From: Pierre Gondois <pierre.gondois@arm.com>
To: Dawei Li <daweilics@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Valentin Schneider <vschneid@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair: simplify __calc_delta()
Date: Wed, 13 Mar 2024 11:21:26 +0100 [thread overview]
Message-ID: <baac77f6-2fd4-4683-a3b8-4bd68fee1ecf@arm.com> (raw)
In-Reply-To: <CAG5MgCpW3xwSGgF6VBPiSMkwOz793NO4_nLNhJeqyBhs+jN30w@mail.gmail.com>
Hello Dawei,
On 3/13/24 00:25, Dawei Li wrote:
> Hi Pierre,
> Thank you for the review!
>
> On Tue, Mar 12, 2024 at 6:18 AM Pierre Gondois <pierre.gondois@arm.com> wrote:
>>
>> Hello Dawei,
>>
>> On 3/6/24 23:28, Dawei Li wrote:
>>> Based on how __calc_delta() is called now, the input parameter, weight
>>> is always NICE_0_LOAD. I think we don't need it as an input parameter
>>> now?
>>
>> Maybe
>> 5e963f2bd4654a202a8a05aa3a86cb0300b10e6c ("sched/fair: Commit to EEVDF")
>> should be referenced to explain that the case where (weight =< lw.weight)
>> doesn't exist anymore and that NICE_0_LOAD could be incorporated in
>> __calc_delta() directly.
>>
>>
>> Also I think indirect forms are preferred in general:
>> "I think we don't need it as an input parameter now ?" ->
>> "The 'weight' parameter doesn't seem to be required anymore"
>> (same note for the whole commit message)
>>
>>>
>>> Also, when weight is always NICE_0_LOAD, the initial fact value is
>>> always 2^10, and the first fact_hi will always be 0. Thus, we can get
>>> rid of the first if bock.
>>>
>>> The previous comment "(delta_exec * (weight * lw->inv_weight)) >>
>>> WMULT_SHIFT" seems to be assuming that lw->weight * lw->inv_weight is
>>> always (approximately) equal to 2^WMULT_SHIFT. However, when
>>> CONFIG_64BIT is set, lw->weight * lw->inv_weight is (approximately)
>>> equal to 2^WMULT_SHIFT * 2^10. What remains true for both CONFIG_32BIT
>>> and CONFIG_64BIT is: scale_load_down(lw->weight) * lw->inv_weight is
>>> (approximately) equal to 2^WMULT_SHIFT. (Correct me if I am wrong.)
>>
>> I think the comment is more about explaining that:
>> X * lw.weight
>> equals:
>> X * lw->inv_weight >> WMULT_SHIFT
>>
> I assume you mean
> X / lw->weight
> equals:
> X * lw->inv_weight >> WMULT_SHIFT
Yes right indeed.
> However, this is not always true, and that's why I'd like to revise
> it. It is true for
> CONFIG_32BIT. However, For CONFIG_64BIT, we have lw->weight * lw->inv_weight =
> 2**WMULT_SHIFT * 2**10. Thus,
> X / lw->weight
> equals:
> X * lw->inv_weight >> (WMULT_SHIFT + 10)
Ok yes, you're correct indeed.
The equality is always correct when scale_load_down() is used,
Regards,
Pierre
>
>
>> Also, if CONFIG_64BIT is set, we should have:
>> weight / lw.weight == scale_load_down(lw->weight) * 2**10 * lw->inv_weight
>>
>
> weight / lw->weight should be equal to scale_load_down(weight) /
> scale_load_down(lw->weight)
> = scale_load_down(weight) * lw->inv_weight / (2**WMULT_SHIFT)
> Right?
>
>> So IIUC, either both lines should be update, either none.
>> (meaning that:
>> delta_exec * NICE_0_LOAD / lw->weight
>> should be changed to
>> delta_exec * scale_load_down(NICE_0_LOAD) / lw->weight
>
> I think this is not correct? scale_load_down(NICE_0_LOAD) is the true
> weight, as mapped
> directly from the task's nice/priority value, while lw->weight is the
> scaled_up load.
> Their units/scales don't match.
>
prev parent reply other threads:[~2024-03-13 10:21 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-06 22:28 [PATCH] sched/fair: simplify __calc_delta() Dawei Li
2024-03-12 13:18 ` Pierre Gondois
2024-03-12 23:25 ` Dawei Li
2024-03-13 10:21 ` Pierre Gondois [this message]
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