linux-kernel - Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

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Date:   Wed, 19 Apr 2023 08:29:47 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

On Wed, Apr 19, 2023 at 08:14:09AM -0300, Marcelo Tosatti wrote:
> This was tried before:
> https://lore.kernel.org/lkml/20220127173037.318440631@fedora.localdomain/
> 
> My conclusion from that discussion (and work) is that a special system
> call:
> 
> 1) Does not allow the benefits to be widely applied (only modified
> applications will benefit). Is not portable across different operating systems. 
> 
> Removing the vmstat_work interruption is a benefit for HPC workloads, 
> for example (in fact, it is a benefit for any kind of application, 
> since the interruption causes cache misses).
> 
> 2) Increases the system call cost for applications which would use
> the interface.
> 
> So avoiding the vmstat_update update interruption, without userspace 
> knowledge and modifications, is a better than solution than a modified
> userspace.

Another important point is this: if an application dirties
its own per-CPU vmstat cache, while performing a system call,
and a vmstat sync event is triggered on a different CPU, you'd have to:

1) Wait for that CPU to return to userspace and sync its stats
(unfeasible).

2) Queue work to execute on that CPU (undesirable, as that causes
an interruption).

3) Remotely sync the vmstat for that CPU.