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

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Message-ID: <c9c86312-09f6-4e03-8934-f5d7e8b36c81@suse.cz>
Date: Wed, 26 Jun 2024 15:16:04 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Michal Hocko <mhocko@...e.com>, 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>,
 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 6/25/24 4:20 PM, Michal Hocko wrote:
> 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.

I guess drain via lru_cache_disable() should be honored, but also rare.

> Please also CC linux MM ML once the core infrastructure is agreed on.