linux-kernel - [PATCH v4 3/3] sched/core: Prevent wakeup of ksoftirqd during idle load balance

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Message-ID: <20241030071557.1422-4-kprateek.nayak@amd.com>
Date: Wed, 30 Oct 2024 07:15:57 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Sebastian Andrzej Siewior
	<bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, "Steven
 Rostedt" <rostedt@...dmis.org>, <linux-kernel@...r.kernel.org>,
	<linux-rt-devel@...ts.linux.dev>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall
	<bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Valentin Schneider
	<vschneid@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, Tejun Heo
	<tj@...nel.org>, Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.de>,
	Zqiang <qiang.zhang1211@...il.com>, Caleb Sander Mateos
	<csander@...estorage.com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
	Chen Yu <yu.c.chen@...el.com>, Julia Lawall <Julia.Lawall@...ia.fr>, "K
 Prateek Nayak" <kprateek.nayak@....com>, Julia Lawall <julia.lawall@...ia.fr>
Subject: [PATCH v4 3/3] sched/core: Prevent wakeup of ksoftirqd during idle load balance

Scheduler raises a SCHED_SOFTIRQ to trigger a load balancing event on
from the IPI handler on the idle CPU. Since the softirq can be raised
from flush_smp_call_function_queue(), it can end up waking up ksoftirqd,
which can give an illusion of the idle CPU being busy when doing an idle
load balancing.

Adding a trace_printk() in nohz_csd_func() at the spot of raising
SCHED_SOFTIRQ and enabling trace events for sched_switch, sched_wakeup,
and softirq_entry (for SCHED_SOFTIRQ vector alone) helps observing the
current behavior:

       <idle>-0   [000] dN.1.:  nohz_csd_func: Raising SCHED_SOFTIRQ from nohz_csd_func
       <idle>-0   [000] dN.4.:  sched_wakeup: comm=ksoftirqd/0 pid=16 prio=120 target_cpu=000
       <idle>-0   [000] .Ns1.:  softirq_entry: vec=7 [action=SCHED]
       <idle>-0   [000] .Ns1.:  softirq_exit: vec=7  [action=SCHED]
       <idle>-0   [000] d..2.:  sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=ksoftirqd/0 next_pid=16 next_prio=120
  ksoftirqd/0-16  [000] d..2.:  sched_switch: prev_comm=ksoftirqd/0 prev_pid=16 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
       ...

ksoftirqd is woken up before the idle thread calls
do_softirq_post_smp_call_flush() which can make the runqueue appear
busy and prevent the idle load balancer from pulling task from an
overloaded runqueue towards itself[1].

Since the softirq raised is guranteed to be serviced in irq_exit() or
via do_softirq_post_smp_call_flush(), set SCHED_SOFTIRQ without checking
the need to wakeup ksoftirq for idle load balancing.

Following are the observations with the changes when enabling the same
set of events:

       <idle>-0       [000] dN.1.: nohz_csd_func: Raising SCHED_SOFTIRQ for nohz_idle_balance
       <idle>-0       [000] dN.1.: softirq_raise: vec=7 [action=SCHED]
       <idle>-0       [000] .Ns1.: softirq_entry: vec=7 [action=SCHED]

No unnecessary ksoftirqd wakeups are seen from idle task's context to
service the softirq.

Fixes: b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()")
Reported-by: Julia Lawall <julia.lawall@...ia.fr>
Closes: https://lore.kernel.org/lkml/fcf823f-195e-6c9a-eac3-25f870cb35ac@inria.fr/ [1]
Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
v3..v4:

o New patch based on Sebastian's suggestion.
---
 kernel/sched/core.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aaf99c0bcb49..2ee3621d6e7e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1244,7 +1244,18 @@ static void nohz_csd_func(void *info)
 	rq->idle_balance = idle_cpu(cpu);
 	if (rq->idle_balance) {
 		rq->nohz_idle_balance = flags;
-		raise_softirq_irqoff(SCHED_SOFTIRQ);
+
+		/*
+		 * Don't wakeup ksoftirqd when raising SCHED_SOFTIRQ
+		 * since the idle load balancer may mistake wakeup of
+		 * ksoftirqd as a genuine task wakeup and bail out from
+		 * load balancing early. Since it is guaranteed that
+		 * pending softirqs will be handled soon, either on
+		 * irq_exit() or via do_softirq_post_smp_call_flush(),
+		 * raise SCHED_SOFTIRQ without checking the need to
+		 * wakeup ksoftirqd.
+		 */
+		__raise_softirq_irqoff(SCHED_SOFTIRQ);
 	}
 }
 
-- 
2.34.1