sched/psi: Fix avgs_work re-arm in psi_avgs_work()

[ Upstream commit 2fcd7bbae90a6d844da8660a9d27079281dfbba2 ]

Pavan reported a problem that PSI avgs_work idle shutoff is not
working at all. Because PSI_NONIDLE condition would be observed in
psi_avgs_work()->collect_percpu_times()->get_recent_times() even if
only the kworker running avgs_work on the CPU.

Although commit 1b69ac6b40eb ("psi: fix aggregation idle shut-off")
avoided the ping-pong wake problem when the worker sleep, psi_avgs_work()
still will always re-arm the avgs_work, so shutoff is not working.

This patch changes to use PSI_STATE_RESCHEDULE to flag whether to
re-arm avgs_work in get_recent_times(). For the current CPU, we re-arm
avgs_work only when (NR_RUNNING > 1 || NR_IOWAIT > 0 || NR_MEMSTALL > 0),
for other CPUs we can just check PSI_NONIDLE delta. The new flag
is only used in psi_avgs_work(), so we check in get_recent_times()
that current_work() is avgs_work.

One potential problem is that the brief period of non-idle time
incurred between the aggregation run and the kworker's dequeue will
be stranded in the per-cpu buckets until avgs_work run next time.
The buckets can hold 4s worth of time, and future activity will wake
the avgs_work with a 2s delay, giving us 2s worth of data we can leave
behind when shut off the avgs_work. If the kworker run other works after
avgs_work shut off and doesn't have any scheduler activities for 2s,
this maybe a problem.

Reported-by: Pavan Kondeti <quic_pkondeti@quicinc.com>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Link: https://lore.kernel.org/r/20221014110551.22695-1-zhouchengming@bytedance.com
Stable-dep-of: aff037078eca ("sched/psi: use kernfs polling functions for PSI trigger polling")
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index 14a1ebb..1e0a0d7 100644 (file)
--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -72,6 +72,9 @@ enum psi_states {
 /* Use one bit in the state mask to track TSK_ONCPU */
 #define PSI_ONCPU      (1 << NR_PSI_STATES)
 
+/* Flag whether to re-arm avgs_work, see details in get_recent_times() */
+#define PSI_STATE_RESCHEDULE   (1 << (NR_PSI_STATES + 1))
+
 enum psi_aggregators {
        PSI_AVGS = 0,
        PSI_POLL,

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index e83c321..02e011c 100644 (file)
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -243,6 +243,8 @@ static void get_recent_times(struct psi_group *group, int cpu,
                             u32 *pchanged_states)
 {
        struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
+       int current_cpu = raw_smp_processor_id();
+       unsigned int tasks[NR_PSI_TASK_COUNTS];
        u64 now, state_start;
        enum psi_states s;
        unsigned int seq;
@@ -257,6 +259,8 @@ static void get_recent_times(struct psi_group *group, int cpu,
                memcpy(times, groupc->times, sizeof(groupc->times));
                state_mask = groupc->state_mask;
                state_start = groupc->state_start;
+               if (cpu == current_cpu)
+                       memcpy(tasks, groupc->tasks, sizeof(groupc->tasks));
        } while (read_seqcount_retry(&groupc->seq, seq));
 
        /* Calculate state time deltas against the previous snapshot */
@@ -281,6 +285,28 @@ static void get_recent_times(struct psi_group *group, int cpu,
                if (delta)
                        *pchanged_states |= (1 << s);
        }
+
+       /*
+        * When collect_percpu_times() from the avgs_work, we don't want to
+        * re-arm avgs_work when all CPUs are IDLE. But the current CPU running
+        * this avgs_work is never IDLE, cause avgs_work can't be shut off.
+        * So for the current CPU, we need to re-arm avgs_work only when
+        * (NR_RUNNING > 1 || NR_IOWAIT > 0 || NR_MEMSTALL > 0), for other CPUs
+        * we can just check PSI_NONIDLE delta.
+        */
+       if (current_work() == &group->avgs_work.work) {
+               bool reschedule;
+
+               if (cpu == current_cpu)
+                       reschedule = tasks[NR_RUNNING] +
+                                    tasks[NR_IOWAIT] +
+                                    tasks[NR_MEMSTALL] > 1;
+               else
+                       reschedule = *pchanged_states & (1 << PSI_NONIDLE);
+
+               if (reschedule)
+                       *pchanged_states |= PSI_STATE_RESCHEDULE;
+       }
 }
 
 static void calc_avgs(unsigned long avg[3], int missed_periods,
@@ -416,7 +442,6 @@ static void psi_avgs_work(struct work_struct *work)
        struct delayed_work *dwork;
        struct psi_group *group;
        u32 changed_states;
-       bool nonidle;
        u64 now;
 
        dwork = to_delayed_work(work);
@@ -427,7 +452,6 @@ static void psi_avgs_work(struct work_struct *work)
        now = sched_clock();
 
        collect_percpu_times(group, PSI_AVGS, &changed_states);
-       nonidle = changed_states & (1 << PSI_NONIDLE);
        /*
         * If there is task activity, periodically fold the per-cpu
         * times and feed samples into the running averages. If things
@@ -438,7 +462,7 @@ static void psi_avgs_work(struct work_struct *work)
        if (now >= group->avg_next_update)
                group->avg_next_update = update_averages(group, now);
 
-       if (nonidle) {
+       if (changed_states & PSI_STATE_RESCHEDULE) {
                schedule_delayed_work(dwork, nsecs_to_jiffies(
                                group->avg_next_update - now) + 1);
        }