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

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 22, 2020 at 10:32:41AM +0100, John Garry wrote:
> > > > > > > 
> > > > > > > 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
> 
> Could the logic of blk_mq_run_hw_queue() -> blk_mq_hctx_has_pending() ->
> sbitmap_any_bit_set(&hctx->ctx_map) be optimised for megaraid scenario?
> 
> As I see, since megaraid will have 1:1 mapping of CPU to hw queue, will
> there only ever possibly a single bit set in ctx_map? If so, it seems a
> waste to always check every sbitmap map. But adding logic for this may
> negate any possible gains.

It really depends on min and max cpu id in the map, then sbitmap
depth can be reduced to (max - min + 1). I'd suggest to double check that
cost of sbitmap_any_bit_set() really matters.

> 
> > > > > 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:
> > 
> 
> I saw that you actually posted the v3:
> https://lore.kernel.org/linux-scsi/BL0PR2101MB11230C5F70151037B23C0C35CE2D0@BL0PR2101MB1123.namprd21.prod.outlook.com/
> And it no longer applies, due to the changes in scsi_mq_get_budget(), I
> think, which look non-trivial. Any chance to repost?

OK, will post V4.


thanks,
Ming