linux-kernel - Re: [patch 1/2] x86, x2apic: minimize IPI register writes using cluster groups v4

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Date:	Mon, 02 May 2011 18:02:18 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Suresh Siddha <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 1/2] x86, x2apic: minimize IPI register writes using cluster
 groups v4

On 05/02/2011 05:22 PM, Ingo Molnar wrote:
> 
> * Cyrill Gorcunov <gorcunov@...nvz.org> 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.
> 
> typo.
> 

ok, will fix, thanks.

>> 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).
> 
> What kind of microbenchmark was this, could the actual results and measurement 
> methods be shared as well?

Suresh, could you please post the microbenchmark?
...
>> Index: tip-linux-2.6/arch/x86/kernel/apic/probe_64.c
>> ===================================================================
>> --- tip-linux-2.6.orig/arch/x86/kernel/apic/probe_64.c
>> +++ tip-linux-2.6/arch/x86/kernel/apic/probe_64.c
>> @@ -55,6 +55,15 @@ static int apicid_phys_pkg_id(int initia
>>  void __init default_setup_apic_routing(void)
>>  {
>>  
>> +	/*
>> +	 * FIXME:
>> +	 *
>> +	 * Cleanup the apic routing selection by having an apic driver specific
>> +	 * selection routine. Then all we need to do here is iterate through
>> +	 * them to finalize the apic selection. That would get rid of the
>> +	 * ifdef mess and most of the code here.
>> +	 */
>> +
>>  	enable_IR_x2apic();
>>  
>>  #ifdef CONFIG_X86_X2APIC
>> @@ -71,7 +80,9 @@ void __init default_setup_apic_routing(v
>>  #endif
>>  
>>  	if (apic == &apic_flat && num_possible_cpus() > 8)
>> -			apic = &apic_physflat;
>> +		apic = &apic_physflat;
>> +	else if (apic == &apic_x2apic_cluster)
>> +		x2apic_init_cpu_notifier();
> 
> 
> Why is there an x2apic specific function in the generic 
> default_setup_apic_routing() function?
> 
> Instead of that it would be cleaner to extend the apic driver functions with an 
> init method, which would be filled in for x2apic and left NULL for the others.
> 
> Thanks,
> 
> 	Ingo

  Ingo, the idea was to merge probe_x.c completely, and put all this not into init()
but rather into apic->probe() or something like that. I don't have a clear picture
in mind yet what the best way would be, so instead of fast designed method I thought
to leave it opencoded with fixme note.

  So lets wait until Suresh post the benchmark and I will make apic->init() meanwhile.

-- 
    Cyrill
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