linux-kernel - Re: [RFT][PATCH v7 6/8] sched: idle: Select idle state before stopping the tick

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Date:   Wed, 28 Mar 2018 12:37:06 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Thomas Ilsche <thomas.ilsche@...dresden.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFT][PATCH v7 6/8] sched: idle: Select idle state before
 stopping the tick

On Wed, Mar 28, 2018 at 10:38 AM, Thomas Ilsche
<thomas.ilsche@...dresden.de> wrote:
> On 2018-03-28 10:13, Rafael J. Wysocki wrote:
>>
>> On Wed, Mar 28, 2018 at 12:10 AM, Rafael J. Wysocki <rjw@...ysocki.net>
>> wrote:
>>>
>>> On Tuesday, March 27, 2018 11:50:02 PM CEST Thomas Ilsche wrote:
>>>>
>>>> On 2018-03-20 16:45, Rafael J. Wysocki wrote:
>>>>>
>>>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>>>
>>>>> In order to address the issue with short idle duration predictions
>>>>> by the idle governor after the tick has been stopped, reorder the
>>>>> code in cpuidle_idle_call() so that the governor idle state selection
>>>>> runs before tick_nohz_idle_go_idle() and use the "nohz" hint returned
>>>>> by cpuidle_select() to decide whether or not to stop the tick.
>>>>>
>>>>> This isn't straightforward, because menu_select() invokes
>>>>> tick_nohz_get_sleep_length() to get the time to the next timer
>>>>> event and the number returned by the latter comes from
>>>>> __tick_nohz_idle_enter().  Fortunately, however, it is possible
>>>>> to compute that number without actually stopping the tick and with
>>>>> the help of the existing code.
>>>>
>>>>
>>>> I think something is wrong with the new tick_nohz_get_sleep_length.
>>>> It seems to return a value that is too large, ignoring immanent
>>>> non-sched timer.
>>>
>>>
>>> That's a very useful hint, let me have a look.
>>>
>>>> I tested idle-loop-v7.3. It looks very similar to my previous results
>>>> on the first idle-loop-git-version [1]. Idle and traditional synthetic
>>>> powernightmares are mostly good.
>>>
>>>
>>> OK
>>>
>>>> But it selects too deep C-states for short idle periods, which is bad
>>>> for power consumption [2].
>>>
>>>
>>> That still needs to be improved, then.
>>>
>>>> I tracked this down with additional tests using
>>>> __attribute__((optimize("O0"))) menu_select
>>>> and perf probe. With this the behavior seems slightly different, but it
>>>> shows that data->next_timer_us is:
>>>> v4.16-rc6: the expected ~500 us [3]
>>>> idle-loop-v7.3: many milliseconds to minutes [4].
>>>> This leads to the governor to wrongly selecting C6.
>>>>
>>>> Checking with 372be9e and 6ea0577, I can confirm that the change is
>>>> introduced by this patch.
>>>
>>>
>>> Yes, that's where the most intrusive reordering happens.
>>
>>
>> Overall, this is an interesting conundrum, because the case in
>> question is when the tick should never be stopped at all during the
>> workload and the code's behavior in that case should not change, so
>> the change was not intentional.
>>
>> Now, from walking through the code, as long as can_stop_idle_tick()
>> returns 'true' all should be fine or at least I don't see why there is
>> any difference in behavior in that case.
>>
>> However, if can_stop_idle_tick() returns 'false' (for example, because
>> need_resched() returns 'true' when it is evaluated), the behavior *is*
>> different in a couple of ways.  I sort of know how that can be
>> addressed, but I'd like to reproduce your results here.
>>
>> Are you still using the same workload as before to trigger this behavior?
>>
>
> Yes, the exact code I use is as follows
>
> $ gcc poller.c -O3 -fopenmp -o poller_omp
> $ GOMP_CPU_AFFINITY=0-35 ./poller_omp 500
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <unistd.h>
>
> int main(int argc, char *argv[])
> {
>     int sleep_us = 10000;
>     if (argc == 2) {
>         sleep_us = atoi(argv[1]);
>     }
>
>     #pragma omp parallel
>     {
>         while (1) {
>             usleep(sleep_us);
>         }
>     }
> }

So I do

$ for cpu in 0 1 2 3; do taskset -c $cpu sh -c 'while true; do usleep
500; done' & done

which is a shell kind of imitation of the above and I cannot see this
issue at all.

I count the number of times data->next_timer_us in menu_select() is
greater than TICK_USEC and while this "workload" is running, that
number is exactly 0.

I'll try with a C program still.