On 12/11/2012 05:23 PM, Alex Shi wrote: > On 12/11/2012 02:30 PM, Preeti U Murthy wrote: >> 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. > > Thanks for question Preeti! :) > > Yes, with more iteration you has more possibility to get task allowed > cpu in select_task_rq_fair. but how many opportunity the situation > happened? how much gain you get here? > With LCPU increasing many many iterations cause scalability issue. that > is the simplified forking patchset for. and that why 10% performance > gain on hackbench process/thread. > > and if you insist want not to miss your chance in strf(), the current > iteration is still not enough. How you know the idlest cpu is still > idlest after this function finished? how to ensure the allowed cpu won't > be changed again? > > A quick snapshot is enough in balancing here. we still has periodic > balacning.
Hmm ok,let me look at this more closely. > >> >> * 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 > > After read the first 4 patches, believe you will find the patchset is > trying to reduce iterations, not increase them. Right,sorry about not noticing this. > >> the wrong path of sched_domains before exit,compared to what is shown >> above.) >> Regards Preeti U Murthy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/