Re: [RFT][PATCH v7 5/8] cpuidle: Return nohz hint from cpuidle_select()

["Rafael J. Wysocki" <rafael@kernel.org>]

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On Wed, Mar 21, 2018 at 6:59 PM, Thomas Ilsche
<thomas.ilsche@tu-dresden.de> wrote:
> On 2018-03-21 15:36, Rafael J. Wysocki wrote:
>>
>>
>> So please disregard this one entirely and take the v7.2 replacement
>> instead of it:https://patchwork.kernel.org/patch/10299429/
>>
>> The current versions (including the above) is in the git branch at
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
>>   idle-loop-v7.2
>
>
> With v7.2 (tested on SKL-SP from git) I see similar behavior in idle
> as with v5: several cores which just keep the sched tick enabled.
> Worse yet, some go only in C1 (not even C1E!?) despite sleeping the
> full sched tick.
> The resulting power consumption is ~105 W instead of ~ 70 W.
>
> https://wwwpub.zih.tu-dresden.de/~tilsche/powernightmares/v7_2_skl_sp_idle.png
>
> I have briefly ran v7 and I believe it was also affected.

Then it looks like menu_select() stubbornly thinks that the idle
duration will be within the tick boundary on those cores.

That may be because the bumping up of the correction factor in
menu_reflect() is too conservative or it may be necessary to do
something radical to measured_us in menu_update() in case of a tick
wakeup combined with a large next_timer_us value.

For starters, please see if the attached patch (on top of the
idle-loop-v7.2 git branch) changes this behavior in any way.

[-- Attachment #2: cpuidle-menu-menu_reflect-debug.patch --]
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---
 drivers/cpuidle/governors/menu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-pm/drivers/cpuidle/governors/menu.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/menu.c
+++ linux-pm/drivers/cpuidle/governors/menu.c
@@ -498,7 +498,7 @@ static void menu_reflect(struct cpuidle_
 		 * correction factor.  Use 0.75 * RESOLUTION (which is easy
 		 * enough to get) that should work fine on the average.
 		 */
-		new_factor += RESOLUTION / 2 + RESOLUTION / 4;
+		new_factor += RESOLUTION;
 		data->correction_factor[data->bucket] = new_factor;
 	} else {
 		data->needs_update = 1;