linux-kernel - Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings of SMT

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Date:	Thu, 24 May 2012 17:02:20 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	borislav.petkov@....com, arnd@...db.de,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	akinobu.mita@...il.com, eric.dumazet@...il.com, fweisbec@...il.com,
	rostedt@...dmis.org, hughd@...gle.com, jeremy@...p.org,
	len.brown@...el.com, tony.luck@...el.com, yongjie.ren@...el.com,
	kamezawa.hiroyu@...fujitsu.com, seto.hidetoshi@...fujitsu.com,
	penberg@...nel.org, yinghai@...nel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, ak@...ux.intel.com, luto@....edu,
	avi@...hat.com, dhowells@...hat.com, mingo@...hat.com,
	riel@...hat.com, cpw@....com, steiner@....com,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	hpa@...or.com
Subject: Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings
 of SMT


>>
>> I just measured the cost of this function on my Romely EP(32 LCPUs) with
>> cpumask_t and NR_CPUS = 32/256/512/4096, the cost are similar with
>> 256/512/4096 and that increased about 20% time cost from 32.
>>
>> I also tried to use cpumask_var_t and alloc it in heap(use
>> CPUMASK_OFFSTACK), actually, it cost same time with cpumask_t in stack.
>> But, the allocation bring another big cost. So, I use cpumask_t in stack.
>> The performance gain data in commit log is getting with NR_CPUS = 256.
> 
> Perhaps using a per-CPU cpumask would be the better choice here


See.

> (I can't see how preemption could validly be enabled when this
> code is utilized).


Sorry, What's your meaning here?, the function is always in pre-empt
safe mode.

> 
> Jan
> 


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