linux-kernel - Re: [PATCH v9 2/3] sched: Move task_mm_cid_work to mm work

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Message-ID: <2bc76f69-901c-4d2c-bd75-64e757ec2230@efficios.com>
Date: Mon, 24 Feb 2025 15:50:17 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Gabriele Monaco <gmonaco@...hat.com>, 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
Cc: Ingo Molnar <mingo@...hat.org>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v9 2/3] sched: Move task_mm_cid_work to mm work_struct

On 2025-02-24 08:28, Gabriele Monaco wrote:
[...]
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 2cb16091ec0ae..936863fe7eb37 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
>   	}
>   	if (unlikely(rseq_update_cpu_node_id(t)))
>   		goto error;
> +	task_queue_mm_cid(t);

Given that task_queue_mm_cid() will be called quite frequently from
__rseq_handle_notify_resume, perhaps it would be best to move at least
the portion responsible for checks (including the time_before()) to
include/linux/sched.h to eliminate a function call from the fast path.

>   	return;
>   
>   error:

[...]

> -void task_tick_mm_cid(struct rq *rq, struct task_struct *curr)
> +void task_queue_mm_cid(struct task_struct *curr)
>   {
> -	struct callback_head *work = &curr->cid_work;
> +	struct work_struct *work = &curr->mm->cid_work;
>   	unsigned long now = jiffies;
>   
> -	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) ||
> -	    work->next != work)
> +	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)))
>   		return;
>   	if (time_before(now, READ_ONCE(curr->mm->mm_cid_next_scan)))
>   		return;
>   
> -	/* No page allocation under rq lock */
> -	task_work_add(curr, work, TWA_RESUME);
> +	/* Ensure the mm exists when we run. */
> +	mmgrab(curr->mm);
> +	queue_work(system_unbound_wq, work);
>   }

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com