linux-kernel - Re: [PATCH 2/2] sched: Move task_mm_cid

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Message-ID: <CAEXW_YT1NLU2xsBHwDjbBPg_rZWc4hNGNgmkdxF8C_0Dw60N1w@mail.gmail.com>
Date: Tue, 3 Dec 2024 10:00:25 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Gabriele Monaco <gmonaco@...hat.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, 
	paulmck <paulmck@...nel.org>, Frederic Weisbecker <frederic@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Josh Triplett <josh@...htriplett.org>, 
	Boqun Feng <boqun.feng@...il.com>, Uladzislau Rezki <urezki@...il.com>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, 
	"rcu@...r.kernel.org" <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched: Move task_mm_cid_work to RCU callback

On Mon, Dec 2, 2024 at 9:34 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> += CC RCU maintainers, reviewers and list.
> += RSEQ maintainers.
>
> On 2024-12-02 09:07, Gabriele Monaco wrote:
> > Currently, the task_mm_cid_work function is called in a task work
> > triggered by a scheduler tick. This can delay the execution of the
> > task for the entire duration of the function.
> >
> > This patch runs the task_mm_cid_work in the RCU callback thread rather
> > than in the task context before returning to userspace.
> >
> > The main advantage of this change is that the function can be offloaded
> > to a different CPU and even preempted by RT tasks.
> >
> > On a busy system, this may mean the function gets called less often, but
> > the current behaviour already doesn't provide guarantees.
>
> I've used the same task work pattern as NUMA here. What makes it
> OK for NUMA and not for mm_cid ?
>
> I wonder why we'd want to piggy-back on call_rcu here when
> this has nothing to do with RCU. There is likely a characteristic
> of the call_rcu worker threads that we want to import into
> task_tick_mm_cid(), or change task_work.c to add a new flag
> that says the work can be dispatched to any CPU.

Also there is no guarantee that RCU callback will run within a thread
context (example, some configurations run it in softirq). Further,
call_rcu() usage as shown in this patch can also delay callback runs
by seconds (with RCU_LAZY enabled).

See also #5 in checklist: https://docs.kernel.org/RCU/checklist.html

thanks,

 - Joel