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SubjectRe: [PATCH v2 2/2] blk-mq: Rerun dispatching in the case of budget contention
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> Il giorno 5 apr 2020, alle ore 18:16, Doug Anderson <dianders@chromium.org> ha scritto:
>
> Hi,
>
> On Sun, Apr 5, 2020 at 7:55 AM Paolo Valente <paolo.valente@linaro.org> wrote:
>>
>>> Il giorno 5 apr 2020, alle ore 16:00, Doug Anderson <dianders@chromium.org> ha scritto:
>>>
>>> Hi,
>>>
>>> On Sun, Apr 5, 2020 at 2:15 AM Ming Lei <ming.lei@redhat.com> wrote:
>>>>
>>>> OK, looks it isn't specific on BFQ any more.
>>>>
>>>> Follows another candidate approach for this issue, given it is so hard
>>>> to trigger, we can make it more reliable by rerun queue when has_work()
>>>> returns true after ops->dispath_request() returns NULL.
>>>>
>>>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
>>>> index 74cedea56034..4408e5d4fcd8 100644
>>>> --- a/block/blk-mq-sched.c
>>>> +++ b/block/blk-mq-sched.c
>>>> @@ -80,6 +80,7 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
>>>> blk_mq_run_hw_queue(hctx, true);
>>>> }
>>>>
>>>> +#define BLK_MQ_BUDGET_DELAY 3 /* ms units */
>>>> /*
>>>> * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
>>>> * its queue by itself in its completion handler, so we don't need to
>>>> @@ -103,6 +104,9 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>>>> rq = e->type->ops.dispatch_request(hctx);
>>>> if (!rq) {
>>>> blk_mq_put_dispatch_budget(hctx);
>>>> +
>>>> + if (e->type->ops.has_work && e->type->ops.has_work(hctx))
>>>> + blk_mq_delay_run_hw_queue(hctx, BLK_MQ_BUDGET_DELAY);
>>>
>>> I agree that your patch should solve the race. With the current BFQ's
>>> has_work() it's a bit of a disaster though. It will essentially put
>>> blk-mq into a busy-wait loop (with a 3 ms delay between each poll)
>>> while BFQ's has_work() says "true" but BFQ doesn't dispatch anything.
>>>
>>> ...so I guess the question that still needs to be answered: does
>>> has_work() need to be exact? If so then we need the patch you propose
>>> plus one to BFQ. If not, we should continue along the lines of my
>>> patch.
>>>
>>
>> Some more comments. BFQ's I/O plugging lasts 9 ms by default. So,
>> with this last Ming's patch, BFQ may happen to be polled every 3ms,
>> for at most three times.
>
> Ah! I did not know this. OK, then Ming's patch seems like it should
> work. If nothing else it should fix the problem. If this ends up
> making BFQ chew up too much CPU time then presumably someone will
> notice and BFQ's has_work() can be improved.
>
> Ming: how do you want to proceed? Do you want to formally post the
> patch? Do you want me to post a v3 of my series where I place patch
> #2 with your patch? Do you want authorship (which implies adding your
> Signed-off-by)?
>
>
>> On the opposite end, making bfq_has_work plugging aware costs more
>> complexity, and possibly one more lock. While avoiding the above
>> occasional polling, this may imply a lot of overhead or CPU stalls on
>> every dispatch.
>
> I still think it would be interesting to run performance tests with my
> proof-of-concept solution for has_work(). Even if it's not ideal,
> knowing whether performance increased, decreased, or stayed the same
> would give information about how much more effort should be put into
> this.
>

Why not? It is however hard to hope that we add only negligible
overhead and CPU stalls if we move from one lock-protected section per
I/O-request dispatch, to two or more lock-protected sections per
request (has_work may be invoked several times per request).

At any rate, if useful, one of the scripts in my S benchmark suite can
also measure max IOPS (when limited only by I/O processing) [1]. The
script is for Linux distros; I don't know whether it works in your
environments of interest, Doug.

Paolo

[1] https://github.com/Algodev-github/S/blob/master/throughput-sync/throughput-sync.sh

> -Doug

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