linux-kernel - Re: Re: Re: [RFC] autonuma: Support to scan page table asynchronously

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Date:   Fri, 17 Apr 2020 14:21:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     SeongJae Park <sjpark@...zon.com>,
        "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...riel.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Tejun Heo <tj@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Aubrey Li <aubrey.li@...el.com>
Subject: Re: Re: Re: [RFC] autonuma: Support to scan page table asynchronously

On Fri, Apr 17, 2020 at 01:16:29PM +0100, Mel Gorman wrote:
> On Fri, Apr 17, 2020 at 12:21:29PM +0200, SeongJae Park wrote:
> > On Fri, 17 Apr 2020 12:04:17 +0200 Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Fri, Apr 17, 2020 at 09:05:08AM +0200, SeongJae Park wrote:
> > > > I think the main idea of DAMON[1] might be able to applied here.  Have you
> > > > considered it?
> > > > 
> > > > [1] https://lore.kernel.org/linux-mm/20200406130938.14066-1-sjpark@amazon.com/
> > > 
> > > I've ignored that entire thing after you said the information it
> > > provides was already available through the PMU.
> > 
> > Sorry if my answer made you confused.  What I wanted to say was that the
> > fundamental access checking mechanism that DAMON depends on is PTE Accessed bit
> > for now, but it could be modified to use PMU or other features instead. 
> 
> I would not be inclined to lean towards either approach for NUMA
> balancing. Fiddling with the accessed bit can have consequences for page
> aging and residency -- fine for debugging a problem, not to fine for
> normal usage. I would expect the PMU approach would have high overhead
> as well as taking over a PMU counter that userspace debugging may expect
> to be available.

Oh, quite agreed; I was just saying I never saw the use of that whole
DAMON thing. AFAICT it's not actually solving a problem, just making
more.