linux-kernel - Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp

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Message-ID: <CAKEwX=MWotmH2YOC-Sdb5Krzt43ogCy8kqJnCLDRm7Db=evDOg@mail.gmail.com>
Date: Wed, 8 Jan 2025 22:49:57 +0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Barry Song <21cnbao@...il.com>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Vitaly Wool <vitalywool@...il.com>, Sam Sun <samsun1006219@...il.com>, 
	"linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"stable@...r.kernel.org" <stable@...r.kernel.org>, 
	"Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Subject: Re: [PATCH v2 2/2] mm: zswap: disable migration while using per-CPU acomp_ctx

On Wed, Jan 8, 2025 at 10:36 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
>
> Oh, I was not talking about my proposed diff, but the existing logic
> that allocates the requests and buffers in the hotplug callbacks
> instead of just using alloc_percpu() to allocate them once for each
> possible CPU. I was wondering if there are actual setups where this
> matters and a significant amount of memory is being saved. Otherwise
> we should simplify things and just rip out the hotplug callbacks.

My vote is for ripping the hotplug callbacks (eventually) :) In
addition to the discrepancy in the number of possible and online CPUs,
we also need a relatively smaller memory size for the discrepancy to
matter, no? Systems with hundreds of CPUs (hopefully) should have
hundreds of GBs worth of memory available (if not more).

Anyhow, we can just go with the diff you sent for now (and for past
kernels). Seems simple enough, and wouldn't get in the way of the
eventual hotplug logic removal (if you decide to pursue it).