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

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Message-ID: <20251020091601.GH3245006@noisy.programming.kicks-ass.net>
Date: Mon, 20 Oct 2025 11:16:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.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>,
	TCMalloc Team <tcmalloc-eng@...gle.com>
Subject: Re: [patch 16/19] sched/mmcid: Provide CID ownership mode fixup
 functions

On Wed, Oct 15, 2025 at 07:29:54PM +0200, Thomas Gleixner wrote:

> +static void __maybe_unused mm_cid_fixup_cpus_to_tasks(struct mm_struct *mm)
> +{
> +	unsigned int cpu;


Alternatively you could take cpus_read_lock() and iterate online_cpus().
No offline CPU should be running a userspace task and all that.

Doesn't really matter much, this also works fine.

> +	/* Walk the CPUs and fixup all stale CIDs */
> +	for_each_possible_cpu(cpu) {
> +		struct mm_cid_pcpu *pcp = per_cpu_ptr(mm->mm_cid.pcpu, cpu);
> +		struct rq *rq = cpu_rq(cpu);
> +
> +		/* Remote access to mm::mm_cid::pcpu requires rq_lock */
> +		guard(rq_lock_irq)(rq);
> +		if (cid_on_cpu(pcp->cid)) {
> +			/* If rq->curr has @mm, fix it up right here */
> +			if (rq->curr->mm == mm && rq->curr->mm_cid.active)
> +				mm_cid_transfer_to_task(rq->curr, pcp);
> +			else
> +				mm_drop_cid_on_cpu(mm, pcp);
> +		}
> +	}
> +}