Re: [RFC PATCH 03/16] sched/pelt: Add a new function to approximate runtime to reach given util

[Christian Loehle <christian.loehle@arm.com>]

On 8/22/24 06:36, Sultan Alsawaf (unemployed) wrote:
> On Tue, Aug 20, 2024 at 05:34:59PM +0100, Qais Yousef wrote:
>> It is basically the ramp-up time from 0 to a given value. Will be used
>> later to implement new tunable to control response time  for schedutil.
>>
>> Signed-off-by: Qais Yousef <qyousef@layalina.io>
>> ---
>>  kernel/sched/pelt.c  | 21 +++++++++++++++++++++
>>  kernel/sched/sched.h |  1 +
>>  2 files changed, 22 insertions(+)
>>
>> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
>> index 2ce83e880bd5..06cb881ba582 100644
>> --- a/kernel/sched/pelt.c
>> +++ b/kernel/sched/pelt.c
>> @@ -487,3 +487,24 @@ unsigned long approximate_util_avg(unsigned long util, u64 delta)
>>  
>>  	return sa.util_avg;
>>  }
>> +
>> +/*
>> + * Approximate the required amount of runtime in ms required to reach @util.
>> + */
>> +u64 approximate_runtime(unsigned long util)
>> +{
>> +	struct sched_avg sa = {};
>> +	u64 delta = 1024; // period = 1024 = ~1ms
>> +	u64 runtime = 0;
>> +
>> +	if (unlikely(!util))
>> +		return runtime;
> 
> Seems like this check can be removed since it's covered by the loop condition.
> 
>> +
>> +	while (sa.util_avg < util) {
>> +		accumulate_sum(delta, &sa, 1, 0, 1);
>> +		___update_load_avg(&sa, 0);
>> +		runtime++;
>> +	}
> 
> I think this could be a lookup table (probably 1024 * u8), for constant-time
> runtime approximation.

Somewhat agreed, given that we don't seem to care about the 2.4% error margin,
we could allow some more errors here even. Something like 50 values should be
more than enough (which might fit nicely in a simple formula, too?).

FWIW
util: approximate_runtime(util)
160: 8
192: 10
224: 12
256: 14
288: 16
320: 18
352: 20
384: 22
416: 25
448: 27
480: 30
512: 32
544: 35
576: 39
608: 42
640: 46
672: 50
704: 54
736: 59
768: 64
800: 71
832: 78
864: 86
896: 96
928: 109
960: 128
992: 159
1024: 323

Fine for a RFC though.