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

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Date:   Wed, 1 Jun 2022 14:45:06 +0800
From:   Ming Wang <wangming01@...ngson.cn>
To:     Peter Zijlstra <peterz@...radead.org>
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 2022/5/31 下午10:46, Peter Zijlstra wrote:
> 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).
ok, thanks! And I will do it in architecture code.