Messages in this thread Patch in this message | | | Date | Fri, 16 Dec 2011 15:23:34 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] PREEMPT_RT_FULL: ARM context switch needs IRQs enabled |
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On Fri, Dec 16, 2011 at 12:49:53PM +0000, Peter Zijlstra wrote: > On Fri, 2011-12-16 at 11:43 +0000, Catalin Marinas wrote: > > From 26d87e955f089fd246bee29bb388f22da1297e0c Mon Sep 17 00:00:00 2001 > > From: Catalin Marinas <catalin.marinas@arm.com> > > Date: Fri, 16 Dec 2011 11:32:26 +0000 > > Subject: [PATCH] sched, mm: Use activate_mm() instead of switch_mm() > > > > The ARM port tries to remove __ARCH_WANT_INTERRUPTS_ON_CTXSW. Since the > > actual pgd switching requires interrupts to be enabled on ARM (for > > latency on ARMv5 and earlier and IPIs on ARMv6+), the solution is to > > defer the pgd switching to a post context switch hook that is run with > > interrupts enabled. There are however two additional direct calls to > > switch_mm() without the additional post-switch hook and ARM would fail > > to set the new pgd. > > > > This patch changes the switch_mm() call with activate_mm() which ensures > > that the required pgd has been set. The activate_mm() function must be > > called with interrupts enabled. > > > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> > > Cc: Ingo Molnar <mingo@elte.hu> > > Cc: Peter Zijlstra <peterz@infradead.org> > > --- > > kernel/sched.c | 2 +- > > mm/mmu_context.c | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 7b46a39..3976157 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -6310,7 +6310,7 @@ void idle_task_exit(void) > > BUG_ON(cpu_online(smp_processor_id())); > > > > if (mm != &init_mm) > > - switch_mm(mm, &init_mm, current); > > + activate_mm(mm, &init_mm); > > mmdrop(mm); > > } > > > > diff --git a/mm/mmu_context.c b/mm/mmu_context.c > > index cf332bc..4e44ac4 100644 > > --- a/mm/mmu_context.c > > +++ b/mm/mmu_context.c > > @@ -32,7 +32,7 @@ void use_mm(struct mm_struct *mm) > > tsk->active_mm = mm; > > } > > tsk->mm = mm; > > - switch_mm(active_mm, mm, tsk); > > + activate_mm(active_mm, mm); > > task_unlock(tsk); > > > > if (active_mm != mm) > > > > alpha, cris, microblaze, powerpc appear to actually use that task > argument of switch_mm(), so I'm not convinced this is actually correct.
We use it on ARM as well but in both cases above the task was current. However, I can see that the activate_mm() implementation on other architectures doesn't just call switch_mm(current), so the two may not be functionally equivalent.
An alternative would be to only defer the pgd switching on ARM if irqs_disabled() and leave the generic code untouched (well, apart from the post-switch hook).
-- Catalin
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