[11/38] posix-cpu-timers: Cure SMP wobbles

Greg KH
2011-10-11T17:03:06Z

From: Peter Zijlstra <a.p.zijlstra@chello.nl>

commit d670ec13178d0fd8680e6742a2bc6e04f28f87d8 upstream.

David reported:

Attached below is a watered-down version of rt/tst-cpuclock2.c from
GLIBC. Just build it with "gcc -o test test.c -lpthread -lrt" or
similar.

Run it several times, and you will see cases where the main thread
will measure a process clock difference before and after the nanosleep
which is smaller than the cpu-burner thread's individual thread clock
difference. This doesn't make any sense since the cpu-burner thread
is part of the top-level process's thread group.

I've reproduced this on both x86-64 and sparc64 (using both 32-bit and
64-bit binaries).

For example:

[davem@boricha build-x86_64-linux]$ ./test
process: before(0.001221967) after(0.498624371) diff(497402404)
thread: before(0.000081692) after(0.498316431) diff(498234739)
self: before(0.001223521) after(0.001240219) diff(16698)
[davem@boricha build-x86_64-linux]$

The diff of 'process' should always be >= the diff of 'thread'.

I make sure to wrap the 'thread' clock measurements the most tightly
around the nanosleep() call, and that the 'process' clock measurements
are the outer-most ones.

---
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <pthread.h>

static pthread_barrier_t barrier;

static void *chew_cpu(void *arg)
{
pthread_barrier_wait(&barrier);
while (1)
__asm__ __volatile__("" : : : "memory");
return NULL;
}

int main(void)
{
clockid_t process_clock, my_thread_clock, th_clock;
struct timespec process_before, process_after;
struct timespec me_before, me_after;
struct timespec th_before, th_after;
struct timespec sleeptime;
unsigned long diff;
pthread_t th;
int err;

err = clock_getcpuclockid(0, &process_clock);
if (err)
return 1;

err = pthread_getcpuclockid(pthread_self(), &my_thread_clock);
if (err)
return 1;

pthread_barrier_init(&barrier, NULL, 2);
err = pthread_create(&th, NULL, chew_cpu, NULL);
if (err)
return 1;

err = pthread_getcpuclockid(th, &th_clock);
if (err)
return 1;

pthread_barrier_wait(&barrier);

err = clock_gettime(process_clock, &process_before);
if (err)
return 1;

err = clock_gettime(my_thread_clock, &me_before);
if (err)
return 1;

err = clock_gettime(th_clock, &th_before);
if (err)
return 1;

sleeptime.tv_sec = 0;
sleeptime.tv_nsec = 500000000;
nanosleep(&sleeptime, NULL);

err = clock_gettime(th_clock, &th_after);
if (err)
return 1;

err = clock_gettime(my_thread_clock, &me_after);
if (err)
return 1;

err = clock_gettime(process_clock, &process_after);
if (err)
return 1;

diff = process_after.tv_nsec - process_before.tv_nsec;
printf("process: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
process_before.tv_sec, process_before.tv_nsec,
process_after.tv_sec, process_after.tv_nsec, diff);
diff = th_after.tv_nsec - th_before.tv_nsec;
printf("thread: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
th_before.tv_sec, th_before.tv_nsec,
th_after.tv_sec, th_after.tv_nsec, diff);
diff = me_after.tv_nsec - me_before.tv_nsec;
printf("self: before(%lu.%.9lu) after(%lu.%.9lu) diff(%lu)\n",
me_before.tv_sec, me_before.tv_nsec,
me_after.tv_sec, me_after.tv_nsec, diff);

return 0;
}

This is due to us using p->se.sum_exec_runtime in
thread_group_cputime() where we iterate the thread group and sum all
data. This does not take time since the last schedule operation (tick
or otherwise) into account. We can cure this by using
task_sched_runtime() at the cost of having to take locks.

This also means we can (and must) do away with
thread_group_sched_runtime() since the modified thread_group_cputime()
is now more accurate and would deadlock when called from
thread_group_sched_runtime().

Aside of that it makes the function safe on 32 bit systems. The old
code added t->se.sum_exec_runtime unprotected. sum_exec_runtime is a
64bit value and could be changed on another cpu at the same time.

Reported-by: David Miller <davem@davemloft.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1314874459.7945.22.camel@twins
Tested-by: David Miller <davem@davemloft.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
include/linux/sched.h | 1 -
kernel/posix-cpu-timers.c | 5 +++--
kernel/sched.c | 24 ------------------------
3 files changed, 3 insertions(+), 27 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1937,7 +1937,6 @@ static inline void disable_sched_clock_i

extern unsigned long long
task_sched_runtime(struct task_struct *task);
-extern unsigned long long thread_group_sched_runtime(struct task_struct *task);

/* sched_exec is called by processes performing an exec */
#ifdef CONFIG_SMP
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -250,7 +250,7 @@ void thread_group_cputime(struct task_st
do {
times->utime = cputime_add(times->utime, t->utime);
times->stime = cputime_add(times->stime, t->stime);
- times->sum_exec_runtime += t->se.sum_exec_runtime;
+ times->sum_exec_runtime += task_sched_runtime(t);
} while_each_thread(tsk, t);
out:
rcu_read_unlock();
@@ -312,7 +312,8 @@ static int cpu_clock_sample_group(const
cpu->cpu = cputime.utime;
break;
case CPUCLOCK_SCHED:
- cpu->sched = thread_group_sched_runtime(p);
+ thread_group_cputime(p, &cputime);
+ cpu->sched = cputime.sum_exec_runtime;
break;
}
return 0;
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3713,30 +3713,6 @@ unsigned long long task_sched_runtime(st
}

/*
- * Return sum_exec_runtime for the thread group.
- * In case the task is currently running, return the sum plus current's
- * pending runtime that have not been accounted yet.
- *
- * Note that the thread group might have other running tasks as well,
- * so the return value not includes other pending runtime that other
- * running tasks might have.
- */
-unsigned long long thread_group_sched_runtime(struct task_struct *p)
-{
- struct task_cputime totals;
- unsigned long flags;
- struct rq *rq;
- u64 ns;
-
- rq = task_rq_lock(p, &flags);
- thread_group_cputime(p, &totals);
- ns = totals.sum_exec_runtime + do_task_delta_exec(p, rq);
- task_rq_unlock(rq, p, &flags);
-
- return ns;
-}
-
-/*
* Account user cpu time to a process.
* @p: the process that the cpu time gets accounted to
* @cputime: the cpu time spent in user space since the last update