linux-kernel - Re: How do I increment a per-CPU variable without warning?

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Message-ID: <20140415224725.GS4496@linux.vnet.ibm.com>
Date:	Tue, 15 Apr 2014 15:47:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Dave Jones <davej@...hat.com>, cl@...ux.com,
	linux-kernel@...r.kernel.org, mingo@...nel.org, tj@...nel.org,
	grygorii.strashko@...com, peterz@...radead.org
Subject: Re: How do I increment a per-CPU variable without warning?

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.

							Thanx, Paul

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