Commit-ID: 87ff19cb2f1aa55a5d8b691e6690cc059a59d2ec Gitweb: https://git.kernel.org/tip/87ff19cb2f1aa55a5d8b691e6690cc059a59d2ec Author: Davidlohr Bueso <dave@xxxxxxxxxxxx> AuthorDate: Sun, 2 Dec 2018 21:31:30 -0800 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Mon, 21 Jan 2019 11:18:50 +0100 sched/wake_q: Add branch prediction hint to wake_q_add() cmpxchg The cmpxchg() will fail when the task is already in the process of waking up, and as such is an extremely rare occurrence. Micro-optimize the call and put an unlikely() around it. To no surprise, when using CONFIG_PROFILE_ANNOTATED_BRANCHES under a number of workloads the incorrect rate was a mere 1-2%. Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Acked-by: Waiman Long <longman@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Will Deacon <will.deacon@xxxxxxx> Cc: Yongji Xie <elohimes@xxxxxxxxx> Cc: andrea.parri@xxxxxxxxxxxxxxxxxxxx Cc: lilin24@xxxxxxxxx Cc: liuqi16@xxxxxxxxx Cc: nixun@xxxxxxxxx Cc: xieyongji@xxxxxxxxx Cc: yuanlinsi01@xxxxxxxxx Cc: zhangyu31@xxxxxxxxx Link: https://lkml.kernel.org/r/20181203053130.gwkw6kg72azt2npb@linux-r8p5 Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/sched/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d8d76a65cfdd..b05eef7d7a1f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -421,7 +421,7 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task) * state, even in the failed case, an explicit smp_mb() must be used. */ smp_mb__before_atomic(); - if (cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)) + if (unlikely(cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL))) return; get_task_struct(task);
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