Date:   Tue, 27 Aug 2019 20:41:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Miguel de Dios <migueldedios@...gle.com>,
        Wei Wang <wvw@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [RFC PATCH] mm: drop mark_page_access from the unmap path

On Tue 27-08-19 12:00:26, Johannes Weiner wrote:
> On Mon, Aug 26, 2019 at 02:06:30PM +0200, Michal Hocko wrote:
> > On Tue 13-08-19 12:51:43, Michal Hocko wrote:
> > > On Mon 12-08-19 11:07:25, Johannes Weiner wrote:
> > > > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote:
> > [...]
> > > > > > Maybe the refaults will be fine - but latency expectations around
> > > > > > mapped page cache certainly are a lot higher than unmapped cache.
> > > > > >
> > > > > > So I'm a bit reluctant about this patch. If Minchan can be happy with
> > > > > > the lock batching, I'd prefer that.
> > > > > 
> > > > > Yes, it seems that the regular lock drop&relock helps in Minchan's case
> > > > > but this is a kind of change that might have other subtle side effects.
> > > > > E.g. will-it-scale has noticed a regression [1], likely because the
> > > > > critical section is shorter and the overal throughput of the operation
> > > > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose
> > > > > much sleep over it normally but we have already seen real regressions
> > > > > when the locking pattern has changed in the past so I would by a bit
> > > > > cautious.
> > > > 
> > > > I'm much more concerned about fundamentally changing the aging policy
> > > > of mapped page cache then about the lock breaking scheme. With locking
> > > > we worry about CPU effects; with aging we worry about additional IO.
> > > 
> > > But the later is observable and debuggable little bit easier IMHO.
> > > People are quite used to watch for major faults from my experience
> > > as that is an easy metric to compare.
> 
> Rootcausing additional (re)faults is really difficult. We're talking
> about a slight trend change in caching behavior in a sea of millions
> of pages. There could be so many factors causing this, and for most
> you have to patch debugging stuff into the kernel to rule them out.
> 
> A CPU regression you can figure out with perf.
> 
> > > > > As I've said, this RFC is mostly to open a discussion. I would really
> > > > > like to weigh the overhead of mark_page_accessed and potential scenario
> > > > > when refaults would be visible in practice. I can imagine that a short
> > > > > lived statically linked applications have higher chance of being the
> > > > > only user unlike libraries which are often being mapped via several
> > > > > ptes. But the main problem to evaluate this is that there are many other
> > > > > external factors to trigger the worst case.
> > > > 
> > > > We can discuss the pros and cons, but ultimately we simply need to
> > > > test it against real workloads to see if changing the promotion rules
> > > > regresses the amount of paging we do in practice.
> > > 
> > > Agreed. Do you see other option than to try it out and revert if we see
> > > regressions? We would get a workload description which would be helpful
> > > for future regression testing when touching this area. We can start
> > > slower and keep it in linux-next for a release cycle to catch any
> > > fallouts early.
> > > 
> > > Thoughts?
> > 
> > ping...
> 
> Personally, I'm not convinced by this patch. I think it's a pretty
> drastic change in aging heuristics just to address a CPU overhead
> problem that has simpler, easier to verify, alternative solutions.
> 
> It WOULD be great to clarify and improve the aging model for mapped
> cache, to make it a bit easier to reason about.

I fully agree with this! Do you have any specific ideas? I am afraid I
am unlikely to find time for a larger project that this sounds to be but
maybe others will find this as a good fit.

> But this patch does
> not really get there either. Instead of taking a serious look at
> mapped cache lifetime and usage scenarios, the changelog is more in
> "let's see what breaks if we take out this screw here" territory.

You know that I tend to be quite conservative. In this case I can see
the cost which is not negligible and likely to hit many workloads
because it is a common path. The immediate benefit is not really clear,
though, at least to me. We can speculate and I would really love to hear
from Nick what exactly led him to this change.
 
> So I'm afraid I don't think the patch & changelog in its current shape
> should go upstream.

I will not insist of course but it would be really great to know and
_document_ why we are doing this. I really hate how often we keep
different heuristics and build more complex solutions on top just
because nobody dares to change that.

Our code is really hard to reason about.

-- 
Michal Hocko
SUSE Labs

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