posix_cpu_timer: Check thread timers only when there are active thread timers

The fastpath_timer_check() contains logic to check for if any timers
are set by checking if !task_cputime_zero(). Similarly, we can do this
before calling check_thread_timers(). In the case where there
are only process-wide timers, this will skip all of the computations for
per-thread timers when there are no per-thread timers.

As suggested by George, we can put the task_cputime_zero() check in
check_thread_timers(), since that is more of an optization to the
function. Similarly, we move the existing check of cputimer->running
to check_process_timers().

Signed-off-by: Jason Low <jason.low2@hp.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: George Spelvin <linux@horizon.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: hideaki.kimura@hpe.com
Cc: terry.rudd@hpe.com
Cc: scott.norton@hpe.com
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1444849677-29330-3-git-send-email-jason.low2@hp.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index aa4b6f4..6f6e252 100644 (file)
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -864,6 +864,13 @@ static void check_thread_timers(struct task_struct *tsk,
        unsigned long long expires;
        unsigned long soft;
 
+       /*
+        * If cputime_expires is zero, then there are no active
+        * per thread CPU timers.
+        */
+       if (task_cputime_zero(&tsk->cputime_expires))
+               return;
+
        expires = check_timers_list(timers, firing, prof_ticks(tsk));
        tsk_expires->prof_exp = expires_to_cputime(expires);
 
@@ -961,6 +968,13 @@ static void check_process_timers(struct task_struct *tsk,
        struct task_cputime cputime;
        unsigned long soft;
 
+       /*
+        * If cputimer is not running, then there are no active
+        * process wide timers (POSIX 1.b, itimers, RLIMIT_CPU).
+        */
+       if (!READ_ONCE(tsk->signal->cputimer.running))
+               return;
+
        /*
         * Collect the current process totals.
         */
@@ -1169,12 +1183,8 @@ void run_posix_cpu_timers(struct task_struct *tsk)
         * put them on the firing list.
         */
        check_thread_timers(tsk, &firing);
-       /*
-        * If there are any active process wide timers (POSIX 1.b, itimers,
-        * RLIMIT_CPU) cputimer must be running.
-        */
-       if (READ_ONCE(tsk->signal->cputimer.running))
-               check_process_timers(tsk, &firing);
+
+       check_process_timers(tsk, &firing);
 
        /*
         * We must release these locks before taking any timer's lock.