linux-kernel - RE: Random high CPU utilization in blk-mq with the none scheduler

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Date:   Tue, 14 Dec 2021 00:43:30 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Jens Axboe <axboe@...nel.dk>,
        "'ming.lei@...hat.com'" <ming.lei@...hat.com>,
        'Christoph Hellwig' <hch@....de>,
        "'linux-block@...r.kernel.org'" <linux-block@...r.kernel.org>
CC:     Long Li <longli@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: RE: Random high CPU utilization in blk-mq with the none scheduler

From: Jens Axboe <axboe@...nel.dk>
> Sent: Monday, December 13, 2021 10:43 AM
> 
> I wonder if this will help, SCSI artifact making us hit the
> mod delayed work path all the time. 

Yes, I think this is what's happening here.

> Might be a race in there, but should be fine for testing.
> 
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 4d8f5fe91588..cb2f4d604bad 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1625,7 +1625,9 @@ EXPORT_SYMBOL(kblockd_schedule_work);
>  int kblockd_mod_delayed_work_on(int cpu, struct delayed_work *dwork,
>  				unsigned long delay)
>  {
> -	return mod_delayed_work_on(cpu, kblockd_workqueue, dwork, delay);
> +	if (!work_pending(&dwork->work))
> +		return mod_delayed_work_on(cpu, kblockd_workqueue, dwork,
> delay);
> +	return true;
>  }
>  EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
> --
> Jens Axboe

Thanks very much! This path works!

In the test, "v5.16-rc4 + reverting dc5fc361d891 
("block: attempt direct issue of plug list") + this patch" shows a good I/O
performance that's about the same as v5.16-rc4.

I suppose "v5.16-rc4 + this patch" would show a slightly even better
performance.

It would be great if you can commit this small patch so that it can be
backported to old stable kernels.

Thanks,
Dexuan