linux-kernel - Re: [PATCH 2/3] x86: mm: Change tlb_flushall

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Message-ID: <52AC3A8E.7040603@linaro.org>
Date:	Sat, 14 Dec 2013 19:01:34 +0800
From:	Alex Shi <alex.shi@...aro.org>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Mel Gorman <mgorman@...e.de>, H Peter Anvin <hpa@...or.com>,
	Linux-X86 <x86@...nel.org>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH 2/3] x86: mm: Change tlb_flushall_shift for IvyBridge

On 12/13/2013 09:43 PM, Ingo Molnar wrote:
> 
> * Alex Shi <alex.shi@...aro.org> wrote:
> 
>> On 12/13/2013 09:02 AM, Alex Shi wrote:
>>>>> You have not replied to this concern of mine: if my concern is valid 
>>>>> then that invalidates much of the current tunings.
>>> The benefit from pretend flush range is not unconditional, since invlpg
>>> also cost time. And different CPU has different invlpg/flush_all
>>> execution time. 
>>
>> TLB refill time is also different on different kind of cpu.
>>
>> BTW,
>> A bewitching idea is till attracting me.
>> https://lkml.org/lkml/2012/5/23/148
>> Even it was sentenced to death by HPA.
>> https://lkml.org/lkml/2012/5/24/143
> 
> I don't think it was sentenced to death by HPA. What do the hardware 
> guys say, is this safe on current CPUs?

This talking is fully public, no any other info I known.
At that time, I tried core2, nhm, wsm, snd, ivb, all kinds of machine I
can get. No issue found.

And assuming a rebase patch is testing in Fengguang's testing system
from last Friday, no bad news till now.
Fengugang, x86-tlb branch on my github tree.
> 
> If yes then as long as we only activate this optimization for known 
> models (and turn it off for unknown models) we should be pretty safe, 
> even if the hw guys (obviously) don't want to promise this 
> indefinitely for all Intel HT implementations in the future, right?

Agree with you.
> 
>> That is that just flush one of thread TLB is enough for SMT/HT, 
>> seems TLB is still shared in core on Intel CPU. This benefit is 
>> unconditional, and if my memory right, Kbuild testing can improve 
>> about 1~2% in average level.
> 
> Oh, a 1-2% kbuild speedup is absolutely _massive_. Don't even think 
> about dropping this idea ... it needs to be explored.
> 
> Alas, that for_each_cpu() loop is obviously disgusting, these values 
> should be precalculated into percpu variables and such.

yes, pr-calcucatied variable would save much time.
> 
>> So could you like to accept some ugly quirks to do this lazy TLB 
>> flush on known working CPU?
> 
> it's not really 'lazy TLB flush' AFAICS but a genuine optimization: 
> only flush the TLB on the logical CPUs that need it, right? I.e. do 
> only one flush per pair of siblings.
> 
>> Forgive me if it's stupid.
> 
> I'd say measurable speedups that are safe are never ever stupid.

Thanks a lot!
> 
> And even the range-flush TLB optimization we are talking about here 
> could still be used IMO, just tone it down a bit and make it less 
> model dependent.
> 
> Thanks,
> 
> 	Ingo
> 


-- 
Thanks
    Alex
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