[PATCH v2 2/4] timer: Check thread timers only when there are active thread timers

[Jason Low]

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@xxxxxx>
Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/time/posix-cpu-timers.c |   22 ++++++++++++++++------
 1 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index aa4b6f4..6f6e252 100644
--- 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);
 
@@ -962,6 +969,13 @@ static void check_process_timers(struct task_struct *tsk,
 	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.
 	 */
 	thread_group_cputimer(tsk, &cputime);
@@ -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.
-- 
1.7.2.5

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