linux-kernel - Re: [PATCH 2/3] posix_timers: Defer per process timer stop after timers processing

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Date:	Fri, 19 Apr 2013 00:30:02 -0400
From:	Olivier Langlois <olivier@...llion01.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.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>,
	Kevin Hilman <khilman@...aro.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] posix_timers: Defer per process timer stop after
 timers processing

Hi Frederic,
>  /*
> @@ -1279,6 +1277,7 @@ void run_posix_cpu_timers(struct task_struct *tsk)
>  	LIST_HEAD(firing);
>  	struct k_itimer *timer, *next;
>  	unsigned long flags;
> +	struct signal_struct *sig;
>  
>  	BUG_ON(!irqs_disabled());
>  
> @@ -1336,6 +1335,10 @@ void run_posix_cpu_timers(struct task_struct *tsk)
>  			cpu_timer_fire(timer);
>  		spin_unlock(&timer->it_lock);
>  	}
> +
> +	sig = tsk->signal;
> +	if (task_cputime_zero(&sig->cputime_expires))
> +		stop_process_timers(sig);
>  }
>  
>  /*
I have proposed a different modification for the same problem yesterday:

https://lkml.org/lkml/2013/4/17/351

I might be mistaken but I believe that firing timers are not rescheduled
in the current interrupt context. They are going to be rescheduled later
from the task context handling the timer generated signal.

Also, if the cputimer is started by posix_cpu_timer_set() without arming
any timer, the stop_process_timers() function will never be reached.

My methodology to test the modifications that I have proposed is by
adding a printk() statement inside thread_group_cputimer() where the
cputimer is initialized and run

glibc-2.17/rt/tst-cputimer1 unittest.

Before the changes, the cputimer is initialized 4 times!
I was expecting with the proposed changes to see that number reduced to
2 but it ended up to be even better and be only 1 because the second
waves of posix cpu timers is set within 1 tick.

If you do the same test with your changes, I think that there is a
chance that it will not get the same positive result for the reason that
I have stated above.

To conclude, I would be very interested to get your input in posix cpu
timer modification proposal that I did last week. My proposal is either
too complex or I am very bad explaining it so since your knowledgable in
the area, your input would be very helpful.

https://lkml.org/lkml/2013/4/5/370

Thank you!!!
Olivier

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