Re: [PATCH v1] sched: there is no need to call switch_mm_irqs_off when sched between two user thread.

[Ming Wang]

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@xxxxxxxxxxx>
> > ---
> >   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.