linux-kernel - [PATCH] sched/core: Fix arch_scale_freq

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Message-Id: <20221130125121.34407-1-ypodemsk@redhat.com>
Date:   Wed, 30 Nov 2022 14:51:21 +0200
From:   Yair Podemsky <ypodemsk@...hat.com>
To:     peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        rafael.j.wysocki@...el.com, ggherdovich@...e.cz, jlelli@...hat.com,
        mtosatti@...hat.com, nsaenz@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     ypodemsk@...hat.com
Subject: [PATCH] sched/core: Fix arch_scale_freq_tick() on tickless systems

In order for the scheduler to be frequency invariant we measure the
ratio between the maximum cpu frequency and the actual cpu frequency.
During long tickless periods of time the calculations that keep track
of that might overflow, in the function scale_freq_tick():

if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
»       goto error;

eventually forcing the kernel to disable the feature for all cpus,
and show the warning message
"Scheduler frequency invariance went wobbly, disabling!".
Let's avoid that by limiting the frequency invariant calculations
to cpus with regular tick.

Fixes: e2b0d619b400 ("x86, sched: check for counters overflow in frequency invariant accounting")
Signed-off-by: Yair Podemsky <ypodemsk@...hat.com>
Suggested-by: "Peter Zijlstra (Intel)" <peterz@...radead.org>
---
V1 -> V2: solution approach was changed from detecting long tickless periods
to frequency invariant measurements on housekeeping cpus only.
Link: ee89073a1e9de11c7bd7726eb5da71a0e8795099.camel@...hat.com
---
 kernel/sched/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index daff72f00385..1bb0a840c817 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5469,7 +5469,9 @@ void scheduler_tick(void)
 	unsigned long thermal_pressure;
 	u64 resched_latency;
 
-	arch_scale_freq_tick();
+	if (housekeeping_cpu(cpu, HK_TYPE_TICK))
+		arch_scale_freq_tick();
+
 	sched_clock_tick();
 
 	rq_lock(rq, &rf);
-- 
2.31.1