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 07:18:29 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Alex Shi <alex.shi@...el.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Lutomirski <luto@....edu>,
	Jan Beulich <JBeulich@...e.com>, borislav.petkov@....com,
	arnd@...db.de, 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, 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 5/24/2012 6:54 AM, Alex Shi wrote:
> On 05/24/2012 09:39 PM, Arjan van de Ven wrote:
> 
>> On 5/24/2012 6:23 AM, Peter Zijlstra wrote:
>>> On Thu, 2012-05-24 at 06:19 -0700, Andrew Lutomirski wrote:
>>>>
>>>> A decent heuristic might be to prefer idle SMT siblings for TLB
>>>> invalidation.  I don't know what effect that would have on power
>>>> consumption (it would be rather bad if idling one SMT thread while the
>>>> other one is busy saves much power). 
>>
>> we really really shouldn't do flushing of tlb's on only one half of SMT.
>> SMT sibblings have their own TLB pool at least on some of Intels chips.
> 
> 
> That is also the biggest question I want to know. Actually, some
> documents, wiki said the SMT sibling just has process registers and
> interrupt part, no any tlb/l1 cache etc, (like intel's doc
> vol6iss1_hyper_threading_technology.pdf).  And the patch runs well on
> NHM EP/WSM EP/NHM EX/SNB EP CPUs.
> 
> But hard to get such clearly per cpu info of SMT/HT, so, what the
> detailed Intel chips has 'TLB pool' on SMT?

all of them.

the TLB pool is shared as physical resource (dynamic or static, that
depends), but each tlb entry will be tagged for which of the two HT
pairs it's for, and on a logical level, they are completely separate as
a result (as they should be)

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