RE: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ

[Kashyap Desai <kashyap.desai@broadcom.com>]

> On Tue, Jul 21, 2020 at 12:23:39PM +0530, Kashyap Desai wrote:
> > > > >
> > > > > Perf top (shared host tag. IOPS = 230K)
> > > > >
> > > > > 13.98%  [kernel]        [k] sbitmap_any_bit_set
> > > > >      6.43%  [kernel]        [k] blk_mq_run_hw_queue
> > > >
> > > > blk_mq_run_hw_queue function take more CPU which is called from "
> > > > scsi_end_request"
> > >
> > > The problem could be that nr_hw_queues is increased a lot so that
> > > sample
> > on
> > > blk_mq_run_hw_queue() can be observed now.
> >
> > Yes. That is correct.
> >
> > >
> > > > It looks like " blk_mq_hctx_has_pending" handles only elevator
> > > > (scheduler) case. If  queue has ioscheduler=none, we can skip. I
> > > > case of scheduler=none, IO will be pushed to hardware queue and it
> > > > by pass
> > > software queue.
> > > > Based on above understanding, I added below patch and I can see
> > > > performance scale back to expectation.
> > > >
> > > > Ming mentioned that - we cannot remove blk_mq_run_hw_queues()
> from
> > > > IO completion path otherwise we may see IO hang. So I have just
> > > > modified completion path assuming it is only required for IO
scheduler
> case.
> > > > https://www.spinics.net/lists/linux-block/msg55049.html
> > > >
> > > > Please review and let me know if this is good or we have to
> > > > address with proper fix.
> > > >
> > > > diff --git a/block/blk-mq.c b/block/blk-mq.c index
> > > > 1be7ac5a4040..b6a5b41b7fc2 100644
> > > > --- a/block/blk-mq.c
> > > > +++ b/block/blk-mq.c
> > > > @@ -1559,6 +1559,9 @@ void blk_mq_run_hw_queues(struct
> > > request_queue
> > > > *q, bool async)
> > > >         struct blk_mq_hw_ctx *hctx;
> > > >         int i;
> > > >
> > > > +       if (!q->elevator)
> > > > +               return;
> > > > +
> > >
> > > This way shouldn't be correct, blk_mq_run_hw_queues() is still
> > > needed
> > for
> > > none because request may not be dispatched successfully by direct
issue.
> >
> > When block layer attempt posting request to h/w queue directly (for
> > ioscheduler=none) and if it fails, it is calling
> > blk_mq_request_bypass_insert().
> > blk_mq_request_bypass_insert function will start the h/w queue from
> > submission context. Do we still have an issue if we skip running hw
> > queue from completion ?
>
> The thing is that we can't guarantee that direct issue or adding request
into
> hctx->dispatch is always done for MQ/none, for example, request still
> can be added to sw queue from blk_mq_flush_plug_list() when mq plug is
> applied.

I see even blk_mq_sched_insert_requests() from blk_mq_flush_plug_list make
sure it run the h/w queue. If all the submission path which deals with s/w
queue make sure they run h/w queue, can't we remove blk_mq_run_hw_queues()
from scsi_end_request ?

>
> Also, I am not sure it is a good idea to add request into hctx->dispatch
via
> blk_mq_request_bypass_insert() in __blk_mq_try_issue_directly() in case
of
> running out of budget, because this way may hurt sequential IO
performance.
>
> Thanks,
> Ming