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    SubjectRe: [PATCH] sched: avoid large irq-latencies in smp-balancing
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    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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