linux-kernel - Re: [PATCH 1/2] sched/mm: add finish_switch

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Date:	Wed, 13 Nov 2013 12:19:09 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/mm: add finish_switch_mm function

On Wed, Nov 13, 2013 at 11:41:43AM +0000, Peter Zijlstra wrote:
> On Wed, Nov 13, 2013 at 09:16:13AM +0100, Martin Schwidefsky wrote:
> > The switch_mm function is called with the task_lock and/or with
> > request queue lock. Add finish_switch_mm to allow an architecture
> > to execute some code after the mm has been switched but without
> > any locks held. One use case is the s390 architecture which will
> > use this to wait for the completion of TLB flush operations.

We have similar needs on arm and arm64 (full cache flushing where we
want interrupts enable or some IPIs for TLB tagging synchronisation).

> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 1deccd7..89409cb 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -32,7 +32,7 @@
> >  #include <linux/init.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/highmem.h>
> > -#include <asm/mmu_context.h>
> > +#include <linux/mmu_context.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/capability.h>
> >  #include <linux/completion.h>
> > @@ -1996,6 +1996,7 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
> >  	perf_event_task_sched_in(prev, current);
> >  	finish_lock_switch(rq, prev);
> >  	finish_arch_post_lock_switch();
> > +	finish_switch_mm(current->mm, current);

This could use the same hook.

> >  
> >  	fire_sched_in_preempt_notifiers(current);
> >  	if (mm)
> > @@ -4140,8 +4141,10 @@ void idle_task_exit(void)
> >  
> >  	BUG_ON(cpu_online(smp_processor_id()));
> >  
> > -	if (mm != &init_mm)
> > +	if (mm != &init_mm) {
> >  		switch_mm(mm, &init_mm, current);
> > +		finish_switch_mm(&init_mm, current);
> > +	}
> >  	mmdrop(mm);
> >  }

Here finish_switch_mm() is called in the same context with switch_mm().
What we have on ARM via switch_mm() is to check for irqs_disabled() and
if yes, defer the actual switching via a flag until the
finish_arch_post_lock_switch() hook. But on ARM we only cared about the
interrupts being enabled.

> > diff --git a/mm/mmu_context.c b/mm/mmu_context.c
> > index 8a8cd02..11b3d47 100644
> > --- a/mm/mmu_context.c
> > +++ b/mm/mmu_context.c
> > @@ -8,8 +8,6 @@
> >  #include <linux/export.h>
> >  #include <linux/sched.h>
> >  
> > -#include <asm/mmu_context.h>
> > -
> >  /*
> >   * use_mm
> >   *	Makes the calling kernel thread take on the specified
> > @@ -31,6 +29,7 @@ void use_mm(struct mm_struct *mm)
> >  	tsk->mm = mm;
> >  	switch_mm(active_mm, mm, tsk);
> >  	task_unlock(tsk);
> > +	finish_switch_mm(mm, tsk);

As above, for ARM we only care about interrupts being enabled, so it
didn't require a hook.

Is s390 switch_mm() ok with only interrupts being enabled but some locks
held?

-- 
Catalin
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