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SubjectRe: sched: Performance of Trade workload running inside VM
On Tue, 2012-02-14 at 16:58 +0530, Srivatsa Vaddagiri wrote:

> This lead me to investigate the wakeup code path closely and in
> particular select_idle_sibling(). select_idle_sibling() looks for a core
> that is fully idle, failing which causes the task to wakeup on prev_cpu
> (or cur_cpu). In particular, it does not go hunt for the least loaded
> cpu, which is what SD_BALANCE_WAKE provides.
> It seemed to me that we could have SD_BALANCE_WAKE enabled in SMT/MC
> domains atleast without losing on cache benefits. However Peterz seems
> to have noted that SD_BALANCE_WAKE can hurt sysbench.

> I have tried coming up with something that allows us to keep
> SD_BALANCE_WAKE enabled at smt/mc domains, not hurt sysbench and
> also help the Trade benchmark that I had begun investigating. The patch
> falls back to SD_BALANCE_WAKE type balance when the cpu returned by
> select_idle_cpu() is not idle.

> Index: linux-3.3-rc3-tip-a80142eb/kernel/sched/fair.c
> ===================================================================
> --- linux-3.3-rc3-tip-a80142eb.orig/kernel/sched/fair.c
> +++ linux-3.3-rc3-tip-a80142eb/kernel/sched/fair.c
> @@ -2783,7 +2783,9 @@ select_task_rq_fair(struct task_struct *
> prev_cpu = cpu;
> new_cpu = select_idle_sibling(p, prev_cpu);
> - goto unlock;
> + if (idle_cpu(new_cpu))
> + goto unlock;
> + sd = rcu_dereference(per_cpu(sd_llc, prev_cpu));
> }
> while (sd) {

Right, so the problem with this is that it might defeat wake_affine,
wake_affine tries to pull a task towards it wakeup source (irrespective
of idleness thereof).

Also, wake_balance is somewhat expensive, which seems like a bad thing
considering your workload is already wakeup heavy.

That said, there was a lot of text in your email which hid what your
actual problem was. So please try again, less words, more actual content

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