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Commit 71a2224d authored by Christoph Lameter's avatar Christoph Lameter Committed by Linus Torvalds
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[PATCH] Optimize sys_times for a single thread process 



Avoid taking the tasklist_lock in sys_times if the process is single
threaded.  In a NUMA system taking the tasklist_lock may cause a bouncing
cacheline if multiple independent processes continually call sys_times to
measure their performance.

Signed-off-by: default avatarChristoph Lameter <christoph@lameter.com>
Signed-off-by: default avatarShai Fultheim <shai@scalex86.org>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent b030a4dd

kernel/exit.c

+5 −0
Original line number Diff line number Diff line
@@ -72,6 +72,11 @@ repeat: 
	BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));

	__exit_signal(p);

	__exit_sighand(p);

	/*

	 * Note that the fastpath in sys_times depends on __exit_signal having

	 * updated the counters before a task is removed from the tasklist of

	 * the process by __unhash_process.

	 */

	__unhash_process(p);



	/*

kernel/sys.c

+60 −26
Original line number Diff line number Diff line
@@ -894,9 +894,43 @@ asmlinkage long sys_times(struct tms __user * tbuf) 
	 */

	if (tbuf) {

		struct tms tmp;

		cputime_t utime, stime, cutime, cstime;



#ifdef CONFIG_SMP

		if (thread_group_empty(current)) {

			/*

			 * Single thread case without the use of any locks.

			 *

			 * We may race with release_task if two threads are

			 * executing. However, release task first adds up the

			 * counters (__exit_signal) before  removing the task

			 * from the process tasklist (__unhash_process).

			 * __exit_signal also acquires and releases the

			 * siglock which results in the proper memory ordering

			 * so that the list modifications are always visible

			 * after the counters have been updated.

			 *

			 * If the counters have been updated by the second thread

			 * but the thread has not yet been removed from the list

			 * then the other branch will be executing which will

			 * block on tasklist_lock until the exit handling of the

			 * other task is finished.

			 *

			 * This also implies that the sighand->siglock cannot

			 * be held by another processor. So we can also

			 * skip acquiring that lock.

			 */

			utime = cputime_add(current->signal->utime, current->utime);

			stime = cputime_add(current->signal->utime, current->stime);

			cutime = current->signal->cutime;

			cstime = current->signal->cstime;

		} else

#endif

		{



			/* Process with multiple threads */

			struct task_struct *tsk = current;

			struct task_struct *t;

		cputime_t utime, stime, cutime, cstime;



			read_lock(&tasklist_lock);

			utime = tsk->signal->utime;
@@ -922,7 +956,7 @@ asmlinkage long sys_times(struct tms __user * tbuf) 
			cstime = tsk->signal->cstime;

			spin_unlock_irq(&tsk->sighand->siglock);

			read_unlock(&tasklist_lock);



		}

		tmp.tms_utime = cputime_to_clock_t(utime);

		tmp.tms_stime = cputime_to_clock_t(stime);

		tmp.tms_cutime = cputime_to_clock_t(cutime);