Messages in this thread | | | Subject | Re: [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE_AFFINE domain with cpu | From | Mike Galbraith <> | Date | Fri, 09 May 2014 17:24:02 +0200 |
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On Fri, 2014-05-09 at 10:22 -0400, Rik van Riel wrote: > On 05/09/2014 03:34 AM, Mike Galbraith wrote: > > On Fri, 2014-05-09 at 01:27 -0400, Rik van Riel wrote: > >> On Thu, 08 May 2014 22:20:25 -0400 > >> Rik van Riel <riel@redhat.com> wrote: > >> > >>> Looks like SD_BALANCE_WAKE is not gotten from the sd flags at > >>> all, but passed into select_task_rq by try_to_wake_up, as a > >>> hard coded sd_flags argument. > >> > >>> Should we do that, if SD_WAKE_BALANCE is not set for any sched domain? > >> > >> I answered my own question. The sd_flag SD_WAKE_BALANCE simply means > >> "this is a wakeup of a previously existing task, please place it > >> properly". > >> > >> However, it appears that the current code will fall back to the large > >> loop with select_idlest_group and friends, if prev_cpu and cpu are not > >> part of the same SD_WAKE_AFFINE sched domain. That is a bug... > > > > ttwu(): cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags); > > > > We pass SD_BALANCE_WAKE for a normal wakeup, so sd will only be set if > > we encounter a domain during traversal where Joe User has told us to do > > (expensive) wake balancing before we hit a domain shared by waker/wakee. > > > > The user can turn SD_WAKE_AFFINE off beyond socket, and we'll not pull > > cross node on wakeup. > > > > Or, you could create an override button to say despite SD_WAKE_AFFINE > > perhaps having been set during domain construction (because of some > > pseudo-random numbers), don't do that if we have a preferred node, or > > just make that automatically part of having numa scheduling enabled, and > > don't bother wasting cycles if preferred && this != preferred. > > That's not the problem. > > The problem is that if we do not do an affine wakeup, due to > SD_WAKE_AFFINE not being set on a top level domain, we will > not try to run p on prev_cpu, but we will fall through into > the loop with find_idlest_group, etc...
If no ->flags & SD_BALANCE_WAKE is encountered during traversal, sd remains NULL, we fall through to return prev_cpu.
-Mike
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