[tip: sched/core] sched: Cache task_struct::flags in sched_submit_work()

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     c1cecf884ad748f63f9139d5a18ee265ee2f70fb
Gitweb:        https://git.kernel.org/tip/c1cecf884ad748f63f9139d5a18ee265ee2f70fb
Author:        Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate:    Wed, 19 Aug 2020 22:00:25 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 26 Aug 2020 12:41:58 +02:00

sched: Cache task_struct::flags in sched_submit_work()

sched_submit_work() is considered to be a hot path. The preempt_disable()
instruction is a compiler barrier and forces the compiler to load
task_struct::flags for the second comparison.
By using a local variable, the compiler can load the value once and keep it in
a register for the second comparison.

Verified on x86-64 with gcc-10.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200819200025.lqvmyefqnbok5i4f@xxxxxxxxxxxxx
---
 kernel/sched/core.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8471a0f..c36dc1a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4551,9 +4551,12 @@ void __noreturn do_task_dead(void)
 
 static inline void sched_submit_work(struct task_struct *tsk)
 {
+	unsigned int task_flags;
+
 	if (!tsk->state)
 		return;
 
+	task_flags = tsk->flags;
 	/*
 	 * If a worker went to sleep, notify and ask workqueue whether
 	 * it wants to wake up a task to maintain concurrency.
@@ -4562,9 +4565,9 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * in the possible wakeup of a kworker and because wq_worker_sleeping()
 	 * requires it.
 	 */
-	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+	if (task_flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
 		preempt_disable();
-		if (tsk->flags & PF_WQ_WORKER)
+		if (task_flags & PF_WQ_WORKER)
 			wq_worker_sleeping(tsk);
 		else
 			io_wq_worker_sleeping(tsk);