linux-kernel - Re: [PATCH] psi: Treat ksm swapping in copy as memstall

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Date:   Fri, 28 Jan 2022 02:31:42 +0000
From:   CGEL <cgel.zte@...il.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     akpm@...ux-foundation.org, sfr@...b.auug.org.au,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yang Yang <yang.yang29@....com.cn>
Subject: Re: [PATCH] psi: Treat ksm swapping in copy as memstall

On Thu, Jan 27, 2022 at 08:29:08PM -0500, Johannes Weiner wrote:
> On Fri, Jan 21, 2022 at 09:51:08AM +0000, CGEL wrote:
> >  Wed, Jan 19, 2022 at 07:58:23AM -0500, Johannes Weiner wrote:
> > > On Wed, Jan 19, 2022 at 06:13:54AM +0000, CGEL wrote:
> > > > I did a test, when we use zram, it takes longer time for ksm copying than
> > > > swap_readpage(). Ksm copying average takes 147263ns, swap_readpage()
> > > > average takes 55639ns. So I think this patch is reasonable.
> > > 
> > > Ok, that sounds reasonable to me as well. Please add the
> > > PageWorkingset() check and resubmit the patch. Thanks!
> > I am a litte confused about adding PageWorkingset(), since I
> > think ksm_might_need_to_copy() memstall is like swap_readpage()
> > memstall and swap_readpage() doesn't add PageWorkingset().
> 
> That's actually a bug! It should do that.
I recently found that too. Please CC to me your new patch, thanks!
And I will send V2 of this patch "psi: Treat ksm swapping in copy
as memstall" with PageWorkingset().

> That psi_memstall_enter() in swap_readpage() was added by commit
> 937790699be9c8100e5358625e7dfa8b32bd33f2. It's for catching the
> scenarios that don't go through submit_bio(). When you look at
> submit_bio(), it counts stalls only if we have workingset pages:
> 
> 	/*
> 	 * If we're reading data that is part of the userspace workingset, count
> 	 * submission time as memory stall.  When the device is congested, or
> 	 * the submitting cgroup IO-throttled, submission can be a significant
> 	 * part of overall IO time.
> 	 */
> 	if (unlikely(bio_op(bio) == REQ_OP_READ &&
> 	    bio_flagged(bio, BIO_WORKINGSET))) {
> 		unsigned long pflags;
> 
> 		psi_memstall_enter(&pflags);
> 		submit_bio_noacct(bio);
> 		psi_memstall_leave(&pflags);
> 		return;
> 	}
> 
> I hope that clarifies it. I'll send a patch to fix up swap_readpage().
> 
> Thanks