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 10:45:36 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alex Shi" <alex.shi@...el.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

>>> On 24.05.12 at 11:02, Alex Shi <alex.shi@...el.com> wrote:

>>>
>>> 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.

That's exactly what I implied (I was merely pointing this out to
prove that using per-CPU data here is possible).

Jan

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