Messages in this thread Patch in this message | | | Subject | Re: [patch 1/2] sched: check for prev_cpu == this_cpu in wake_affine() | From | Mike Galbraith <> | Date | Fri, 05 Mar 2010 20:36:32 +0100 |
| |
On Fri, 2010-03-05 at 10:39 -0800, Suresh Siddha wrote: > plain text document attachment (fix_wake_affine.patch) > On a single cpu system with SMT, in the scenario of one SMT thread being > idle with another SMT thread running a task and doing a non sync wakeup of > another task, we see (from the traces) that the woken up task ends up running > on the busy thread instead of the idle thread. Idle balancing that comes > in little bit later is fixing the scernaio.
Yup, wake_affine() fails for non sync wakeup when 1 task is running. That's annoying, but making it succeed globally worries me. We need a high quality hint, and avg_overlap ain't it unfortunately, because to get accurate overlap info cross cpu, you have to double clock and update_curr() overhead. We need dirt cheap.
> But fixing this wake balance and running the woken up task directly on the > idle SMT thread improved the performance (phoronix 7zip compression workload) > by ~9% on an atom platform.
So there is profit to be had.
> During the process wakeup, select_task_rq_fair() and wake_affine() makes > the decision to wakeup the task either on the previous cpu that the task > ran or the cpu that the task is currently woken up. > > select_task_rq_fair() also goes through to see if there are any idle siblings > for the cpu that the task is woken up on. This is to ensure that we select > any idle sibling rather than choose a busy cpu.
Yeah, but with the 1 task + non-sync wakeup scenario, we miss the boat because select_idle_sibling() uses wake_affine() success as it's enabler. I did that because I couldn't think up something else which did not harm multiple buddy pairs. You can globally say sibling is idle, go for it, but that _does_ cause throughput loss during ramp up.
Best alternative I've found is to only check for an idle sibling/cache when there is exactly one task on the current cpu (ie put some faith in load balancing), then force idle sibling selection. Also not optimal.
> In the above load scenario, it so happens that the prev_cpu (that the > task ran before) and this_cpu (where it is woken up currently) are the same. And > in this case, it looks like wake_affine() returns 0 and ultimately not selecting > the idle sibling chosen by select_idle_sibling() in select_task_rq_fair(). > Further down the path of select_task_rq_fair(), we ultimately select > the currently running cpu (busy SMT thread instead of the idle SMT thread). > > Check for prev_cpu == this_cpu in wake_affine() and no need to do > any fancy stuff(and ultimately taking wrong decisions) in this case.
I have a slightly different patch for that in my tree. There's no need to even call wake_affine() since the result is meaningless.
--- kernel/sched_fair.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
Index: linux-2.6.34.git/kernel/sched_fair.c =================================================================== --- linux-2.6.34.git.orig/kernel/sched_fair.c +++ linux-2.6.34.git/kernel/sched_fair.c @@ -1547,8 +1547,14 @@ static int select_task_rq_fair(struct ta } #endif - if (affine_sd && wake_affine(affine_sd, p, sync)) - return cpu; + if (affine_sd) { + if (cpu == prev_cpu) + return cpu; + if (wake_affine(affine_sd, p, sync)) + return cpu; + if (!(affine_sd->flags & SD_BALANCE_WAKE)) + return prev_cpu; + } while (sd) { int load_idx = sd->forkexec_idx;
| |