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

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Message-ID: <20131214141902.GA16438@laptop.programming.kicks-ass.net>
Date:	Sat, 14 Dec 2013 15:19:02 +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 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.
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