Re: [PATCH -next RFC v2 4/8] blk-mq: don't preempt tag under heavy load

[yukuai (C)]

在 2022/04/08 22:24, Bart Van Assche 写道:
> On 4/8/22 00:39, Yu Kuai wrote:
> > The idle way to disable tag preemption is to track how many tags are
>
> idle -> ideal?
>
> > available, and wait directly in blk_mq_get_tag() if free tags are
> > very little. However, this is out of reality because fast path is
> > affected.
> >
> > As 'ws_active' is only updated in slow path, this patch disable tag
> > preemption if 'ws_active' is greater than 8, which means there are many
> > threads waiting for tags already.
> >
> > Once tag preemption is disabled, there is a situation that can cause
> > performance degration(or io hung in extreme scenarios): the waitqueue
>
> degration -> degradation?
>
> > diff --git a/block/blk-mq.h b/block/blk-mq.h
> > index 2615bd58bad3..b49b20e11350 100644
> > --- a/block/blk-mq.h
> > +++ b/block/blk-mq.h
> > @@ -156,6 +156,7 @@ struct blk_mq_alloc_data {
> >       /* allocate multiple requests/tags in one go */
> >       unsigned int nr_tags;
> > +    bool preemption;
> >       struct request **cached_rq;
>
> Please change "preemption" into "preempt".

Thanks for your advice, will change that and previous spelling mistakes.

Kuai
> Thanks,
>
> Bart.
>
> .