linux-kernel - Re: [RFT][PATCH v5 4/7] cpuidle: Return nohz hint from cpuidle

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Date:   Mon, 19 Mar 2018 10:39:50 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        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 v5 4/7] cpuidle: Return nohz hint from cpuidle_select()

On Mon, Mar 19, 2018 at 10:11 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Mar 15, 2018 at 11:11:35PM +0100, Rafael J. Wysocki wrote:
>
> I would suggest s/nohz_ret/stop_tick/ throughout, because I keep
> forgetting which way around the boolean works and the new name doesn't
> confuse.

OK

>> Index: linux-pm/drivers/cpuidle/governors/menu.c
>> ===================================================================
>> --- linux-pm.orig/drivers/cpuidle/governors/menu.c
>> +++ linux-pm/drivers/cpuidle/governors/menu.c
>> @@ -275,12 +275,16 @@ again:
>>       goto again;
>>  }
>>
>> +#define TICK_USEC_HZ   ((USEC_PER_SEC + HZ/2) / HZ)
>
> Do we want to put that next to TICK_NSEC?

That would be one change too far IMHO.

> Also, there are only 2 users of the existing TICK_USEC, do we want to:
>
>   s/TICK_USEC/USER_&/
>
> and then rename the new thing to TICK_USEC ?

Well, that can be done.

>>  /**
>>   * menu_select - selects the next idle state to enter
>>   * @drv: cpuidle driver containing state data
>>   * @dev: the CPU
>> + * @nohz_ret: indication on whether or not to stop the tick
>>   */
>> +static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>> +                    bool *nohz_ret)
>>  {
>>       struct menu_device *data = this_cpu_ptr(&menu_devices);
>>       struct device *device = get_cpu_device(dev->cpu);
>> @@ -303,8 +307,10 @@ static int menu_select(struct cpuidle_dr
>>               latency_req = resume_latency;
>>
>>       /* Special case when user has set very strict latency requirement */
>> +     if (unlikely(latency_req == 0)) {
>> +             *nohz_ret = false;
>>               return 0;
>> +     }
>>
>>       /* determine the expected residency time, round up */
>>       data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length());
>> @@ -354,6 +360,7 @@ static int menu_select(struct cpuidle_dr
>>       if (latency_req > interactivity_req)
>>               latency_req = interactivity_req;
>>
>> +     expected_interval = data->predicted_us;
>>       /*
>>        * Find the idle state with the lowest power while satisfying
>>        * our constraints.
>> @@ -369,15 +376,30 @@ static int menu_select(struct cpuidle_dr
>>                       idx = i; /* first enabled state */
>>               if (s->target_residency > data->predicted_us)
>>                       break;
>> +             if (s->exit_latency > latency_req) {
>> +                     /*
>> +                      * If we break out of the loop for latency reasons, use
>> +                      * the target residency of the selected state as the
>> +                      * expected idle duration so that the tick is retained
>> +                      * as long as that target residency is low enough.
>> +                      */
>> +                     expected_interval = drv->states[idx].target_residency;
>>                       break;
>> +             }
>>               idx = i;
>>       }
>>
>>       if (idx == -1)
>>               idx = 0; /* No states enabled. Must use 0. */
>>
>> +     /*
>> +      * Don't stop the tick if the selected state is a polling one or if the
>> +      * expected idle duration is shorter than the tick period length.
>> +      */
>> +     if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
>> +         expected_interval < TICK_USEC_HZ)
>> +             *nohz_ret = false;
>> +
>>       data->last_state_idx = idx;
>>
>>       return data->last_state_idx;
>
> Yes, much clearer, Thanks!

But this has regressed since the v4, please see
https://patchwork.kernel.org/patch/10291097/

Thanks!