linux-kernel - Re: [PATCH v4 4/5] page_idle: Drain all LRU pagevec before idle tracking

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Date:   Tue, 6 Aug 2019 09:48:53 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brendan Gregg <bgregg@...flix.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Christian Hansen <chansen3@...co.com>, dancol@...gle.com,
        fmayer@...gle.com, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>, kernel-team@...roid.com,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>, minchan@...nel.org,
        namhyung@...gle.com, paulmck@...ux.ibm.com,
        Robin Murphy <robin.murphy@....com>,
        Roman Gushchin <guro@...com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, surenb@...gle.com,
        Thomas Gleixner <tglx@...utronix.de>, tkjos@...gle.com,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 4/5] page_idle: Drain all LRU pagevec before idle
 tracking

On Tue, Aug 06, 2019 at 01:44:02PM +0200, Michal Hocko wrote:
[snip]
> > > > This operation even if expensive is only done once during the access of the
> > > > page_idle file. Did you have a better fix in mind?
> > > 
> > > Can we set the idle bit also for non-lru pages as long as they are
> > > reachable via pte?
> > 
> > Not at the moment with the current page idle tracking code. PageLRU(page)
> > flag is checked in page_idle_get_page().
> 
> yes, I am aware of the current code. I strongly suspect that the PageLRU
> check was there to not mark arbitrary page looked up by pfn with the
> idle bit because that would be unexpected. But I might be easily wrong
> here.

Yes, quite possible.

> > Even if we could set it for non-LRU, the idle bit (page flag) would not be
> > cleared if page is not on LRU because page-reclaim code (page_referenced() I
> > believe) would not clear it.
> 
> Yes, it is either reclaim when checking references as you say but also
> mark_page_accessed. I believe the later might still have the page on the
> pcp LRU add cache. Maybe I am missing something something but it seems
> that there is nothing fundamentally requiring the user mapped page to be
> on the LRU list when seting the idle bit.
> 
> That being said, your big hammer approach will work more reliable but if
> you do not feel like changing the underlying PageLRU assumption then
> document that draining should be removed longterm.

Yes, at the moment I am in preference of keeping the underlying assumption
same. I am Ok with adding of a comment on the drain call that it is to be
removed longterm.

thanks,

 - Joel