linux-kernel - Re: [RFC PATCH] sched: Fix performance regression introduced by mm

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Date:   Mon, 3 Apr 2023 20:13:08 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>
Subject: Re: [RFC PATCH] sched: Fix performance regression introduced by
 mm_cid

On 2023-04-03 14:13, Mathieu Desnoyers wrote:
[...]
> +/*
> + * Migration to dst cpu. Called with dst_rq lock held.
> + */
> +void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t, int src_cid)
> +{
> +	struct mm_struct *mm = t->mm;
> +	int dst_cid, *dst_pcpu_cid;
> +
> +	lockdep_assert_rq_held(dst_rq);
> +
> +	if (!mm || src_cid == -1)
> +		return;
> +
> +	dst_pcpu_cid = per_cpu_ptr(mm->pcpu_cid, cpu_of(dst_rq));
> +
> +	/*
> +	 * If destination cpu cid is greater than the source cpu cid, unset it
> +	 * so it can be reallocated.
> +	 */
> +	dst_cid = *dst_pcpu_cid;
> +	if (dst_cid == -1 || dst_cid < src_cid)

Small detail: I plan to change this from "dst_cid < src_cid" to
"dst_cid  <= src_cid" in my next version of the patch to handle the
unlikely case where a task would be migrated back and forth between
two runqueues without being scheduled. It would be possible that the
task's last_mm_cid is equal to the dst_cid here, in which case it
would be better to leave the mm's destination cpu cid set.

> +		return;
> +	*dst_pcpu_cid = -1;
> +	/*
> +	 * Put dst_cid if it is not currently in use, else it will be lazy put
> +	 * on the next context switch.
> +	 */
> +	if (dst_rq->curr->mm != mm)
> +		__mm_cid_put(mm, dst_cid);
> +}

[...]

Thanks,

Mathieu


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