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SubjectRe: [PATCH 01/18] sched: select_task_rq_fair clean up
On 12/11/2012 10:58 AM, Alex Shi wrote:
> On 12/11/2012 12:23 PM, Preeti U Murthy wrote:
>> Hi Alex,
>>
>> On 12/10/2012 01:52 PM, Alex Shi wrote:
>>> It is impossible to miss a task allowed cpu in a eligible group.
>>
>> The one thing I am concerned with here is if there is a possibility of
>> the task changing its tsk_cpus_allowed() while this code is running.
>>
>> i.e find_idlest_group() finds an idle group,then the tsk_cpus_allowed()
>> for the task changes,perhaps by the user himself,which might not include
>> the cpus in the idle group.After this find_idlest_cpu() is called.I mean
>> a race condition in short.Then we might not have an eligible cpu in that
>> group right?
>
> your worry make sense, but the code handle the situation, in
> select_task_rq(), it will check the cpu allowed again. if the answer is
> no, it will fallback to old cpu.
>>
>>> And since find_idlest_group only return a different group which
>>> excludes old cpu, it's also imporissible to find a new cpu same as old
>>> cpu.

I doubt this will work correctly.Consider the following situation:sched
domain begins with sd that encloses both socket1 and socket2

cpu0 cpu1 | cpu2 cpu3
-----------|-------------
socket1 | socket2

old cpu = cpu1

Iteration1:
1.find_idlest_group() returns socket2 to be idlest.
2.task changes tsk_allowed_cpus to 0,1
3.find_idlest_cpu() returns cpu2

* without your patch
1.the condition after find_idlest_cpu() returns -1,and sd->child is
chosen which happens to be socket1
2.in the next iteration, find_idlest_group() and find_idlest_cpu()
will probably choose cpu0 which happens to be idler than cpu1,which is
in tsk_allowed_cpu.

* with your patch
1.the condition after find_idlest_cpu() does not exist,therefore
a sched domain to which cpu2 belongs to is chosen.this is socket2.(under
the for_each_domain() loop).
2.in the next iteration, find_idlest_group() return NULL,because
there is no cpu which intersects with tsk_allowed_cpus.
3.in select task rq,the fallback cpu is chosen even when an idle cpu
existed.

So my concern is though select_task_rq() checks the
tsk_allowed_cpus(),you might end up choosing a different path of
sched_domains compared to without this patch as shown above.

In short without the "if(new_cpu==-1)" condition we might get misled
doing unnecessary iterations over the wrong sched domains in
select_task_rq_fair().(Think about situations when not all the cpus of
socket2 are disallowed by the task,then there will more iterations in
the wrong path of sched_domains before exit,compared to what is shown
above.)

Regards
Preeti U Murthy




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