linux-kernel - Re: [PATCH] blk-mq: Use request queue-wide tags for tagset-wide sbitmap

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Date:   Tue, 11 May 2021 09:47:34 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Douglas Gilbert <dgilbert@...erlog.com>
Cc:     John Garry <john.garry@...wei.com>, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, kashyap.desai@...adcom.com,
        chenxiang66@...ilicon.com, yama@...hat.com
Subject: Re: [PATCH] blk-mq: Use request queue-wide tags for tagset-wide
 sbitmap

On Mon, May 10, 2021 at 09:35:01PM -0400, Douglas Gilbert wrote:
> On 2021-05-10 8:52 p.m., Ming Lei wrote:
> > On Mon, May 03, 2021 at 06:22:13PM +0800, John Garry wrote:
> > > The tags used for an IO scheduler are currently per hctx.
> > > 
> > > As such, when q->nr_hw_queues grows, so does the request queue total IO
> > > scheduler tag depth.
> > > 
> > > This may cause problems for SCSI MQ HBAs whose total driver depth is
> > > fixed.
> > > 
> > > Ming and Yanhui report higher CPU usage and lower throughput in scenarios
> > > where the fixed total driver tag depth is appreciably lower than the total
> > > scheduler tag depth:
> > > https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@huawei.com/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b
> > > 
> > 
> > No difference any more wrt. fio running on scsi_debug with this patch in
> > Yanhui's test machine:
> > 
> > 	modprobe scsi_debug host_max_queue=128 submit_queues=32 virtual_gb=256 delay=1
> > vs.
> > 	modprobe scsi_debug max_queue=128 submit_queues=1 virtual_gb=256 delay=1
> > 
> > Without this patch, the latter's result is 30% higher than the former's.
> > 
> > note: scsi_debug's queue depth needs to be updated to 128 for avoiding io hang,
> > which is another scsi issue.
> 
> "scsi_debug: Fix cmd_per_lun, set to max_queue" made it into lk 5.13.0-rc1 as
> commit fc09acb7de31badb2ea9e85d21e071be1a5736e4 . Is this the issue you are
> referring to, or is there a separate issue in the wider scsi stack?

OK, that is it, then it isn't necessary to update scsi_debug's queue
depth for the test.


Thanks,
Ming