[PATCH] psi: fix randomized calculation in record_times()

[Yafang Shao <laoar.shao@gmail.com>]

In record_times() we use 'now' and groupc->state_start to calculate the
delta as bellow,
	delta = now - groupc->state_start;
But note that groupc->state_start may be not initialized yet, IOW, the
state_start may be 0 currently. If state_start is 0, this calculation is
same with assigning the lower 32-bit of 'now' to delta, that is a random
value. To fix this value, we should initialize groupc->state_start before.

After we calculate the delta, we will assign 'now' to
groupc->state_start then,
	groupc->state_start = now;
This will cause the same issue if groupc->state_start will not be used in a
long period. Let's take an example. We create a cgroup foo and run tasks
in it. Some of these tasks enter into memstall and state_start is set.
Then we move all of these tasks out of cgroup foo for more than (1 << 32)
nsecs, and then move them in. That will cause the same issue as above.

The root cause of these issues is that we don't initialize the state_start
properly. To fix it, we should record how many tasks in this per cpu
psi_group. If there's no task in it, we just set state_start and don't
calculate the delta, that means it is the begin of the pressure. To
avoid redundant calculating the total number of tasks in this per cpu
psi_group, a new member 'total_tasks' is introduced in struct
psi_group_cpu, which is the sum of array members in tasks[].

Fixes: eb414681d5a0 ("psi: pressure stall information for CPU, memory, and IO")
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Daniel Drake <drake@endlessm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 include/linux/psi_types.h |  2 ++
 kernel/sched/psi.c        | 13 ++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index 4b7258495a04..b42cbfdb15e9 100644
--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -69,6 +69,8 @@ struct psi_group_cpu {
 
 	/* States of the tasks belonging to this group */
 	unsigned int tasks[NR_PSI_TASK_COUNTS];
+	/* Sum of above array members */
+	unsigned int total_tasks;
 
 	/* Aggregate pressure state derived from the tasks */
 	u32 state_mask;
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 8f45cdb6463b..7061529dc406 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -690,7 +690,10 @@ static void psi_group_change(struct psi_group *group, int cpu,
 	 */
 	write_seqcount_begin(&groupc->seq);
 
-	record_times(groupc, cpu, false);
+	if (groupc->total_tasks)
+		record_times(groupc, cpu, false);
+	else
+		groupc->state_start = cpu_clock(cpu);
 
 	for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
 		if (!(m & (1 << t)))
@@ -703,11 +706,15 @@ static void psi_group_change(struct psi_group *group, int cpu,
 			psi_bug = 1;
 		}
 		groupc->tasks[t]--;
+		groupc->total_tasks--;
 	}
 
-	for (t = 0; set; set &= ~(1 << t), t++)
-		if (set & (1 << t))
+	for (t = 0; set; set &= ~(1 << t), t++) {
+		if (set & (1 << t)) {
 			groupc->tasks[t]++;
+			groupc->total_tasks++;
+		}
+	}
 
 	/* Calculate state mask representing active states */
 	for (s = 0; s < NR_PSI_STATES; s++) {
-- 
2.18.2