Re: PSI idle-shutoff

From: Hillf Danton <hdanton@sina.com>
To: Pavan Kondeti <quic_pkondeti@quicinc.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Suren Baghdasaryan <surenb@google.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	quic_charante@quicinc.com
Subject: Re: PSI idle-shutoff
Date: Mon, 10 Oct 2022 18:57:10 +0800	[thread overview]
Message-ID: <20221010105710.171-1-hdanton@sina.com> (raw)
In-Reply-To: <20220913140817.GA9091@hu-pkondeti-hyd.qualcomm.com>

On 13 Sep 2022 19:38:17 +0530 Pavan Kondeti <quic_pkondeti@quicinc.com>
> Hi
> 
> The fact that psi_avgs_work()->collect_percpu_times()->get_recent_times()
> run from a kworker thread, PSI_NONIDLE condition would be observed as
> there is a RUNNING task. So we would always end up re-arming the work.
> 
> If the work is re-armed from the psi_avgs_work() it self, the backing off
> logic in psi_task_change() (will be moved to psi_task_switch soon) can't
> help. The work is already scheduled. so we don't do anything there.
> 
> Probably I am missing some thing here. Can you please clarify how we
> shut off re-arming the psi avg work?

Instead of open coding schedule_delayed_work() in bid to check if timer
hits the idle task (see delayed_work_timer_fn()), the idle task is tracked
in psi_task_switch() and checked by kworker to see if it preempted the idle
task.

Only for thoughts now.

Hillf

+++ b/kernel/sched/psi.c
@@ -412,6 +412,8 @@ static u64 update_averages(struct psi_gr
 	return avg_next_update;
 }
 
+static DEFINE_PER_CPU(int, prev_task_is_idle);
+
 static void psi_avgs_work(struct work_struct *work)
 {
 	struct delayed_work *dwork;
@@ -439,7 +441,7 @@ static void psi_avgs_work(struct work_st
 	if (now >= group->avg_next_update)
 		group->avg_next_update = update_averages(group, now);
 
-	if (nonidle) {
+	if (nonidle && 0 == per_cpu(prev_task_is_idle, raw_smp_processor_id())) {
 		schedule_delayed_work(dwork, nsecs_to_jiffies(
 				group->avg_next_update - now) + 1);
 	}
@@ -859,6 +861,7 @@ void psi_task_switch(struct task_struct
 	if (prev->pid) {
 		int clear = TSK_ONCPU, set = 0;
 
+		per_cpu(prev_task_is_idle, cpu) = 0;
 		/*
 		 * When we're going to sleep, psi_dequeue() lets us
 		 * handle TSK_RUNNING, TSK_MEMSTALL_RUNNING and
@@ -888,7 +891,8 @@ void psi_task_switch(struct task_struct
 			for (; group; group = iterate_groups(prev, &iter))
 				psi_group_change(group, cpu, clear, set, now, true);
 		}
-	}
+	} else
+		per_cpu(prev_task_is_idle, cpu) = 1;
 }
 
 /**

