linux-kernel - Re: [PATCH v11 2/3] sched: Move task_mm_cid_work to mm work

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Message-ID: <1c161066bcbc916ae9d97e7d1753ee12511da085.camel@redhat.com>
Date: Mon, 10 Mar 2025 15:46:30 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Ingo Molnar <mingo@...hat.org>, Shuah Khan <shuah@...nel.org>, 
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney"	 <paulmck@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v11 2/3] sched: Move task_mm_cid_work to mm work_struct

On Thu, 2025-02-27 at 16:33 +0100, Gabriele Monaco wrote:
> Currently, the task_mm_cid_work function is called in a task work
> triggered by a scheduler tick to frequently compact the mm_cids of
> each
> process. This can delay the execution of the corresponding thread for
> the entire duration of the function, negatively affecting the
> response
> in case of real time tasks. In practice, we observe task_mm_cid_work
> increasing the latency of 30-35us on a 128 cores system, this order
> of
> magnitude is meaningful under PREEMPT_RT.
> 
> Run the task_mm_cid_work in a new work_struct connected to the
> mm_struct rather than in the task context before returning to
> userspace.
> 
> This work_struct is initialised with the mm and disabled before
> freeing
> it. The queuing of the work happens while returning to userspace in
> __rseq_handle_notify_resume, maintaining the checks to avoid running
> more frequently than MM_CID_SCAN_DELAY.
> To make sure this happens predictably also on long running tasks, we
> trigger a call to __rseq_handle_notify_resume also from the scheduler
> tick if the runtime exceeded a 100ms threshold.
> [...]
> 
> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by
> mm_cid")
> Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>

Is this patch missing anything?

I refactored a bit to have it build in configurations without RSEQ
and/or MM_CID (which was failing v10)

Thanks,
Gabriele