| 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
| ||
|
Message-ID: <CAKfTPtB_4bQxBOUJ4tCFRVqzHDRVXj3YPkyzyTv_h-sgm8NwEQ@mail.gmail.com>
Date: Wed, 28 May 2014 08:49:39 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>, npiggin@...il.com
Cc: Ingo Molnar <mingo@...nel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Russell King - ARM Linux <linux@....linux.org.uk>,
LAK <linux-arm-kernel@...ts.infradead.org>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Mike Galbraith <efault@....de>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 02/11] sched: remove a wake_affine condition
Using another email address for Nick
On 27 May 2014 18:14, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> On 27 May 2014 17:39, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, May 27, 2014 at 05:19:02PM +0200, Vincent Guittot wrote:
>>> On 27 May 2014 14:48, Peter Zijlstra <peterz@...radead.org> wrote:
>>> > On Fri, May 23, 2014 at 05:52:56PM +0200, Vincent Guittot wrote:
>>> >> I have tried to understand the meaning of the condition :
>>> >> (this_load <= load &&
>>> >> this_load + target_load(prev_cpu, idx) <= tl_per_task)
>>> >> but i failed to find a use case that can take advantage of it and i haven't
>>> >> found description of it in the previous commits' log.
>>> >
>>> > commit 2dd73a4f09beacadde827a032cf15fd8b1fa3d48
>>> >
>>> > int try_to_wake_up():
>>> >
>>> > in this function the value SCHED_LOAD_BALANCE is used to represent the load
>>> > contribution of a single task in various calculations in the code that
>>> > decides which CPU to put the waking task on. While this would be a valid
>>> > on a system where the nice values for the runnable tasks were distributed
>>> > evenly around zero it will lead to anomalous load balancing if the
>>> > distribution is skewed in either direction. To overcome this problem
>>> > SCHED_LOAD_SCALE has been replaced by the load_weight for the relevant task
>>> > or by the average load_weight per task for the queue in question (as
>>> > appropriate).
>>> >
>>> > if ((tl <= load &&
>>> > - tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>>> > - 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>>> > + tl + target_load(cpu, idx) <= tl_per_task) ||
>>> > + 100*(tl + p->load_weight) <= imbalance*load) {
>>>
>>> The oldest patch i had found was: https://lkml.org/lkml/2005/2/24/34
>>> where task_hot had been replaced by
>>> + if ((tl <= load &&
>>> + tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>>> + 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>>>
>>> but as explained, i haven't found a clear explanation of this condition
>>
>> Yeah, that's the commit I had below; but I suppose we could ask Nick if
>> we really want, I've heard he still replies to email, even though he's
>> locked up in a basement somewhere :-)
ok, I have added him in the list
Nick,
While doing some rework on the wake affine part of the scheduler, i
failed to catch the use case that takes advantage of a condition that
you added some while ago with the commit
a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
Could you help us to clarify the 2 first lines of the test that you added ?
+ if ((tl <= load &&
+ tl + target_load(cpu, idx) <=
SCHED_LOAD_SCALE) ||
+ 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
Regards,
Vincent
>
>>
>>> > commit a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
>>> >
>>> >
>>> > + if ((tl <= load &&
>>> > + tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>>> > + 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>>> >
>>> >
>>> > So back when the code got introduced, it read:
>>> >
>>> > target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE < source_load(this_cpu, idx) &&
>>> > target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE + target_load(this_cpu, idx) < SCHED_LOAD_SCALE
>>> >
>>> > So while the first line makes some sense, the second line is still
>>> > somewhat challenging.
>>> >
>>> > I read the second line something like: if there's less than one full
>>> > task running on the combined cpus.
>>>
>>> ok. your explanation makes sense
>>
>> Maybe, its still slightly weird :-)
>>
>>> >
>>> > Now for idx==0 this is hard, because even when sync=1 you can only make
>>> > it true if both cpus are completely idle, in which case you really want
>>> > to move to the waking cpu I suppose.
>>>
>>> This use case is already taken into account by
>>>
>>> if (this_load > 0)
>>> ..
>>> else
>>> balance = true
>>
>> Agreed.
>>
>>> > One task running will have it == SCHED_LOAD_SCALE.
>>> >
>>> > But for idx>0 this can trigger in all kinds of situations of light load.
>>>
>>> target_load is the max between load for idx == 0 and load for the
>>> selected idx so we have even less chance to match the condition : both
>>> cpu are completely idle
>>
>> Ah, yes, I forgot to look at the target_load() thing and missed the max,
>> yes that all makes it entirely less likely.
--
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/
Powered by blists - more mailing lists