linux-kernel - Re: [patch 16/19] sched/mmcid: Provide CID ownership mode fixup functions

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Message-ID: <87h5vuytc1.ffs@tglx>
Date: Mon, 20 Oct 2025 08:34:38 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Gabriele Monaco
 <gmonaco@...hat.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Jeanson <mjeanson@...icios.com>, Jens Axboe <axboe@...nel.dk>,
 "Paul E. McKenney" <paulmck@...nel.org>, "Gautham R. Shenoy"
 <gautham.shenoy@....com>, Florian Weimer <fweimer@...hat.com>, Tim Chen
 <tim.c.chen@...el.com>
Subject: Re: [patch 16/19] sched/mmcid: Provide CID ownership mode fixup
 functions

On Wed, Oct 15 2025 at 19:29, Thomas Gleixner wrote:
> As the goal is to avoid serialization of the scheduler hotpath, this
> requires that the switch back threshold is maximally nr_cpu_ids / 2.
> Otherwise the CID space might become exhausted when tasks are scheduled in
> on CPUs which already transferred ownership before the fixup function was
> able to free or transfer enough CIDs. That would result in a live lock
> because the task loops in mm_get_cid() with runqueue lock held and the
> fixup function is stuck on that runqueue lock.

I've found a way how to lift that limit and to address a problem I've
overlooked in the initial implementation.