linux-kernel - Re: Stopping the tick on a fully loaded system

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Message-ID: <20230726200949.GA3869356@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Jul 2023 22:09:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Gautham R. Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: Stopping the tick on a fully loaded system

On Wed, Jul 26, 2023 at 08:30:01PM +0200, Rafael J. Wysocki wrote:

> > - The governors teo and menu do the tick_nohz_next_event() check even if
> >   the CPU is fully loaded and but the check is not for free.
> 
> Let me have a loot at teo in that respect.
> 
> The problem is when tick_nohz_get_sleep_length() should not be called.
> The easy case is when the governor would select the shallowest idle
> state without taking it into account, but what about the deeper ones?
> I guess this depends on the exit latency of the current candidate idle
> state, but what exit latency would be low enough?  I guess 2 us would
> be fine, but what about 10 us, or even 20 us for that matter?

The patch I send here:

  https://lkml.kernel.org/r/20230726164958.GV38236@hirez.programming.kicks-ass.net

(which was stuck in a mailqueue :/) tries to address that.

Additionally, I think we can do something like this on top of all that,
stop going deeper when 66% of wakeups is at or below the current state.


--- a/drivers/cpuidle/governors/teo.c
+++ b/drivers/cpuidle/governors/teo.c
@@ -362,6 +362,7 @@ static int teo_select(struct cpuidle_dri
 	unsigned int idx_hit_sum = 0;
 	unsigned int hit_sum = 0;
 	unsigned int tick_sum = 0;
+	unsigned int thresh_sum = 0;
 	int constraint_idx = 0;
 	int idx0 = 0, idx = -1;
 	bool alt_intercepts, alt_recent;
@@ -396,6 +397,8 @@ static int teo_select(struct cpuidle_dri
 		duration_ns = tick_nohz_get_sleep_length(&delta_tick);
 	cpu_data->sleep_length_ns = duration_ns;
 
+	thresh_sum = 2 * cpu_data->total / 3; /* 66% */
+
 	/*
 	 * Find the deepest idle state whose target residency does not exceed
 	 * the current sleep length and the deepest idle state not deeper than
@@ -426,6 +429,9 @@ static int teo_select(struct cpuidle_dri
 		if (s->target_residency_ns > duration_ns)
 			break;
 
+		if (intercept_sum + hit_sum > thresh_sum)
+			break;
+
 		idx = i;
 
 		if (s->exit_latency_ns <= latency_req)