linux-kernel - Re: sched_yield() on 2.6.25

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Message-Id: <20080609023518.4a7d9868.akpm@linux-foundation.org>
Date:	Mon, 9 Jun 2008 02:35:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Jakub W. Jozwicki" <jozwicki@...er.pl>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: sched_yield() on 2.6.25

On Sun, 8 Jun 2008 13:34:10 +0200 "Jakub W. Jozwicki" <jozwicki@...er.pl> wrote:

> Hello,
> I observe strange behavior of sched_yield() on 2.6.25 (strange comparing to 
> 2.6.24). Here is the code (available at 
> http://systest.googlecode.com/files/systest20080119.tgz):
> 
> ------------------------------------------------------
> timer_t timer;
> sig_atomic_t cnt = 0;
> long long sum = 0;
> long times[21], min, max;
> pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
> struct timespec ts = { 0, 0 };
> pthread_t last_th = 0;
> 
> void *th_proc(void* p) {
>     int n = SIZE(times) -1;
>     pthread_t th;
>         
>     while(1) {    
> 	pthread_mutex_lock(&mutex);
> 	th = pthread_self();
> 	if (pthread_equal(th,last_th)) {
> 	    pthread_mutex_unlock(&mutex);
> 	    sched_yield();
> 	    continue;
> 	}
> 	rt_timer_stop(&ts);
> 	last_th = th;
> 	if (cnt>=1) {
> 	    times[cnt-1] = ts_sum(&ts);
> 	    if (cnt <= n) {
> 		sum += times[cnt-1];
> 		box(times[cnt-1],min,max);
> 		#define uint unsigned int
> 		printf("[%u] Thread switching time: %ldns\n",(uint)th, times[cnt-1]);
> 	    }
> 	    else {
> 		printf("[%u] Thread switching time (not counted): %ldns\n",(uint)th, 
> times[cnt-1]);
> 	    }
> 	    cnt--;
> 	}
>         ....
> -----------------------------------------------------
> and here are the results:
> 
> Setting cpu mask to 1
> 
> -- SYSTEM INFO -------------------
> 
> localhost: Linux 2.6.24-rt4 #8 SMP PREEMPT RT Mon Jan 21 18:45:00 CET 2008
> 
> Setting priority SCHED_OTHER to 0 (normal process) for 20802
> [3084102544] Thread switching time (not counted): 10709015ns
> [3075709840] Thread switching time: 35468301ns
> [3084102544] Thread switching time: 2793ns
> [3075709840] Thread switching time 30725ns
> [3084102544] Thread switching time: 10405ns
> [3075709840] Thread switching time: 2724ns
> [3084102544] Thread switching time: 2654ns
> [3075709840] Thread switching time: 2653ns
> [3084102544] Thread switching time: 3352ns
> [3075709840] Thread switching time: 2583ns
> [3084102544] Thread switching time: 2514ns
> [3075709840] Thread switching time: 2514ns

It would save us some time if you were to tell us what these results
mean, please.

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