On 02/21/2013 04:10 PM, Mike Galbraith wrote: > On Thu, 2013-02-21 at 15:00 +0800, Michael Wang wrote: >> On 02/21/2013 02:11 PM, Mike Galbraith wrote: >>> On Thu, 2013-02-21 at 12:51 +0800, Michael Wang wrote: >>>> On 02/20/2013 06:49 PM, Ingo Molnar wrote: >>>> [snip] >> [snip] >>>> >>>> if wake_affine() >>>> new_cpu = select_idle_sibling(curr_cpu) >>>> else >>>> new_cpu = select_idle_sibling(prev_cpu) >>>> >>>> return new_cpu >>>> >>>> Actually that doesn't make sense. >>>> >>>> I think wake_affine() is trying to check whether move a task from >>>> prev_cpu to curr_cpu will break the balance in affine_sd or not, but why >>>> won't break balance means curr_cpu is better than prev_cpu for searching >>>> the idle cpu? >>> >>> You could argue that it's impossible to break balance by moving any task >>> to any idle cpu, but that would mean bouncing tasks cross node on every >>> wakeup is fine, which it isn't. >> >> I don't get it... could you please give me more detail on how >> wake_affine() related with bouncing? > > If we didn't ever ask if it's ok, we'd always pull, and stack load up on > one package if there's the tiniest of holes to stuff a task into, > periodic balance forcibly rips buddies back apart, repeat. At least > with wake_affine() in the loop, there's somewhat of a damper.
Oh, I think I got the reason why old logical check the affine_sd firstly, so when prev_cpu and curr_cpu belong to different package, there will be a chance to enter balance path, that seems like a solution for this problem, amazing ;-) You are right, with out this logical, chances to balance load between packages will missed, I will apply it in next version. Regards, Michael Wang > >>>> So the new logical in this patch set is: >>>> >>>> new_cpu = select_idle_sibling(prev_cpu) >>>> if idle_cpu(new_cpu) >>>> return new_cpu >>> >>> So you tilted the scales in favor of leaving tasks in their current >>> package, which should benefit large footprint tasks, but should also >>> penalize light communicating tasks. >> >> Yes, I'd prefer to wakeup the task on a cpu which: >> 1. idle >> 2. close to prev_cpu >> >> So if both curr_cpu and prev_cpu have idle cpu in their topology, which >> one is better? that depends on how task benefit from cache and the >> balance situation, whatever, I don't think the benefit worth the high >> cost of wake_affine() in most cases... > > We've always used wake_affine() before, yet been able to schedule at > high frequency, so I don't see that it can be _that_ expensive. I > haven't actually measured lately (loooong time) though. > > WRT cost/benefit of migration, yeah, it depends entirely on the tasks, > some will gain, some will lose. On a modern single processor box, it > just doesn't matter, there's only one llc (two s_i_s() calls = oopsie), > but on my beloved old Q6600 or a big box, it'll matter a lot to > something. NUMA balance will deal with big boxen, my trusty old Q6600 > will likely get all upset with some localhost stuff. > > -Mike > > -- > 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/ > -- 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/
