linux-kernel - Re: [RFC PATCH 6/6] mm: Drain LRUs upon resume to userspace on nohz

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Date: Tue, 25 Jun 2024 16:20:01 +0200
From: Michal Hocko <mhocko@...e.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC PATCH 6/6] mm: Drain LRUs upon resume to userspace on
 nohz_full CPUs

On Tue 25-06-24 15:52:44, Frederic Weisbecker wrote:
> LRUs can be drained through several ways. One of them may add disturbances
> to isolated workloads while queuing a work at any time to any target,
> whether running in nohz_full mode or not.
> 
> Prevent from that on isolated tasks with draining LRUs upon resuming to
> userspace using the isolated task work framework.
> 
> It's worth noting that this is inherently racy against
> lru_add_drain_all() remotely queueing the per CPU drain work and
> therefore it prevents from the undesired disturbance only
> *most of the time*.

Can we simply not schedule flushing on remote CPUs and leave that to the
"return to the userspace" path?

I do not think we rely on LRU cache flushing for correctness purposes anywhere.

Please also CC linux MM ML once the core infrastructure is agreed on.
-- 
Michal Hocko
SUSE Labs