linux-kernel - Re: [PATCH v1] sched: there is no need to call switch_mm_irqs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YpYqOC9Wx84oC2z5@hirez.programming.kicks-ass.net>
Date:   Tue, 31 May 2022 16:46:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ming Wang <wangming01@...ngson.cn>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] sched: there is no need to call switch_mm_irqs_off
 when sched between two user thread.

On Tue, May 31, 2022 at 07:56:41PM +0800, Ming Wang wrote:
> When condition (prev->active_mm == next->mm && !prev->mm) is met,
> the situation is as follows:
> 
> user thread -> user thread
> 
> There is not need switch_mm when sched between two user thread.
> Because they share the mm_struct. This can provide better
> performance when testing UnixBench.
> 
> Signed-off-by: Ming Wang <wangming01@...ngson.cn>
> ---
>  kernel/sched/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 696c649..9d7f6fb 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5099,7 +5099,8 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
>  		 * case 'prev->active_mm == next->mm' through
>  		 * finish_task_switch()'s mmdrop().
>  		 */
> -		switch_mm_irqs_off(prev->active_mm, next->mm, next);
> +		if ((prev->active_mm != next->mm) || (!prev->mm))
> +			switch_mm_irqs_off(prev->active_mm, next->mm, next);

I think this needs to be inside switch_mm(). Architectures are free to
play silly games with what the current active mm is (and iirc x86
actually does this).