Message-ID: <20200801011821.GA859734@carbon.dhcp.thefacebook.com>
Date:   Fri, 31 Jul 2020 18:18:21 -0700
From:   Roman Gushchin <guro@...com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: vmstat: fix /proc/sys/vm/stat_refresh generating
 false warnings

On Thu, Jul 30, 2020 at 09:06:55PM -0700, Hugh Dickins wrote:
> On Thu, 30 Jul 2020, Roman Gushchin wrote:
> > On Wed, Jul 29, 2020 at 08:45:47PM -0700, Hugh Dickins wrote:
> > > 
> > > But a better idea is perhaps to redefine the behavior of
> > > "echo >/proc/sys/vm/stat_refresh".  What if
> > > "echo someparticularstring >/proc/sys/vm/stat_refresh" were to
> > > disable or enable the warning (permanently? or just that time?):
> > > disable would be more "back-compatible", but I think it's okay
> > > if you prefer enable.  Or "someparticularstring" could actually
> > > specify the warning threshold you want to use - you might echo
> > > 125 or 16000, I might echo 0.  We can haggle over the default.
> > 
> > May I ask you, what kind of problems you have in your in mind,
> > which can be revealed by these warnings? Or maybe there is some
> > history attached?
> 
> Yes: 52b6f46bc163 mentions finding a bug of mine in NR_ISOLATED_FILE
> accounting, but IIRC (though I might be making this up) there was
> also a bug in the NR_ACTIVE or NR_INACTIVE FILE or ANON accounting.
> 
> When one of the stats used for balancing or limiting in vmscan.c
> trends increasingly negative, it becomes increasingly difficult
> for those heuristics (adding on to others, comparing with others)
> to do what they're intended to do: they behave increasingly weirdly.
> 
> Now the same (or the opposite) is true if one of those stats trends
> increasingly positive: but if it leaks positive, it's visible in
> /proc/vmstat; whereas if it leaks negative, it's presented there as 0.
> 
> And most of the time (when unsynchronized) showing 0 is much better
> than showing a transient negative.  But to help fix bugs, we do need
> some way of seeing the negatives, and vm/stat_refresh provides an
> opportunity to do so, when it synchronizes.
> 
> I'd be glad not to show the transients if I knew them: set a flag
> on any that go negative, and only show if negative twice or more
> in a row?  Perhaps, but I don't relish adding that, and think it
> would be over-engineering.
> 
> It does sound to me like echoing the warning threshold into
> /proc/sys/vm/stat_refresh is the best way to satisfy us both.
> 
> Though another alternative did occur to me overnight: we could
> scrap the logged warning, and show "nr_whatever -53" as output
> from /proc/sys/vm/stat_refresh: that too would be acceptable
> to me, and you redirect to /dev/null.

It sounds like a good idea to me. Do you want me to prepare a patch?

> 
> (Why did I choose -53 in my example?  An in-joke: when I looked
> through our machines for these warnings, on old kernels with my
> old shmem hugepage implementation, there were a striking number
> with "nr_shmem_freeholes -53"; but I'm a few years too late to
> investigate what was going on there.)

:)

> 
> > 
> > If it's all about some particular counters, which are known to be
> > strictly positive, maybe we should do the opposite, and check only
> > those counters? Because in general it's not an indication of a problem.
> 
> Yet it's very curious how few stats ever generate such warnings:
> you're convinced they're just transient noise, and you're probably right;
> but I am a little suspicious of whether they are accounted correctly.

Yeah, I was initially very suspicious too, but I didn't find any issues
and still think it's not an indication of a problem.

Thank you!

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