[lkml]   [2018]   [Feb]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] cpufreq: schedutil: rate limits for SCHED_DEADLINE

Il 09/02/2018 13:56, Rafael J. Wysocki ha scritto:
> On Fri, Feb 9, 2018 at 1:52 PM, Juri Lelli <> wrote:
>> On 09/02/18 13:08, Rafael J. Wysocki wrote:
>>> On Fri, Feb 9, 2018 at 12:51 PM, Juri Lelli <> wrote:
>>>> On 09/02/18 12:37, Rafael J. Wysocki wrote:
>>>>> On Fri, Feb 9, 2018 at 12:26 PM, Juri Lelli <> wrote:
>>>>>> On 09/02/18 12:04, Rafael J. Wysocki wrote:
>>>>>>> On Fri, Feb 9, 2018 at 11:53 AM, Juri Lelli <> wrote:
>>>>>>>> Hi,
>>>>>>>> On 09/02/18 11:36, Rafael J. Wysocki wrote:
>>>>>>>>> On Friday, February 9, 2018 9:02:34 AM CET Claudio Scordino wrote:
>>>>>>>>>> Hi Viresh,
>>>>>>>>>> Il 09/02/2018 04:51, Viresh Kumar ha scritto:
>>>>>>>>>>> On 08-02-18, 18:01, Claudio Scordino wrote:
>>>>>>>>>>>> When the SCHED_DEADLINE scheduling class increases the CPU utilization,
>>>>>>>>>>>> we should not wait for the rate limit, otherwise we may miss some deadline.
>>>>>>>>>>>> Tests using rt-app on Exynos5422 have shown reductions of about 10% of deadline
>>>>>>>>>>>> misses for tasks with low RT periods.
>>>>>>>>>>>> The patch applies on top of the one recently proposed by Peter to drop the
>>>>>>>>>>>> SCHED_CPUFREQ_* flags.
>>>>>>>>> [cut]
>>>>>>>>>>> Is it possible to (somehow) check here if the DL tasks will miss
>>>>>>>>>>> deadline if we continue to run at current frequency? And only ignore
>>>>>>>>>>> rate-limit if that is the case ?
>>>>>>>> Isn't it always the case? Utilization associated to DL tasks is given by
>>>>>>>> what the user said it's needed to meet a task deadlines (admission
>>>>>>>> control). If that task wakes up and we realize that adding its
>>>>>>>> utilization contribution is going to require a frequency change, we
>>>>>>>> should _theoretically_ always do it, or it will be too late. Now, user
>>>>>>>> might have asked for a bit more than what strictly required (this is
>>>>>>>> usually the case to compensate for discrepancies between theory and real
>>>>>>>> world, e.g. hw transition limits), but I don't think there is a way to
>>>>>>>> know "how much". :/
>>>>>>> You are right.
>>>>>>> I'm somewhat concerned about "fast switch" cases when the rate limit
>>>>>>> is used to reduce overhead.
>>>>>> Mmm, right. I'm thinking that in those cases we could leave rate limit
>>>>>> as is. The user should then be aware of it and consider it as proper
>>>>>> overhead when designing her/his system.
>>>>>> But then, isn't it the same for "non fast switch" platforms? I mean,
>>>>>> even in the latter case we can't go faster than hw limits.. mmm, maybe
>>>>>> the difference is that in the former case we could go as fast as theory
>>>>>> would expect.. but we shouldn't. :)
>>>>> Well, in practical terms that means "no difference" IMO. :-)
>>>>> I can imagine that in some cases this approach may lead to better
>>>>> results than reducing the rate limit overall, but the general case I'm
>>>>> not sure about.
>>>>> I mean, if overriding the rate limit doesn't take place very often,
>>>>> then it really should make no difference overhead-wise. Now, of
>>>>> course, how to define "not very often" is a good question as that
>>>>> leads to rate-limiting the overriding of the original rate limit and
>>>>> that scheme may continue indefinitely ...
>>>> :)
>>>> My impression is that rate limit helps a lot for CFS, where the "true"
>>>> utilization is not known in advance, and being too responsive might
>>>> actually be counterproductive.
>>>> For DEADLINE (and RT, with differences) we should always respond as
>>>> quick as we can (and probably remember that a frequency transition was
>>>> requested if hw was already performing one, but that's another patch)
>>>> because, if we don't, a task belonging to a lower priority class might
>>>> induce deadline misses in highest priority activities. E.g., a CFS task
>>>> that happens to trigger a freq switch right before a DEADLINE task wakes
>>>> up and needs an higher frequency to meet its deadline: if we wait for
>>>> the rate limit of the CFS originated transition.. deadline miss!
>>> Fair enough, but if there's too much overhead as a result of this, you
>>> can't guarantee the deadlines to be met anyway.
>> Indeed. I guess this only works if corner cases as the one above don't
>> happen too often.
> Well, that's the point.
> So there is a tradeoff: do we want to allow deadlines to be missed
> because of excessive overhead or do we want to allow deadlines to be
> missed because of the rate limit.

For a very few tasks, the tests have indeed shown that the approach pays off: we get a significant reduction of misses with a negligible increase of energy consumption.
I still need to check what happens for a high amount of tasks, trying to reproduce the "ramp up" pattern (in which DL keeps increasing the utilization, ignoring the rate limit and adding overhead)



 \ /
  Last update: 2018-02-09 14:21    [W:0.045 / U:12.656 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site