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SubjectRe: [PATCH] sched: avoid large irq-latencies in smp-balancing
From
Peter Zijlstra wrote:
> Bah, missed a hunk
>
> ---
> Subject: sched: avoid large irq-latencies in smp-balancing
>
> SMP balancing is done with IRQs disabled and can iterate the full rq. When rqs
> are large this can cause large irq-latencies. Limit the nr of iterations on
> each run.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> CC: Peter Williams <pwil3058@bigpond.net.au>

Tested-by: Gregory Haskins <ghaskins@novell.com> (as part of 23.1-rt11)

> ---
> include/linux/sched.h | 1 +
> kernel/sched.c | 15 ++++++++++-----
> kernel/sysctl.c | 8 ++++++++
> 3 files changed, 19 insertions(+), 5 deletions(-)
>
> Index: linux-2.6-2/kernel/sched.c
> ===================================================================
> --- linux-2.6-2.orig/kernel/sched.c
> +++ linux-2.6-2/kernel/sched.c
> @@ -474,6 +474,12 @@ const_debug unsigned int sysctl_sched_fe
> #define sched_feat(x) (sysctl_sched_features & SCHED_FEAT_##x)
>
> /*
> + * Number of tasks to iterate in a single balance run.
> + * Limited because this is done with IRQs disabled.
> + */
> +const_debug unsigned int sysctl_sched_nr_migrate = 32;
> +
> +/*
> * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
> * clock constructed from sched_clock():
> */
> @@ -2237,7 +2243,7 @@ balance_tasks(struct rq *this_rq, int th
> enum cpu_idle_type idle, int *all_pinned,
> int *this_best_prio, struct rq_iterator *iterator)
> {
> - int pulled = 0, pinned = 0, skip_for_load;
> + int loops = 0, pulled = 0, pinned = 0, skip_for_load;
> struct task_struct *p;
> long rem_load_move = max_load_move;
>
> @@ -2251,10 +2257,10 @@ balance_tasks(struct rq *this_rq, int th
> */
> p = iterator->start(iterator->arg);
> next:
> - if (!p)
> + if (!p || loops++ > sysctl_sched_nr_migrate)
> goto out;
> /*
> - * To help distribute high priority tasks accross CPUs we don't
> + * To help distribute high priority tasks across CPUs we don't
> * skip a task if it will be the highest priority task (i.e. smallest
> * prio value) on its new queue regardless of its load weight
> */
> @@ -2271,8 +2277,7 @@ next:
> rem_load_move -= p->se.load.weight;
>
> /*
> - * We only want to steal up to the prescribed number of tasks
> - * and the prescribed amount of weighted load.
> + * We only want to steal up to the prescribed amount of weighted load.
> */
> if (rem_load_move > 0) {
> if (p->prio < *this_best_prio)
> Index: linux-2.6-2/kernel/sysctl.c
> ===================================================================
> --- linux-2.6-2.orig/kernel/sysctl.c
> +++ linux-2.6-2/kernel/sysctl.c
> @@ -298,6 +298,14 @@ static struct ctl_table kern_table[] = {
> .mode = 0644,
> .proc_handler = &proc_dointvec,
> },
> + {
> + .ctl_name = CTL_UNNUMBERED,
> + .procname = "sched_nr_migrate",
> + .data = &sysctl_sched_nr_migrate,
> + .maxlen = sizeof(unsigned int),
> + .mode = 644,
> + .proc_handler = &proc_dointvec,
> + },
> #endif
> {
> .ctl_name = CTL_UNNUMBERED,
> Index: linux-2.6-2/include/linux/sched.h
> ===================================================================
> --- linux-2.6-2.orig/include/linux/sched.h
> +++ linux-2.6-2/include/linux/sched.h
> @@ -1466,6 +1466,7 @@ extern unsigned int sysctl_sched_batch_w
> extern unsigned int sysctl_sched_child_runs_first;
> extern unsigned int sysctl_sched_features;
> extern unsigned int sysctl_sched_migration_cost;
> +extern unsigned int sysctl_sched_nr_migrate;
> #endif
>
> extern unsigned int sysctl_sched_compat_yield;
>
>
> -
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