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

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Date:	Sat, 14 Dec 2013 15:27:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Shi <alex.shi@...aro.org>
Cc:	Ingo Molnar <mingo@...nel.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

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.
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