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SubjectRe: [patch] restore sched_exec load balance heuristics

* Ken Chen <kenchen@google.com> wrote:

> We've seen long standing performance regression on sys_execve for several
> upstream kernels, largely on workload that does heavy execve. The main
> reason for the regression was due to a change in sched_exec load balance
> heuristics. For example, on 2.6.11 kernel, the "exec" task will run on
> the same cpu if that is the only task running. However, 2.6.13 and onward
> kernels will go around the sched-domain looking for most idle CPU (which
> doesn't treat task exec'ing as an idle CPU). Thus bouncing the exec'ing
> task all over the place which leads to poor CPU cache and numa locality.
> (The workload happens to share common data between subsequent exec program).
>
> This execve heuristic was removed in upstream kernel by this git commit:
>
> commit 68767a0ae428801649d510d9a65bb71feed44dd1
> Author: Nick Piggin <nickpiggin@yahoo.com.au>
> Date: Sat Jun 25 14:57:20 2005 -0700
>
> [PATCH] sched: schedstats update for balance on fork
> Add SCHEDSTAT statistics for sched-balance-fork.
>
> >From the commit description, it appears that deleting the heuristics
> was an accident, as the commit is supposedly just for schedstats.
>
> So, restore the sched-exec load balancing if exec'ing task is the only
> task running on that specific CPU. The logic make sense: newly exec
> program should continue to run on current CPU as it doesn't change any
> load imbalance nor does it help anything by bouncing to another idle
> CPU. By keeping on the same CPU, it preserves cache and numa locality.
>
> Signed-off-by: Ken Chen <kenchen@google.com>
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index e8819bc..4ad1907 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2873,7 +2873,12 @@ out:
> */
> void sched_exec(void)
> {
> - int new_cpu, this_cpu = get_cpu();
> + int new_cpu, this_cpu;
> +
> + if (this_rq()->nr_running <= 1)
> + return;
> +
> + this_cpu = get_cpu();
> new_cpu = sched_balance_self(this_cpu, SD_BALANCE_EXEC);
> put_cpu();
> if (new_cpu != this_cpu)

ok, this should be solved - but rather at the level of
sched_balance_self(): it should never migrate this task over to
another cpu, it should take away this task's load from the current
CPU's load when considering migration.

Ingo


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