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Message-ID: <1385745340.1073.17.camel@Wailaba2>
Date: Fri, 29 Nov 2013 12:15:40 -0500
From: Olivier Langlois <olivier@...vierlanglois.net>
To: tglx@...utronix.de
Cc: linux-kernel@...r.kernel.org, fweisbec@...il.com,
kosaki.motohiro@...il.com
Subject: Re: [PATCH] Avoid process cputimer state oscillation
One more detail useful to note to appreciate this patch is that the call
to posix_cpu_timer_schedule() from cpu_timer_fire() is done
exceptionnaly when the timer signal generation fails.
The normal processing will call posix_cpu_timer_schedule() from a
different context in do_schedule_next_timer() (posix-timers.c)
I believe this misunderstanding may partly explain the current state of
the code.
On Mon, 2013-11-25 at 14:25 -0500, Olivier Langlois wrote:
> When a periodic process timer is fired, a signal is generated.
> Rearming the timer, if necessary, will be performed in a second step
> when the signal will be delivered.
>
> Hence, checking the list of process timers list to decide to stop to cpu
> timer right after generating the signal is premature and is leading the
> cputimer to oscillate between the on/off states when a single process timer
> is present.
>
> The following posix-cpu-timers module intrumentation
>
> In void thread_group_cputimer(struct task_s
> raw_spin_lock_irqsave(&cputimer->lock, flags);
> cputimer->running = 1;
> update_gt_cputime(&cputimer->cputime, &sum);
> + printk(KERN_DEBUG "cputimer started\n");
> } else
> raw_spin_lock_irqsave(&cputimer->lock, flags);
> *times = cputimer->cputime;
> and in static void stop_process_timers(struct s
>
> raw_spin_lock_irqsave(&cputimer->lock, flags);
> cputimer->running = 0;
> + printk(KERN_DEBUG "cputimer stopped\n");
> raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> }
>
> with the small program:
>
> \#include <signal.h>
> \#include <time.h>
> \#include <stddef.h>
> \#include <stdio.h>
> \#include <string.h>
> \#include <fcntl.h>
> \#include <unistd.h>
>
> static void chew_cpu()
> {
> static volatile char buf[4096];
> for (size_t i = 0; i < 100; ++i)
> for (size_t j = 0; j < sizeof buf; ++j)
> buf[j] = 0xaa;
> int nullfd = open("/dev/null", O_WRONLY);
> for (size_t i = 0; i < 100; ++ i)
> for (size_t j = 0; j < sizeof buf; ++j)
> buf[j] = 0xbb;
> write(nullfd, (char*)buf, sizeof buf);
> close(nullfd);
> }
>
> timer_t t;
> volatile int sig_cnt;
>
> static void
> sig_handler (int sig, siginfo_t *info, void *ctx)
> {
> if (sig_cnt >= 5) {
> struct itimerspec it = {};
> timer_settime(t, 0, &it, NULL);
> }
> ++sig_cnt;
> }
>
> int main( int argc, char *argv[] )
> {
> clockid_t my_process_clock;
> int e;
>
> struct sigaction sa = { .sa_sigaction = sig_handler,
> .sa_flags = SA_SIGINFO };
> sigemptyset(&sa.sa_mask);
> sigaction(SIGRTMIN, &sa, NULL);
>
> struct sigevent ev;
> memset( &ev, 0, sizeof (ev));
> ev.sigev_notify = SIGEV_SIGNAL;
> ev.sigev_signo = SIGRTMIN;
> ev.sigev_value.sival_ptr = &ev;
>
> e = clock_getcpuclockid( 0, &my_process_clock);
> if (e) {
> printf("clock_getcpuclockid: (%d) %s\n", e, strerror(e));
> return 1;
> }
> if (timer_create(my_process_clock, &ev, &t) != 0) {
> printf("timer_create: %m\n");
> return 1;
> }
>
> struct itimerspec it;
> it.it_value.tv_sec = 0;
> it.it_value.tv_nsec = 100000;
> it.it_interval.tv_sec = 0;
> it.it_interval.tv_nsec = 100000;
>
> timer_settime(t,0,&it,NULL);
>
> while (sig_cnt < 5)
> chew_cpu();
>
> // process timer should stop
> struct timespec req = { .tv_sec = 0, .tv_nsec = 400000 };
> nanosleep(&req,NULL);
>
> // restart timer.
> sig_cnt = 0;
> timer_settime(t,0,&it,NULL);
> while (sig_cnt < 5)
> chew_cpu();
>
> timer_delete(t);
>
> return 0;
> }
>
> demonstrate the issue. Here are the results before and after applying the patch:
>
> Before:
>
> [ 181.904571] cputimer started
> [ 181.905013] cputimer stopped
> [ 181.905027] cputimer started
> [ 181.906009] cputimer stopped
> [ 181.906016] cputimer started
> [ 181.907008] cputimer stopped
> [ 181.907014] cputimer started
> [ 181.908008] cputimer stopped
> [ 181.908013] cputimer started
> [ 181.909008] cputimer stopped
> [ 181.909020] cputimer started
> [ 181.910007] cputimer stopped
> [ 181.910050] cputimer started
> [ 181.911011] cputimer stopped
> [ 181.913619] cputimer started
> [ 181.914007] cputimer stopped
> [ 181.914015] cputimer started
> [ 181.915006] cputimer stopped
> [ 181.915011] cputimer started
> [ 181.916006] cputimer stopped
> [ 181.916010] cputimer started
> [ 181.917008] cputimer stopped
> [ 181.917058] cputimer started
> [ 181.918009] cputimer stopped
> [ 181.918029] cputimer started
> [ 181.919006] cputimer stopped
> [ 181.919010] cputimer started
> [ 181.920006] cputimer stopped
>
> After:
>
> [ 859.722119] cputimer started
> [ 859.730004] cputimer stopped
> [ 859.731354] cputimer started
> [ 859.739004] cputimer stopped
>
> Signed-off-by: Olivier Langlois <olivier@...llion01.com>
> ---
> kernel/posix-cpu-timers.c | 34 +++++++++++++++-------------------
> 1 file changed, 15 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index c7f31aa..972f5cb 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/posix-cpu-timers.c
> @@ -1049,8 +1049,6 @@ static void check_process_timers(struct task_struct *tsk,
> sig->cputime_expires.prof_exp = expires_to_cputime(prof_expires);
> sig->cputime_expires.virt_exp = expires_to_cputime(virt_expires);
> sig->cputime_expires.sched_exp = sched_expires;
> - if (task_cputime_zero(&sig->cputime_expires))
> - stop_process_timers(sig);
> }
>
> /*
> @@ -1158,34 +1156,32 @@ static inline int task_cputime_expired(const struct task_cputime *sample,
> */
> static inline int fastpath_timer_check(struct task_struct *tsk)
> {
> - struct signal_struct *sig;
> - cputime_t utime, stime;
> + struct signal_struct *sig = tsk->signal;
>
> - task_cputime(tsk, &utime, &stime);
> + if (sig->cputimer.running) {
> + if (likely(!task_cputime_zero(&sig->cputime_expires))) {
> + struct task_cputime group_sample;
> +
> + raw_spin_lock(&sig->cputimer.lock);
> + group_sample = sig->cputimer.cputime;
> + raw_spin_unlock(&sig->cputimer.lock);
> +
> + if (task_cputime_expired(&group_sample, &sig->cputime_expires))
> + return 1;
> + } else
> + stop_process_timers(sig);
> + }
>
> if (!task_cputime_zero(&tsk->cputime_expires)) {
> struct task_cputime task_sample = {
> - .utime = utime,
> - .stime = stime,
> .sum_exec_runtime = tsk->se.sum_exec_runtime
> };
> + task_cputime(tsk, &task_sample.utime, &task_sample.stime);
>
> if (task_cputime_expired(&task_sample, &tsk->cputime_expires))
> return 1;
> }
>
> - sig = tsk->signal;
> - if (sig->cputimer.running) {
> - struct task_cputime group_sample;
> -
> - raw_spin_lock(&sig->cputimer.lock);
> - group_sample = sig->cputimer.cputime;
> - raw_spin_unlock(&sig->cputimer.lock);
> -
> - if (task_cputime_expired(&group_sample, &sig->cputime_expires))
> - return 1;
> - }
> -
> return 0;
> }
>
--
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