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SubjectRe: How do I increment a per-CPU variable without warning?
On Tue, Apr 15, 2014 at 03:47:26PM -0700, Paul E. McKenney wrote:
> On Tue, Apr 15, 2014 at 06:29:51PM -0400, Dave Jones wrote:
> > On Tue, Apr 15, 2014 at 03:17:55PM -0700, Paul E. McKenney wrote:
> >
> > > My current admittedly crude workaround is as follows:
> > >
> > > static inline bool rcu_should_resched(void)
> > > {
> > > int t;
> > >
> > > #ifdef CONFIG_DEBUG_PREEMPT
> > > preempt_disable();
> > > #endif /* #ifdef CONFIG_DEBUG_PREEMPT */
> > > t = __this_cpu_read(rcu_cond_resched_count) + 1;
> > > if (t < RCU_COND_RESCHED_LIM) {
> > > __this_cpu_write(rcu_cond_resched_count, t);
> > > #ifdef CONFIG_DEBUG_PREEMPT
> > > preempt_enable();
> > > #endif /* #ifdef CONFIG_DEBUG_PREEMPT */
> > > return false;
> > > }
> > > #ifdef CONFIG_DEBUG_PREEMPT
> > > preempt_enable();
> > > #endif /* #ifdef CONFIG_DEBUG_PREEMPT */
> > > return true;
> > > }
> >
> > Won't using DEBUG_PREEMPT instead of just CONFIG_PREEMPT here make this
> > silently do the wrong thing if preemption is enabled, but debugging isn't ?
>
> If preemption is enabled, but debugging is not, then yes, the above code
> might force an unnecessary schedule() if the above code was preempted
> between the __this_cpu_read() and the __this_cpu_write(). Which does
> not cause a problem, especially given that it won't happen very often.
>
> > I'm not seeing why you need the ifdefs at all, unless the implied
> > barrier() is a problem ?
>
> I don't think that Peter Zijlstra would be too happy about an extra
> unneeded preempt_disable()/preempt_enable() pair in the cond_resched()
> fastpath. Not that I necessarily expect him to be particularly happy
> with the above, but perhaps someone has a better approach.

But falling back on the old ways of doing this at least looks a bit
nicer:

static inline bool rcu_should_resched(void)
{
int t;
int *tp = &per_cpu(rcu_cond_resched_count, raw_smp_processor_id());

t = ACCESS_ONCE(*tp) + 1;
if (t < RCU_COND_RESCHED_LIM) {
ACCESS_ONCE(*tp) = t;
return false;
}
return true;
}

Other thoughts?

Thanx, Paul



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