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Date:   Tue, 21 Aug 2018 10:37:42 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        linux-kernel@...r.kernel.org, Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v1 0/5] Improvement stopping tick decision making in 'menu' idle governor

On Sunday, August 12, 2018 6:09:26 PM CEST Leo Yan wrote:
> We found the CPU cannot stay in deepest idle state as expected with
> running synthetic workloads with mainline kernel on Arm platform
> (96boards Hikey620 with octa CA53 CPUs).
> 
> The main issue is the criteria for decision stopping tick; now
> the criteria is checking expected interval is less than TICK_USEC, but
> this doesn't consider the next tick detla is float due CPU randomly
> eneters and exits idle states; furthermore, it's stick to checking
> TICK_USEC as boundary for decision stopping tick, unfortunately this has
> hole to select a shallow state with stopping tick, so the CPU stays in
> shallow state for long time.
> 
> This patch series is to explore more reasonable making decision for
> stopping tick and the most important fixing is to avoid powernightmares
> issue after we apply these criterias for making decisions.  Patches
> 0001 ~ 0003 are used to refactor the variables and structures for more
> readable code, it also provides a function menu_decide_stopping_tick()
> which can be used to encapsulate the making decision logics.  The last
> two patches are primary for improvement, patch 0004 'cpuidle: menu:
> Don't stay in shallow state for a long time' introduces a new criteria
> (it's a more strict criteria than before) for not stopping tick for
> shallow state cases; patch 0005 is use the dynamic tick detla to replace
> the static value TICK_USEC for decision if the tick is expired before or
> after the prediction, according this comparison we can get conclusion if
> need to stop tick or not.
> 
> With more accurate decision for stopping tick, one immediate benefit is
> the CPUs have more chance to stay in deepest state, it also can avoid to
> run tick unnecessarily and so avoid a shallower state introduced by tick
> event.  For the testing result in below table, we can see the result
> proves the improvement by better stopping tick decision making in this
> patch series, we run the workload generated by rt-app (a single task
> with period 5ms and duty cycle 1%/3%/5%/10%/20%/30%/40%), the total
> running time is 60s.  We do statistics for all CPUs for all idle states
> duration, the unit is second (s), for cases (dutycycle=1%/3%/5%/10%/20%)
> we can see the shallow state C0/C1 duration are reduced and the time
> has been moved to deepest state, so the deepest state C2 duration can
> have improvement for ~9s to ~21s.  for cases (dutycycle=30%/40%) though
> we can see the deepest state durations are parity between with and
> without patch series, but it has a minor improvement for C1 state
> duration by stealing C0 state duration.
> 
> Some notations are used in the table:
> 
> state: C0: WFI; C1: CPU OFF; C2: Cluster OFF
> 
> All testing cases have single task with 5ms period:
> 
> 		 Without patches           With patches               Difference
>             -----------------------  -----------------------   --------------------------
> Duty cycle    C0     C1       C2       C0      C1      C2        C0        C1        C2
>   1%        2.397  16.528  471.905   0.916    2.688  487.328   -1.481   -13.840   +15.422
>   3%        3.957  20.541  464.434   1.510    2.398  485.914   -2.447   -18.143   +21.480
>   5%        2.866   8.609  474.777   1.166    2.250  483.983   -1.699    -6.359    +9.205
>  10%        2.893  28.753  453.277   1.147   14.134  469.190   -1.745   -14.618   +15.913
>  20%        7.620  41.086  431.735   1.595   35.055  442.482   -6.024    -6.030   +10.747
>  30%        4.394  38.328  431.442   1.964   40.857  430.973   -2.430    +2.529    -0.468
>  40%        7.390  29.415  430.914   1.789   34.832  431.588   -5.600    +5.417    -0.673
> 
> 
> P.s. for the testing, applied Rafael's patch 'cpuidle: menu: Handle
> stopped tick more aggressively' [1] to avoid select unexpected shallow
> state after tick has been stopped.
> 
> [1] https://lkml.org/lkml/2018/8/10/259
> 
> Leo Yan (5):
>   cpuidle: menu: Clean up variables usage in menu_select()
>   cpuidle: menu: Record tick delta value in struct menu_device
>   cpuidle: menu: Provide menu_decide_stopping_tick()
>   cpuidle: menu: Don't stay in shallow state for a long time
>   cpuidle: menu: Change to compare prediction with tick delta
> 
>  drivers/cpuidle/governors/menu.c | 104 ++++++++++++++++++++++++++++-----------
>  1 file changed, 76 insertions(+), 28 deletions(-)
> 
> 

Overall, I don't like this series, sorry about that.

The majority of changes in it are code reorganization, quite questionable
in a couple of cases, and a similar goal can be achieved with a very simple
patch that I'm going to post shortly.

Thanks,
Rafael


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