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 21:54:29 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Arjan van de Ven <arjan@...ux.intel.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 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?

> 
> Also, note that on anything sane, we flush the tlb's in software before
> going to an Idle state, so that we don't have to wake idle cpus up to
> flush their TLBs (except for "global tlbs", but those change very very
> very rarely hopefully)
> 
>>
>> Right, I've never really understood how C-states and SMT go together.
>> Arjan recently implied waking a thread sibling from C-state was
>> 'expensive' which on first thought seems daft, the core is running
>> already.
> 
> in order to wake *anything* you need to send an IPI to it, it has to
> exit the idle loop etc etc. It's not expensive-expensive, but it
> certainly is not free either.


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