linux-kernel - Re: [PATCH] mm: Proportional memory.{low,min} reclaim

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Date:   Mon, 28 Jan 2019 21:52:40 +0000
From:   Roman Gushchin <guro@...com>
To:     Chris Down <chris@...isdown.name>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Dennis Zhou <dennis@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH] mm: Proportional memory.{low,min} reclaim

On Mon, Jan 28, 2019 at 04:42:13PM -0500, Chris Down wrote:
> Roman Gushchin writes:
> > Hm, it looks a bit suspicious to me.
> > 
> > Let's say memory.low = 3G, memory.min = 1G and memory.current = 2G.
> > cgroup_size / protection == 1, so scan doesn't depend on memory.min at all.
> > 
> > So, we need to look directly at memory.emin in memcg_low_reclaim case, and
> > ignore memory.(e)low.
> 
> Hmm, this isn't really a common situation that I'd thought about, but it
> seems reasonable to make the boundaries when in low reclaim to be between
> min and low, rather than 0 and low. I'll add another patch with that. Thanks

It's not a stopper, so I'm perfectly fine with a follow-up patch.

> 
> > > +			scan = clamp(scan, SWAP_CLUSTER_MAX, lruvec_size);
> > 
> > Idk, how much sense does it have to make it larger than SWAP_CLUSTER_MAX,
> > given that it will become 0 on default (and almost any other) priority.
> 
> In my testing, setting the scan target to 0 and thus reducing scope for
> reclaim can result in increasing the scan priority more than is desirable,
> and since we base some vm heuristics based on that, that seemed concerning.
> 
> I'd rather start being a bit more cautious, erring on the side of scanning
> at least some pages from this memcg when priority gets elevated.
> 
> Thanks for the review!

For the rest of the patch:
Reviewed-by: Roman Gushchin <guro@...com>

Thanks!