linux-kernel - Re: [RFC 1/2 -tip/master] x86, x2apic: minimize IPI register writes using cluster groups

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Date:	Tue, 15 Feb 2011 04:22:59 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/2 -tip/master] x86, x2apic: minimize IPI register writes
 using cluster groups


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> On 02/14/2011 02:45 PM, Ingo Molnar wrote:
> >
> >* Cyrill Gorcunov<gorcunov@...il.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
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