Re: [PATCH] tracing/sched: Check preempt_count() for current when reading task->state
From: Ingo Molnar
Date: Thu Dec 11 2014 - 07:31:31 EST
* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Thu, 11 Dec 2014 07:38:11 +0100
> Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > > Cc: stable@xxxxxxxxxxxxxxx # 3.13+
> > > Fixes: 01028747559a "sched: Create more preempt_count accessors"
> > > Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > ---
> > > include/trace/events/sched.h | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> > > index 0a68d5ae584e..13fbadcc172b 100644
> > > --- a/include/trace/events/sched.h
> > > +++ b/include/trace/events/sched.h
> > > @@ -97,10 +97,14 @@ static inline long __trace_sched_switch_state(struct task_struct *p)
> > > long state = p->state;
> > >
> > > #ifdef CONFIG_PREEMPT
> > > + unsigned long pc;
> > > +
> > > + pc = (p == current) ? preempt_count() : task_preempt_count(p);
> > > +
> > > /*
> > > * For all intents and purposes a preempted task is a running task.
> > > */
> > > - if (task_preempt_count(p) & PREEMPT_ACTIVE)
> > > + if (pc & PREEMPT_ACTIVE)
> > > state = TASK_RUNNING | TASK_STATE_MAX;
> > I really don't like the overhead around here.
> Hi Ingo!
> What overhead are you worried about? Note, this is in the
> schedule tracepoint and does not affect the scheduler itself
> (as long as the tracepoint is not enabled).
Scheduler tracepoints are pretty popular, so I'm worried about
their complexity when they are activated.
> I'm also thinking that as long as "prev" is always guaranteed
> to be "current" we can remove the check and just use
> preempt_count() always. But I'm worried that we can't
> guaranteed that.
You could add a WARN_ON_ONCE() or so to double check that
> What other ideas do you have? Because wrong data is worse than
> the overhead of the above code. If Thomas taught me anything,
> it's that!
My idea is to have simpler, yet correct code. And ponies!
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