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

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Message-ID: <c97730cdf59fa4b32ee46bd22f3cc514deb0f51e.camel@redhat.com>
Date: Mon, 10 Mar 2025 16:57:02 +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 Mon, 2025-03-10 at 11:50 -0400, Mathieu Desnoyers wrote:
> On 2025-03-10 10:46, Gabriele Monaco wrote:
> > 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)
> 
> Found a small nit. Please fix and resend with my reviewed-by, and
> that version will be ready for inclusion.

Perfect, thank you!
I'm changing that jiffies thing, testing a bit and sending it.

Gabriele