linux-kernel - Re: [PATCH v9 07/20] mm/lru: introduce TestClearPageLRU

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Date:   Tue, 3 Mar 2020 16:46:59 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     cgroups@...r.kernel.org, mgorman@...hsingularity.net,
        tj@...nel.org, hughd@...gle.com, khlebnikov@...dex-team.ru,
        daniel.m.jordan@...cle.com, yang.shi@...ux.alibaba.com,
        willy@...radead.org, hannes@...xchg.org, lkp@...el.com,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v9 07/20] mm/lru: introduce TestClearPageLRU

On Tue, 3 Mar 2020 12:11:34 +0800 Alex Shi <alex.shi@...ux.alibaba.com> wrote:

> 
> 
> 在 2020/3/3 上午6:11, Andrew Morton 写道:
> >> -		if (PageLRU(page)) {
> >> +		if (TestClearPageLRU(page)) {
> >>  			lruvec = mem_cgroup_page_lruvec(page, pgdat);
> >> -			ClearPageLRU(page);
> >>  			del_page_from_lru_list(page, lruvec, page_lru(page));
> >>  		} else
> > 
> > The code will now get exclusive access of the page->flags cacheline and
> > will dirty that cacheline, even for !PageLRU() pages.  What is the
> > performance impact of this?
> > 
> 
> Hi Andrew,
> 
> Thanks a lot for comments!
> 
> I was tested the whole patchset with fengguang's case-lru-file-readtwice
> https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git/
> which is most sensitive case on PageLRU I found. There are no clear performance
> drop.
> 
> On this single patch, I just test the same case again, there is still no perf
> drop. some data is here on my 96 threads machine:
> 
> 		no lock_dep	w lock_dep and few other debug option
> w this patch, 	50.96MB/s		32.93MB/s
> w/o this patch, 50.50MB/s		33.53MB/s
> 
> 

Well, any difference would be small and the numbers did get a bit
lower, albeit probably within the margin of error.

But you know, if someone were to send a patch which micro-optimized
some code by replacing 'TestClearXXX()' with `if PageXXX()
ClearPageXXX()', I would happily merge it!

Is this change essential to the overall patchset?  If not, I'd be
inclined to skip it?