Messages in this thread Patch in this message |  | | Date | Wed, 13 Nov 2013 17:05:56 +0100 | From | Martin Schwidefsky <> | Subject | Re: [PATCH 1/2] sched/mm: add finish_switch_mm function |
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On Wed, 13 Nov 2013 12:19:09 +0000 Catalin Marinas <catalin.marinas@arm.com> wrote:
> 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).
On s390 we need to wait for the completion of a TLB flush.
> > > 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.
Yes. > > > > > > 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.
The guarantee s390 needs is that the rq-lock is not taken. What I have seen with the wait loop in switch_mm is a dead lock because one CPU #0 was looping in switch_mm to wait for the TLB flush of another CPU #1. CPU #1 got an interrupt that tried to wake-up a task which happened to be on the run-queue of CPU #0.
> > > 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?
Interrupts on/off is not the problem for s390.
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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