linux-kernel - Re: [PATCH 2/2] posix-cpu-timers: fix wrong timer initialization

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Date:	Mon, 29 Apr 2013 14:53:07 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Olivier Langlois <olivier@...llion01.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] posix-cpu-timers: fix wrong timer initialization

>>> @@ -749,7 +756,13 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
>>>  	}
>>>  
>>>  	if (new_expires.sched != 0 && !(flags & TIMER_ABSTIME)) {
>>> -		cpu_time_add(timer->it_clock, &new_expires, val);
>>> +		union cpu_time_count now;
>>> +
>>> +		if (CPUCLOCK_PERTHREAD(timer->it_clock))
>>> +			cpu_clock_sample(timer->it_clock, p, &now);
>>> +		else
>>> +			cpu_clock_sample_group(timer->it_clock, p, &now);
>>
>> This triggered a pattern match against earlier in this function; but they're
>> different now; timer vs clock. So nothing to merge...
> 
> Not different, I think.
> Relative timeout need to calculate "now + timeout" by definition.
> 
> But which time is "now"? 
> 
> Example, thread1 has 10ms sum_exec_runtime and 4ms delta and call timer_settime(4ms).
> Old code calculate an expire is 10+4=14. New one calculate  10+4+4=18.
> 
> Which expire is correct? When using old one, timer will fire just after syscall. This
> is posix violation. 
> 
> In the other words,
> 
> 	sighandler(){
> 		t1 = clock_gettime()
> 	}
> 
> 	t0 = clock_gettime()
> 	timer_settime(timeout);
> 	 ... wait to fire
> 	
> 	assert (t1 - t0 >= timeout)
> 
> This pseudo code must be true. it is snippest what glibc rt/tst-cputimer1 test and failed.

In the other hands, following two calculations need to timer time (aka time without delta).

1) Initialization signal->cputimer for avoiding double delta count.
2) calculating old tiemr because timer firing logic (run_posix_cpu_timers) don't care delta_exec. 



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