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Date: Fri, 14 Oct 2022 19:05:51 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: hannes@...xchg.org, surenb@...gle.com, quic_pkondeti@...cinc.com
Cc: peterz@...radead.org, quic_charante@...cinc.com,
linux-kernel@...r.kernel.org,
Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH v2] sched/psi: Fix avgs_work re-arm in psi_avgs_work()
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@...cinc.com>
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
v2:
- need to check NR_IOWAIT and NR_MEMSTALL of the current CPU per Pavan.
- copy groupc->tasks[] out of the retry loop per Suren.
- use new flag PSI_STATE_RESCHEDULE instead of PSI_NONIDLE per Suren.
- test current_work() is avgs_work to only use PSI_STATE_RESCHEDULE
for re-arming avgs_work per Johannes.
---
include/linux/psi_types.h | 3 +++
kernel/sched/psi.c | 30 +++++++++++++++++++++++++++---
2 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index 6e4372735068..325981833587 100644
--- 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 ee2ecc081422..e347e06fdd68 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -242,6 +242,8 @@ 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;
@@ -256,6 +258,8 @@ 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 */
@@ -280,6 +284,28 @@ 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,
@@ -415,7 +441,6 @@ 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);
@@ -426,7 +451,6 @@ 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
@@ -437,7 +461,7 @@ 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);
}
--
2.37.2
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