Messages in this thread Patch in this message |  | | | Subject | Re: [patch 5/6] sched: disable sched feature TTWU_QUEUE by default | | From | Mike Galbraith <> | | Date | Sat, 19 Nov 2011 05:30:09 +0100 |
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On Fri, 2011-11-18 at 15:03 -0800, Suresh Siddha wrote: > plain text document attachment (disable_sched_ttwu_queue.patch) > Context-switch intensive microbenchmark on a 8-socket system had > ~600K times more resched IPI's on each logical CPU with this feature enabled > by default. Disabling this features makes that microbenchmark perform 5 times > better. > > Also disabling this feature showed 2% performance improvement on a 8-socket > OLTP workload. > > More heurestics are needed when and how to use this feature by default. > For now, disable it by default.
Yeah, the overhead for very hefty switchers is high enough to increase TCP_RR latency up to 13% in my testing. I used a trylock() to generally not eat that, but leave the contended case improvement intact.
Peter suggested trying doing the IPI only when crossing cache boundaries, which worked for me as well.
--- kernel/sched.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-) Index: linux-3.0-tip/kernel/sched.c =================================================================== --- linux-3.0-tip.orig/kernel/sched.c +++ linux-3.0-tip/kernel/sched.c @@ -2784,12 +2784,34 @@ static int ttwu_activate_remote(struct t #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */ #endif /* CONFIG_SMP */ +static int ttwu_share_cache(int this_cpu, int cpu) +{ +#ifndef CONFIG_X86 + struct sched_domain *sd; + int ret = 0; + + rcu_read_lock(); + for_each_domain(this_cpu, sd) { + if (!cpumask_test_cpu(cpu, sched_domain_span(sd))) + continue; + + ret = (sd->flags & SD_SHARE_PKG_RESOURCES); + break; + } + rcu_read_unlock(); + + return ret; +#else + return per_cpu(cpu_llc_id, this_cpu) == per_cpu(cpu_llc_id, cpu); +#endif +} + static void ttwu_queue(struct task_struct *p, int cpu) { struct rq *rq = cpu_rq(cpu); #if defined(CONFIG_SMP) - if (sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) { + if (sched_feat(TTWU_QUEUE) && !ttwu_share_cache(smp_processor_id(), cpu)) { sched_clock_cpu(cpu); /* sync clocks x-cpu */ ttwu_queue_remote(p, cpu); return;
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