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

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Date:	Mon, 16 Dec 2013 14:59:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alex Shi <alex.shi@...aro.org>, 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>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH 2/3] x86: mm: Change tlb_flushall_shift for IvyBridge


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Sat, Dec 14, 2013 at 03:19:02PM +0100, Peter Zijlstra wrote:
> > On Fri, Dec 13, 2013 at 10:11:05AM +0800, Alex Shi wrote:
> > > 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
> > > 
> > > 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.
> > > 
> > > So could you like to accept some ugly quirks to do this lazy TLB flush
> > > on known working CPU?
> > > Forgive me if it's stupid.
> > 
> > I think there's a further problem with that patch -- aside of it being
> > right from a hardware point of view.
> > 
> > We currently rely on the tlb flush IPI to synchronize with lockless page
> > table walkers like gup_fast().
> > 
> > By not sending an IPI to all CPUs you can get into trouble and crash the
> > kernel.
> > 
> > We absolutely must keep sending the IPI to all relevant CPUs, we can
> > choose not to actually do the flush on some CPUs, but we must keep
> > sending the IPI.
> 
> The alternative is switching x86 over to use HAVE_RCU_TABLE_FREE.

So if the kbuild speedup of 1-2% is true and reproducable then that 
might be worth doing.

Building the kernel is obviously a prime workload - and given that the 
kernel is active only about 10% of the time for a typical kernel 
build, a 1-2% speedup means a 10-20% speedup in kernel performance 
(which sounds a bit too good at first glance).

Thanks,

	Ingo
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