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From:   Chengming Zhou <zhouchengming@bytedance.com>
To:     hannes@cmpxchg.org, surenb@google.com, quic_pkondeti@quicinc.com
Cc:     peterz@infradead.org, quic_charante@quicinc.com,
        linux-kernel@vger.kernel.org,
        Chengming Zhou <zhouchengming@bytedance.com>
Subject: [PATCH] sched/psi: Fix avgs_work re-arm in psi_avgs_work()
Date:   Mon, 10 Oct 2022 18:42:06 +0800
Message-Id: <20221010104206.12184-1-zhouchengming@bytedance.com>
In-Reply-To: <20220913140817.GA9091@hu-pkondeti-hyd.qualcomm.com>
References: <20220913140817.GA9091@hu-pkondeti-hyd.qualcomm.com>
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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 consider current CPU groupc as IDLE if the
kworker running avgs_work is the only task running and no IOWAIT
or MEMSTALL sleep tasks, in which case we will shut off the avgs_work
if other CPUs' groupc are also IDLE.

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>
---
 kernel/sched/psi.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index ee2ecc081422..f4cdf6f184ba 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();
+	bool only_avgs_work = false;
 	u64 now, state_start;
 	enum psi_states s;
 	unsigned int seq;
@@ -256,6 +258,15 @@ 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;
+		/*
+		 * This CPU has only avgs_work kworker running, snapshot the
+		 * newest times then don't need to re-arm for this groupc.
+		 * Normally this kworker will sleep soon and won't wake
+		 * avgs_work back up in psi_group_change().
+		 */
+		if (current_cpu == cpu && groupc->tasks[NR_RUNNING] == 1 &&
+		    !groupc->tasks[NR_IOWAIT] && !groupc->tasks[NR_MEMSTALL])
+			only_avgs_work = true;
 	} while (read_seqcount_retry(&groupc->seq, seq));
 
 	/* Calculate state time deltas against the previous snapshot */
@@ -280,6 +291,10 @@ static void get_recent_times(struct psi_group *group, int cpu,
 		if (delta)
 			*pchanged_states |= (1 << s);
 	}
+
+	/* Clear PSI_NONIDLE so avgs_work won't be re-armed for this groupc */
+	if (only_avgs_work)
+		*pchanged_states &= ~(1 << PSI_NONIDLE);
 }
 
 static void calc_avgs(unsigned long avg[3], int missed_periods,
-- 
2.37.2