linux-kernel - Re: [PATCH v2 2/4] rseq: Run the mm_cid_compaction from rseq_handle_notify

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Message-ID: <2bf22dc5213ada5339d85744c348da9558f25e3d.camel@redhat.com>
Date: Tue, 30 Sep 2025 12:18:18 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>, Mathieu Desnoyers
	 <mathieu.desnoyers@...icios.com>, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
  David Hildenbrand	 <david@...hat.com>, linux-mm@...ck.org, "Paul E.
 McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v2 2/4] rseq: Run the mm_cid_compaction from
 rseq_handle_notify_resume()

On Tue, 2025-09-30 at 00:01 +0200, Thomas Gleixner wrote:
> 
> The current overhead of RSEQ is way too high. People have reported 3%
> regressions just because glibc uses RSEQ now. So that needs to be
> addressed. Moving the RSEQ fastpath to the last point before going back
> to user space is the correct thing to do.
> 
> mm cid compaction is a related but orthogonal problem. I just skimmed
> your patches and I'm really not convinced that this is the right
> approach to solve the problem.
> 
> The main issue with task_mm_cid_work() is that it iterates over all
> possible CPUs twice. Your batching just makes the impact smaller, but it
> does not really try to change the overall approach to this.

Thanks for the thorough explanation and sketch implementation. As I'm not really
confident with rseq I didn't dare changing the compaction logic too much, but
clearly your approach of getting compact cid by construction seems the way to
go.

I confirm your patch passes the selftest in 4/4 of this series and, obviously,
removes the latency I was observing.

Thanks,
Gabriele