Re: [PATCH BUGFIX 1/1] block, bfq: honor already-setup queue merges

[Paolo Valente]

> Il giorno 2 ago 2021, alle ore 16:13, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 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.
> 

Hi Jens,
did you have time to look at this fix?

Thanks,
Paolo


> Signed-off-by: Davide Zini <davidezini2@xxxxxxxxx>
> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
> ---
> block/bfq-iosched.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 727955918563..08d9122dd4c0 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2659,6 +2659,15 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct bfq_queue *new_bfqq)
> 	 * are likely to increase the throughput.
> 	 */
> 	bfqq->new_bfqq = new_bfqq;
> +	/*
> +	 * The above assignment schedules the following redirections:
> +	 * each time some I/O for bfqq arrives, the process that
> +	 * generated that I/O is disassociated from bfqq and
> +	 * associated with new_bfqq. Here we increases new_bfqq->ref
> +	 * in advance, adding the number of processes that are
> +	 * expected to be associated with new_bfqq as they happen to
> +	 * issue I/O.
> +	 */
> 	new_bfqq->ref += process_refs;
> 	return new_bfqq;
> }
> @@ -2721,6 +2730,10 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> {
> 	struct bfq_queue *in_service_bfqq, *new_bfqq;
> 
> +	/* if a merge has already been setup, then proceed with that first */
> +	if (bfqq->new_bfqq)
> +		return bfqq->new_bfqq;
> +
> 	/*
> 	 * Check delayed stable merge for rotational or non-queueing
> 	 * devs. For this branch to be executed, bfqq must not be
> @@ -2822,9 +2835,6 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> 	if (bfq_too_late_for_merging(bfqq))
> 		return NULL;
> 
> -	if (bfqq->new_bfqq)
> -		return bfqq->new_bfqq;
> -
> 	if (!io_struct || unlikely(bfqq == &bfqd->oom_bfqq))
> 		return NULL;
> 
> -- 
> 2.20.1
>