linux-kernel - Re: [PATCH 1/1] sched: fix the PREEMPT_ACTIVE check in __trace_sched_switch

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Date:	Fri, 10 Oct 2014 11:56:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched: fix the PREEMPT_ACTIVE check in
 __trace_sched_switch_state()

On Fri, Oct 10, 2014 at 11:54:52AM +0200, Peter Zijlstra wrote:
> On Tue, Oct 07, 2014 at 09:51:08PM +0200, Oleg Nesterov wrote:
> > task_preempt_count() has nothing to do with the actual preempt counter,
> > thread_info->saved_preempt_count is only valid right after switch_to().
> > 
> > __trace_sched_switch_state() can use preempt_count(), prev is still the
> > current task when trace_sched_switch() is called.
> 
> This is true, but the paranoid in me would like a BUG_ON(p != current)
> right along with that to make sure we don't accidentally change this.
> 
> But yes, its good to get rid of task_preempt_count(), its horrible.

Like so then?

---
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -97,16 +97,19 @@ static inline long __trace_sched_switch_
 	long state = p->state;
 
 #ifdef CONFIG_PREEMPT
+#ifdef CONFIG_SCHED_DEBUG
+	BUG_ON(p != current);
+#endif /* CONFIG_SCHED_DEBUG */
 	/*
 	 * For all intents and purposes a preempted task is a running task.
 	 */
-	if (task_preempt_count(p) & PREEMPT_ACTIVE)
+	if (preempt_count() & PREEMPT_ACTIVE)
 		state = TASK_RUNNING | TASK_STATE_MAX;
-#endif
+#endif /* CONFIG_PREEMPT */
 
 	return state;
 }
-#endif
+#endif /* CREATE_TRACE_POINTS */
 
 /*
  * Tracepoint for task switches, performed by the scheduler:
--
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