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

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Message-ID: <CAJD7tkaiGYSRXkG8eYRTkm_BBkPSHi2VAVzTsTHNgHydAzPZNQ@mail.gmail.com>
Date: Wed, 8 Jan 2025 08:17:54 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.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 7:50 AM Nhat Pham <nphamcs@...il.com> wrote:
>
> 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).

Yeah I sent that out just now:
https://lore.kernel.org/lkml/20250108161529.3193825-1-yosryahmed@google.com/

I will look into sending an RFC to rip out the hotplug callbacks later.