Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[Ming Lei]

On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang
<jianchao.w.wang@xxxxxxxxxx> wrote:
>
> Hi Jens
>
> Thanks for your kindly response.
>
> On 11/13/18 9:44 PM, Jens Axboe wrote:
> > On 11/13/18 2:56 AM, Jianchao Wang wrote:
> >> When issue request directly and the task is migrated out of the
> >> original cpu where it allocates request, hctx could be ran on
> >> the cpu where it is not mapped.
> >> To fix this,
> >>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
> >>  - check whether the current is mapped to the hctx, if not, insert
> >>    forcibly.
> >>  - invoke __blk_mq_issue_directly under preemption disabled.
> >
> > I'm not too crazy about this one, adding a get/put_cpu() in the hot
> > path, and a cpumask test. The fact is that most/no drivers care
> > about strict placement. We always try to do so, if convenient,
> > since it's faster, but this seems to be doing the opposite.
> >
> > I'd be more inclined to have a driver flag if it needs guaranteed
> > placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.
> >
> > What do you think?
> >
>
> I'd inclined blk-mq should comply with a unified rule, no matter the
> issuing directly path or inserting one. Then blk-mq would have a simpler
> model. And also this guarantee could be a little good for drivers,
> especially the case where cpu and hw queue mapping is 1:1.

I guess it is quite hard to respect this rule 100%, such as in case of
CPU hotplug.

Thanks,
Ming Lei