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

Kashyap Desai
2020-07-21T06:53:43Z

> > >
> > > 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 ?

Kashyap

>
>
> Thanks,
> Ming