Commit 1f2ea083 authored Feb 16, 2007 by Paul E. McKenney Committed by Linus Torvalds Feb 16, 2007

[PATCH] posix timers: RCU optimization for clock_gettime()



Use RCU to avoid the need to acquire tasklist_lock in the single-threaded
case of clock_gettime().  It still acquires tasklist_lock when for a
(potentially multithreaded) process.  This change allows realtime
applications to frequently monitor CPU consumption of individual tasks, as
requested (and now deployed) by some off-list users.

This has been in Ingo Molnar's -rt patchset since late 2005 with no
problems reported, and tests successfully on 2.6.20-rc6, so I believe that
it is long-since ready for mainline adoption.

[paulmck@linux.vnet.ibm.com: fix exit()/posix_cpu_clock_get() race spotted by Oleg]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 7460ed28

kernel/posix-cpu-timers.c

+10 −5

Original line number	Original line	Diff line number	Diff line
	@@ -304,7 +304,7 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
	* should be able to see it.		* should be able to see it.
	*/		*/
	struct task_struct *p;		struct task_struct *p;
	read_lock(&tasklist_lock);		rcu_read_lock();
	p = find_task_by_pid(pid);		p = find_task_by_pid(pid);
	if (p) {		if (p) {
	if (CPUCLOCK_PERTHREAD(which_clock)) {		if (CPUCLOCK_PERTHREAD(which_clock)) {
	@@ -312,13 +312,18 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
	error = cpu_clock_sample(which_clock,		error = cpu_clock_sample(which_clock,
	p, &rtn);		p, &rtn);
	}		}
	} else if (p->tgid == pid && p->signal) {		} else {
	error = cpu_clock_sample_group(which_clock,		read_lock(&tasklist_lock);
			if (p->tgid == pid && p->signal) {
			error =
			cpu_clock_sample_group(which_clock,
	p, &rtn);		p, &rtn);
	}		}
	}
	read_unlock(&tasklist_lock);		read_unlock(&tasklist_lock);
	}		}
			}
			rcu_read_unlock();
			}

	if (error)		if (error)
	return error;		return error;