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

[Paolo Valente]

> Il giorno 26 ago 2021, alle ore 11:16, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
> 
> 
> 
>> 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?
> 

ping ...


> 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