linux-kernel - Re: [PATCH v2][RFC] tracing/context-tracking: Add preempt_schedule

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Message-ID: <20130605114559.GE8923@twins.programming.kicks-ass.net>
Date:	Wed, 5 Jun 2013 13:45:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2][RFC] tracing/context-tracking: Add
 preempt_schedule_context() for tracing

On Tue, Jun 04, 2013 at 10:16:56AM -0400, Steven Rostedt wrote:
> index 65349f0..73b89d9 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -71,6 +71,46 @@ void user_enter(void)
>  	local_irq_restore(flags);
>  }
>  
> +#ifdef CONFIG_PREEMPT
> +/**
> + * preempt_schedule_context - preempt_schedule called by tracing
> + *
> + * The tracing infrastructure uses preempt_enable_notrace to prevent
> + * recursion and tracing preempt enabling caused by the tracing
> + * infrastructure itself. But as tracing can happen in areas coming
> + * from userspace or just about to enter userspace, a preempt enable
> + * can occur before user_exit() is called. This will cause the scheduler
> + * to be called when the system is still in usermode.
> + *
> + * To prevent this, the preempt_enable_notrace will use this function
> + * instead of preempt_schedule() to exit user context if needed before
> + * calling the scheduler.
> + */

If preempt_enable_notrace() is the only user, why does this live in
kernel/context_tracking.c and not in kernel/sched/core.c?

> +void __sched notrace preempt_schedule_context(void)
> +{
> +	struct thread_info *ti = current_thread_info();
> +	enum ctx_state prev_ctx;
> +
> +	if (likely(ti->preempt_count || irqs_disabled()))
> +		return;
> +
> +	/*
> +	 * Need to disable preemption in case user_exit() is traced
> +	 * and the tracer calls preempt_enable_notrace() causing
> +	 * an infinite recursion.
> +	 */
> +	preempt_disable_notrace();
> +	prev_ctx = exception_enter();
> +	preempt_enable_no_resched_notrace();
> +
> +	preempt_schedule();
> +
> +	preempt_disable_notrace();
> +	exception_exit(prev_ctx);
> +	preempt_enable_notrace();
> +}
> +EXPORT_SYMBOL(preempt_schedule_context);

Do we really need to export this?

> +#endif /* CONFIG_PREEMPT */
>  
>  /**
>   * user_exit - Inform the context tracking that the CPU is
> -- 
> 1.7.10.4
> 
> 
> 
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