psi: Reduce calls to sched_clock() in psi

We noticed that the cost of psi increases with the increase in the
levels of the cgroups. Particularly the cost of cpu_clock() sticks out
as the kernel calls it multiple times as it traverses up the cgroup
tree. This patch reduces the calls to cpu_clock().

Performed perf bench on Intel Broadwell with 3 levels of cgroup.

Before the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.747 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.516 [sec]

       3.516689 usecs/op
         284358 ops/sec

After the patch:

$ perf bench sched all
 # Running sched/messaging benchmark...
 # 20 sender and receiver processes per group
 # 10 groups == 400 processes run

     Total time: 0.640 [sec]

 # Running sched/pipe benchmark...
 # Executed 1000000 pipe operations between two processes

     Total time: 3.329 [sec]

       3.329820 usecs/op
         300316 ops/sec
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20210321205156.4186483-1-shakeelb@google.com

kernel/sched/psi.c

@@ -644,12 +644,10 @@ static void poll_timer_fn(struct timer_list *t)
 	wake_up_interruptible(&group->poll_wait);
 }
 
-static void record_times(struct psi_group_cpu *groupc, int cpu)
+static void record_times(struct psi_group_cpu *groupc, u64 now)
 {
 	u32 delta;
-	u64 now;
 
-	now = cpu_clock(cpu);
 	delta = now - groupc->state_start;
 	groupc->state_start = now;
 
@@ -676,7 +674,7 @@ static void record_times(struct psi_group_cpu *groupc, int cpu)
 }
 
 static void psi_group_change(struct psi_group *group, int cpu,
-			     unsigned int clear, unsigned int set,
+			     unsigned int clear, unsigned int set, u64 now,
 			     bool wake_clock)
 {
 	struct psi_group_cpu *groupc;
@@ -696,7 +694,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
 	 */
 	write_seqcount_begin(&groupc->seq);
 
-	record_times(groupc, cpu);
+	record_times(groupc, now);
 
 	for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
 		if (!(m & (1 << t)))
@@ -788,12 +786,14 @@ void psi_task_change(struct task_struct *task, int clear, int set)
 	struct psi_group *group;
 	bool wake_clock = true;
 	void *iter = NULL;
+	u64 now;
 
 	if (!task->pid)
 		return;
 
 	psi_flags_change(task, clear, set);
 
+	now = cpu_clock(cpu);
 	/*
 	 * Periodic aggregation shuts off if there is a period of no
 	 * task changes, so we wake it back up if necessary. However,
@@ -806,7 +806,7 @@ void psi_task_change(struct task_struct *task, int clear, int set)
 		wake_clock = false;
 
 	while ((group = iterate_groups(task, &iter)))
-		psi_group_change(group, cpu, clear, set, wake_clock);
+		psi_group_change(group, cpu, clear, set, now, wake_clock);
 }
 
 void psi_task_switch(struct task_struct *prev, struct task_struct *next,
@@ -815,6 +815,7 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 	struct psi_group *group, *common = NULL;
 	int cpu = task_cpu(prev);
 	void *iter;
+	u64 now = cpu_clock(cpu);
 
 	if (next->pid) {
 		bool identical_state;
@@ -836,7 +837,7 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 				break;
 			}
 
-			psi_group_change(group, cpu, 0, TSK_ONCPU, true);
+			psi_group_change(group, cpu, 0, TSK_ONCPU, now, true);
 		}
 	}
 
@@ -858,7 +859,7 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 
 		iter = NULL;
 		while ((group = iterate_groups(prev, &iter)) && group != common)
-			psi_group_change(group, cpu, clear, set, true);
+			psi_group_change(group, cpu, clear, set, now, true);
 
 		/*
 		 * TSK_ONCPU is handled up to the common ancestor. If we're tasked
@@ -867,7 +868,7 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
 		if (sleep) {
 			clear &= ~TSK_ONCPU;
 			for (; group; group = iterate_groups(prev, &iter))
-				psi_group_change(group, cpu, clear, set, true);
+				psi_group_change(group, cpu, clear, set, now, true);
 		}
 	}
 }