linux-kernel - Re: [PATCH v1] cpuidle: governors: menu: Predict longer idle time when in doubt

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Message-ID: <5f0aa630-b30a-44c4-a52c-e08179cd3bf9@arm.com>
Date: Fri, 17 Oct 2025 09:22:51 +0100
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
 LKML <linux-kernel@...r.kernel.org>,
 Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
 Tomasz Figa <tfiga@...omium.org>, Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v1] cpuidle: governors: menu: Predict longer idle time
 when in doubt

On 10/16/25 17:25, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> It is reported that commit 85975daeaa4d ("cpuidle: menu: Avoid discarding
> useful information") led to a performance regression on Intel Jasper Lake
> systems because it reduced the time spent by CPUs in idle state C7 which
> is correlated to the maximum frequency the CPUs can get to because of an
> average running power limit [1].
> 
> Before that commit, get_typical_interval() would have returned UINT_MAX
> whenever it had been unable to make a high-confidence prediction which
> had led to selecting the deepest available idle state too often and
> both power and performance had been inadequate as a result of that in
> some cases.  This was not a problem on systems with relatively
> aggressive average running power limits, like the Jasper Lake systems
> in question, because on those systems it was compensated by the ability
> to run CPUs at relatively higher frequencies.
> 
> Commit 85975daeaa4d addressed that by causing get_typical_interval() to
> return a number based on the recent idle duration information available
> to it in those cases, but that number is usually smaller than the
> maximum idle duration observed recently which may be regarded as an
> overly optimistic choice.
> 
> Namely, it may be argued that when the samples considered by
> get_typical_interval() are spread too much for a high-confidence
> prediction to be made, the function should fall back to returning a
> number that is likely to be an upper bound for the duration of the
> upcoming idle interval and that number needs to be at least equal to
> the maximum recently observed idle time.  Otherwise, the governor may
> miss an oportunity to reduce power without hurting performance in a
> noticeable way.  Of course, it may also be argued the other way around,
> but the available data indicate that get_typical_interval() should
> rather tend to return larger numbers as that causes the governor to
> behave more closely to its past behavior from before the problematic
> commit.
> 
> Accordingly, modify get_typical_interval() to return the maximum
> recently observed idle time when it is unable to make a high-
> confidence prediction.
> 
> Fixes: 85975daeaa4d ("cpuidle: menu: Avoid discarding useful information")
> Closes: https://lore.kernel.org/linux-pm/36iykr223vmcfsoysexug6s274nq2oimcu55ybn6ww4il3g3cv@cohflgdbpnq7/ [1]
> Reported-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Tested-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Cc: All applicable <stable@...r.kernel.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/cpuidle/governors/menu.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -116,6 +116,7 @@ static void menu_update(struct cpuidle_d
>  static unsigned int get_typical_interval(struct menu_device *data)
>  {
>  	s64 value, min_thresh = -1, max_thresh = UINT_MAX;
> +	unsigned int max_overall = 0;

nit: for reverse xmas this should be one further down?
Maybe s/max_overall/max_first_pass/?

>  	unsigned int max, min, divisor;
>  	u64 avg, variance, avg_sq;
>  	int i;
> @@ -151,6 +152,9 @@ again:
>  	if (!max)
>  		return UINT_MAX;
>  

Or alternatively a comment:
/* Save the max before we discard any intervals */
or something.

> +	if (max_overall < max)
> +		max_overall = max;
> +>  	if (divisor == INTERVALS) {
>  		avg >>= INTERVAL_SHIFT;
>  		variance >>= INTERVAL_SHIFT;
> @@ -198,7 +202,7 @@ again:
>  		 * maximum, so return the latter in that case.
>  		 */
>  		if (divisor >= INTERVALS / 2)
> -			return max;
> +			return max_overall;
>  
>  		return UINT_MAX;
>  	}
> 

Anyway, the patch makes sense, let me run some tests and get back.