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 11:54:52 +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 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.

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  include/trace/events/sched.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 0a68d5a..a7d67bc 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -100,7 +100,7 @@ static inline long __trace_sched_switch_state(struct task_struct *p)
>  	/*
>  	 * 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
>  
> -- 
> 1.5.5.1
> 
> 
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