Messages in this thread | | | Date | Tue, 15 Feb 2011 11:39:10 +0300 | Subject | Re: [RFC 1/2 -tip/master] x86, x2apic: minimize IPI register writes using cluster groups | From | Cyrill Gorcunov <> |
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On 2/15/11, Ingo Molnar <mingo@elte.hu> wrote: > > * Cyrill Gorcunov <gorcunov@gmail.com> wrote: > >> On 02/14/2011 02:45 PM, Ingo Molnar wrote: >> > >> >* Cyrill Gorcunov<gorcunov@gmail.com> wrote: >> > >> >>In the case of x2apic cluster mode we can group IPI register writes >> >> based on the >> >>cluster group instead of individual per-cpu destiantion messages. This >> >> reduces the >> >>apic register writes and reduces the amount of IPI messages (in the best >> >> case we >> >>can reduce it by a factor of 16). >> >> >> >>With this change, microbenchmark measuring the cost of >> >> flush_tlb_others(), with >> >>the flush tlb IPI being sent from a cpu in the socket-1 to all the >> >> logical cpus in >> >>socket-2 (on a Westmere-EX system that has 20 logical cpus in a socket) >> >> is 3x >> >>times better now (compared to the former 'send one-by-one' algorithm). >> > >> >Pretty nice! >> > >> >I have a few structural and nitpicking comments: >> >> Thanks a lot for review, Ingo! I'll address all the nits during this week. >> >> ... >> > >> >>+void x2apic_init_cpu_notifier(void) >> >>+{ >> >>+ int cpu = smp_processor_id(); >> >> >> >>+ zalloc_cpumask_var(&per_cpu(cpus_in_cluster, cpu), GFP_KERNEL); >> >>+ zalloc_cpumask_var(&per_cpu(ipi_mask, cpu), GFP_KERNEL); >> >>+ BUG_ON(!per_cpu(cpus_in_cluster, cpu) || !per_cpu(ipi_mask, cpu)); >> > >> >Such a BUG_ON() is not particularly user friendly - and this could >> > trigger during >> >CPU hotplug events, i.e. while the system is fully booted up, right? >> > >> >Thanks, >> > >> > Ingo >> >> Yup is not that much friendly but it's called during system bootup, >> hotplug events are handled by >> >> +static int __cpuinit >> +cluster_setup(struct notifier_block *nfb, unsigned long action, void >> *hcpu) >> +{ >> + unsigned int cpu = (unsigned long)hcpu; >> + int err = 0; >> + >> + switch (action) { >> + case CPU_UP_PREPARE: >> + zalloc_cpumask_var(&per_cpu(cpus_in_cluster, cpu), GFP_KERNEL); >> + zalloc_cpumask_var(&per_cpu(ipi_mask, cpu), GFP_KERNEL); >> + if (!per_cpu(cpus_in_cluster, cpu) || !per_cpu(ipi_mask, cpu)) { >> + free_cpumask_var(per_cpu(cpus_in_cluster, cpu)); >> + free_cpumask_var(per_cpu(ipi_mask, cpu)); >> + err = -ENOMEM; >> + } >> + break; >> >> so it returns -ENOMEM if failed. And btw just noted that we forgot to make >> x2apic_init_cpu_notifier being in __init section. >> >> Or I miss something? > > Is there no GFP_NOFAIL or GFP_FAIL_ON_PANIC variant that could be used the > 'must not > fail' property of the boot-time allocation? > > Thanks, > > Ingo >
If only i'm not missing something obvious we can set GFP_NOFAIL ending in endless loop if allocation failed (slab should be already running at this point of boot). Probably another option might be to switch to no-apic mode if there is no enough memory to allocate this masks (though i guess if allocation failed at this point we likely to fail in further allocations in kernel anyway)
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