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

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On Tue, Jul 21, 2020 at 12:23:39PM +0530, Kashyap Desai wrote:

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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

"

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scsi_end_request"

The problem could be that nr_hw_queues is increased a lot so
that sample

on

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blk_mq_run_hw_queue() can be observed now.

Yes. That is correct.

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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.

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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

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IO completion path otherwise we may see IO hang. So I have
just modified completion path assuming it is only required for
IO

scheduler

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case.

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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

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*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

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none because request may not be dispatched successfully by
direct

issue.

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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

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hctx->dispatch is always done for MQ/none, for example, request
hctx->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/

Ming - I think above patch will fix the issue of performance on VD.
I fix some hunk failure and ported to 5.8 kernel -
I am testing this patch on my setup. If you post V4, I will use that.

So far looks good.  I have reduced device queue depth so that I hit budget
busy code path frequently.

Kashyap

Thanks,
Ming