente wrote:
> The function bfq_setup_merge prepares the merging between two
> bfq_queues, say bfqq and new_bfqq. To this goal, it assigns
> bfqq->new_bfqq = new_bfqq. Then, each time some I/O for bfqq arrives,
> the process that generated that I/O is disassociated from bfqq and
> associated with new_bfqq (merging is actually a redirection). In this
> respect, bfq_setup_merge increases new_bfqq->ref in advance, adding
> the number of processes that are expected to be associated with
> new_bfqq.
> 
> Unfortunately, the stable-merging mechanism interferes with this
> setup. After bfqq->new_bfqq has been set by bfq_setup_merge, and
> before all the expected processes have been associated with
> bfqq->new_bfqq, bfqq may happen to be stably merged with a different
> queue than the current bfqq->new_bfqq. In this case, bfqq->new_bfqq
> gets changed. So, some of the processes that have been already
> accounted for in the ref counter of the previous new_bfqq will not be
> associated with that queue.  This creates an unbalance, because those
> references will never be decremented.
> 
> This commit fixes this issue by reestablishing the previous, natural
> behaviour: once bfqq->new_bfqq has been set, it will not be changed
> until all expected redirections have occurred.

Applied, thanks.

-- 
Jens Axboe