linux-kernel - Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKfTPtCKAAhU4hbkRG2-m1n=koYts4t6ozmHj=P+6GTRNkkiuQ@mail.gmail.com>
Date:	Thu, 10 Sep 2015 13:11:01 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Benjamin Segall <bsegall@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"mturquette@...libre.com" <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"sgurrappadi@...dia.com" <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

On 10 September 2015 at 13:06, Morten Rasmussen
<morten.rasmussen@....com> wrote:
> On Wed, Sep 09, 2015 at 03:23:43PM -0700, bsegall@...gle.com wrote:
>> Peter Zijlstra <peterz@...radead.org> writes:
>>
>> > On Tue, Sep 08, 2015 at 03:31:58PM +0100, Morten Rasmussen wrote:
>> >> On Tue, Sep 08, 2015 at 02:52:05PM +0200, Peter Zijlstra wrote:
>> >> > > Tricky that, LOAD_AVG_MAX very much relies on the unit being 1<<10.
>> >>
>> >> I don't get why LOAD_AVG_MAX relies on the util_avg shifting being
>> >> 1<<10, it is just the sum of the geometric series and the upper bound of
>> >> util_sum?
>> >
>> > It needs a 1024, it might just have been the 1024 ns we use a period
>> > instead of the scale unit though.
>> >
>> > The LOAD_AVG_MAX is the number where adding a next element to the series
>> > doesn't change the result anymore, so scaling it up will allow more
>> > significant elements to the series before we bottom out, which is the _N
>> > thing.
>> >
>>
>> Yes, as the comments say, the 1024ns unit is arbitrary (and is an
>> average of not-quite-microseconds instead of just nanoseconds to allow
>> more bits to load.weight when we multiply load.weight by this number).
>> In fact there are two arbitrary 1024 units here, which are technically
>> unrelated and are both unrelated to SCHED_LOAD_RESOLUTION/etc - we
>> operate on units of almost-microseconds and we also do decays every
>> almost-millisecond.
>>
>> There appears to be a bunch of confusion in the current code around
>> util_sum/util_avg which appears to using SCHED_LOAD_SCALE
>> for a fixed-point percentage or something, which is at least reasonable,
>> but is initializing it as scale_load_down(SCHED_LOAD_SCALE), which
>> results in either initializing as 100% or .1% depending on RESOLUTION.
>> This'll get clobbered on first update, but if it needs to be
>> initialized, it should either get initialized to something sane or at
>> least consistent.
>
> This is what I thought too. The whole geometric series math is completely
> independent of the scale used for priority in load_avg and the fixed
> point shifting used for util_avg.
>
>> load_sum/load_avg appear to be scale_load_down()ed properly, and appear
>> to be used as such at a quick glance.
>
> I don't think shifting by SCHED_LOAD_SHIFT in __update_load_avg() is
> right:
>
>         sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX;
>
> util_avg is initialized to low resolution (>> SCHED_LOAD_RESOLUTION):
>
>         sa->util_avg = scale_load_down(SCHED_LOAD_SCALE);
>
> so it appear to be intended to be using low resolution like load_avg
> (weight is scaled down before it is passed into __update_load_avg()),
> but util_avg is shifted up to high resolution. It should be:
>
>         sa->util_avg = (sa->util_sum << (SCHED_LOAD_SHIFT -
>                                         SCHED_LOAD_SHIFT)) / LOAD_AVG_MAX;

you probably mean (SCHED_LOAD_SHIFT -  SCHED_LOAD_RESOLUTION)

The goal of this patchset is to be able to scale util_avg in the range
of cpu capacity so why don't we directly initialize it with
sa->util_avg = SCHED_CAPACITY_SCALE;

and then use

 sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX;

so we don't have to take care of high and low load resolution
Regards,

>
> to be consistent.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/