linux-kernel - Re: [PATCH v4 1/1] memcg/hugetlb: Add hugeTLB counters to memcg

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Message-ID: <20241114052624.GD1564047@cmpxchg.org>
Date: Thu, 14 Nov 2024 00:26:24 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: David Rientjes <rientjes@...gle.com>
Cc: Joshua Hahn <joshua.hahnjy@...il.com>, akpm@...ux-foundation.org,
	nphamcs@...il.com, shakeel.butt@...ux.dev, roman.gushchin@...ux.dev,
	muchun.song@...ux.dev, chris@...isdown.name, tj@...nel.org,
	lizefan.x@...edance.com, mkoutny@...e.com, corbet@....net,
	lnyng@...a.com, cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [PATCH v4 1/1] memcg/hugetlb: Add hugeTLB counters to memcg

On Wed, Nov 13, 2024 at 02:42:29PM -0800, David Rientjes wrote:
> On Mon, 11 Nov 2024, David Rientjes wrote:
> 
> > > The reason that I opted not to include a breakdown of each hugetlb
> > > size in memory.stat is only because I wanted to keep the addition that
> > > this patch makes as minimal as possible, while still addressing
> > > the goal of bridging the gap between memory.stat and memory.current.
> > > Users who are curious about this breakdown can see how much memory
> > > is used by each hugetlb size by enabling the hugetlb controller as well.
> > > 
> > 
> > While the patch may be minimal, this is solidifying a kernel API that 
> > users will start to count on.  Users who may be interested in their 
> > hugetlb usage may not have control over the configuration of their kernel?
> > 
> > Does it make sense to provide a breakdown in memory.stat so that users can 
> > differentiate between mapping one 1GB hugetlb page and 512 2MB hugetlb 
> > pages, which are different global resources?
> > 
> > > It's true that this is the case as well for total hugeltb usage, but
> > > I felt that not including hugetlb memory usage in memory.stat when it
> > > is accounted by memory.current would cause confusion for the users
> > > not being able to see that memory.current = sum of memory.stat. On the
> > > other hand, seeing the breakdown of how much each hugetlb size felt more
> > > like an optimization, and not a solution that bridges a confusion.
> > > 
> > 
> > If broken down into hugetlb_2048kB and hugetlb_1048576kB on x86, for 
> > example, users could still do sum of memory.stat, no?>
> > 
> 
> Friendly ping on this, would there be any objections to splitting the 
> memory.stat metrics out to be per hugepage size?

I don't think it has to be either/or. We can add the total here, and a
per-size breakdown in a separate patch (with its own changelog)?

That said, a per-size breakdown might make more sense in the hugetlb
cgroup controller. You're mentioning separate global resources, which
suggests this is about more explicitly controlled hugetlb use.

>From a memcg POV, all hugetlb is the same. It's just (non-swappable)
memory consumed by the cgroup.