linux-kernel - Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime accounting

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Message-ID: <1357676771.5190.29.camel@gandalf.local.home>
Date:	Tue, 08 Jan 2013 15:26:11 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime
 accounting

On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:

> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index c952770..bd461ad 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -56,7 +56,7 @@ void user_enter(void)
>  	local_irq_save(flags);
>  	if (__this_cpu_read(context_tracking.active) &&
>  	    __this_cpu_read(context_tracking.state) != IN_USER) {
> -		__this_cpu_write(context_tracking.state, IN_USER);
> +		vtime_user_enter(current);
>  		/*
>  		 * At this stage, only low level arch entry code remains and
>  		 * then we'll run in userspace. We can assume there won't be
> @@ -65,6 +65,7 @@ void user_enter(void)
>  		 * on the tick.
>  		 */
>  		rcu_user_enter();

Hmm, the rcu_user_enter() can do quite a bit. Too bad we are accounting
it as user time. I wonder if we could move the vtime_user_enter() below
it. But then if vtime_user_enter() calls rcu_read_lock() it breaks.

The notorious chicken vs egg ordeal!

-- Steve
 
> +		__this_cpu_write(context_tracking.state, IN_USER);
>  	}
>  	local_irq_restore(flags);
>  }
> @@ -90,12 +91,13 @@ void user_exit(void)
>  
>  	local_irq_save(flags);
>  	if (__this_cpu_read(context_tracking.state) == IN_USER) {
> -		__this_cpu_write(context_tracking.state, IN_KERNEL);
>  		/*
>  		 * We are going to run code that may use RCU. Inform
>  		 * RCU core about that (ie: we may need the tick again).
>  		 */
>  		rcu_user_exit();
> +		vtime_user_exit(current);
> +		__this_cpu_write(context_tracking.state, IN_KERNEL);
>  	}
>  	local_irq_restore(flags);
>  }


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