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

Ming Lei
2020-07-22T08:04:34Z

On Wed, Jul 22, 2020 at 11:00:45AM +0530, Kashyap Desai wrote:
> > 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 ?

No, one purpose of blk_mq_run_hw_queues() is for rerun queue in case that
dispatch budget is running out of in submission path, and sdev->device_busy is
shared by all hw queues on this scsi device.

I posted one patch for avoiding it in scsi_end_request() before, looks it
never lands upstream:

https://lore.kernel.org/linux-block/20191118100640.3673-1-ming.lei@redhat.com/

Thanks,
Ming