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

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Message-ID: <4D5945C8.4080108@gmail.com>
Date:	Mon, 14 Feb 2011 18:10:00 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

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?

-- 
     Cyrill
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