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	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.
		*/
		struct task_struct *p;
		read_lock(&tasklist_lock);
		rcu_read_lock();
		p = find_task_by_pid(pid);
		if (p) {
		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,
		p, &rtn);
		}
		} else if (p->tgid == pid && p->signal) {
		error = cpu_clock_sample_group(which_clock,
		} else {
		read_lock(&tasklist_lock);
		if (p->tgid == pid && p->signal) {
		error =
		cpu_clock_sample_group(which_clock,
		p, &rtn);
		}
		}
		read_unlock(&tasklist_lock);
		}
		}
		rcu_read_unlock();
		}

		if (error)
		return error;

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