From 1f2ea0837dbc263ce2a2512c4e73c83df68a6a55 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 16 Feb 2007 01:28:22 -0800 Subject: [PATCH] [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 Signed-off-by: Ingo Molnar Cc: Thomas Gleixner Cc: john stultz Cc: Roman Zippel Cc: Oleg Nesterov Signed-off-by: Paul E. McKenney Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/posix-cpu-timers.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c index 7c3e1e6dfb5b..657f77697415 100644 --- a/kernel/posix-cpu-timers.c +++ b/kernel/posix-cpu-timers.c @@ -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)) { @@ -312,12 +312,17 @@ 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, - p, &rtn); + } else { + read_lock(&tasklist_lock); + if (p->tgid == pid && p->signal) { + error = + cpu_clock_sample_group(which_clock, + p, &rtn); + } + read_unlock(&tasklist_lock); } } - read_unlock(&tasklist_lock); + rcu_read_unlock(); } if (error) -- 2.20.1