Message-ID: <20200521142231.GA812233@cmpxchg.org>
Date:   Thu, 21 May 2020 10:22:31 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Chris Down <chris@...isdown.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm, memcg: reclaim more aggressively before high
 allocator throttling

On Thu, May 21, 2020 at 09:51:55AM -0400, Johannes Weiner wrote:
> On Thu, May 21, 2020 at 09:32:45AM +0200, Michal Hocko wrote:
> > I wouldn't mind to loop over try_to_free_pages to meet the requested
> > memcg_nr_pages_over_high target.
> 
> Should we do the same for global reclaim? Move reclaim to userspace
> resume where there are no GFP_FS, GFP_NOWAIT etc. restrictions and
> then have everybody just reclaim exactly what they asked for, and punt
> interrupts / kthread allocations to a worker/kswapd?

Oof, typo: I meant limit reclaim by memory.max and
memory.limit_in_bytes. Not physical memory reclaim of course.

> > > > > > Also if the current high reclaim scaling is insufficient then we should
> > > > > > be handling that via memcg_nr_pages_over_high rather than effectivelly
> > > > > > unbound number of reclaim retries.
> > > > > 
> > > > > ???
> > > > 
> > > > I am not sure what you are asking here.
> > > 
> > > You expressed that some alternate solution B would be preferable,
> > > without any detail on why you think that is the case.
> > > 
> > > And it's certainly not obvious or self-explanatory - in particular
> > > because Chris's proposal *is* obvious and self-explanatory, given how
> > > everybody else is already doing loops around page reclaim.
> > 
> > Sorry, I could have been less cryptic. I hope the above and my response
> > to Chris goes into more details why I do not like this proposal and what
> > is the alternative. But let me summarize. I propose to use memcg_nr_pages_over_high
> > target. If the current calculation of the target is unsufficient - e.g.
> > in situations where the high limit excess is very large then this should
> > be reflected in memcg_nr_pages_over_high.
> > 
> > Is it more clear?
> 
> Well you haven't made a good argument why memory.high is actually
> different than any other form of reclaim, and why it should be the
> only implementation of page reclaim that has special-cased handling
> for the inherent "unfairness" or rather raciness of that operation.
> 
> You cut these lines from the quote:
> 
>   Under pressure, page reclaim can struggle to satisfy the reclaim
>   goal and may return with less pages reclaimed than asked to.
> 
>   Under concurrency, a parallel allocation can invalidate the reclaim
>   progress made by a thread.
> 
> Even if we *could* invest more into trying to avoid any unfairness,
> you haven't made a point why we actually should do that here
> specifically, yet not everywhere else.
> 
> (And people have tried to do it for global reclaim[1], but clearly
> this isn't a meaningful problem in practice.)
> 
> I have a good reason why we shouldn't: because it's special casing
> memory.high from other forms of reclaim, and that is a maintainability
> problem. We've recently been discussing ways to make the memory.high
> implementation stand out less, not make it stand out even more. There
> is no solid reason it should be different from memory.max reclaim,
> except that it should sleep instead of invoke OOM at the end. It's
> already a mess we're trying to get on top of and straighten out, and
> you're proposing to add more kinks that will make this work harder.
> 
> I have to admit, I'm baffled by this conversation. I consider this a
> fairly obvious, idiomatic change, and I cannot relate to the
> objections or counter-proposals in the slightest.
> 
> [1] http://lkml.iu.edu/hypermail//linux/kernel/0810.0/0169.html

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