linux-kernel - Re: [PATCH 2/3] x86/flush_tlb: try flush_tlb_single one by one in flush_tlb

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Date:	Wed, 02 May 2012 11:21:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Alex Shi <alex.shi@...el.com>
CC:	andi.kleen@...el.com, tim.c.chen@...ux.intel.com, jeremy@...p.org,
	chrisw@...s-sol.org, akataria@...are.com, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, rostedt@...dmis.org,
	fweisbec@...il.com, luto@....edu, avi@...hat.com,
	len.brown@...el.com, paul.gortmaker@...driver.com,
	dhowells@...hat.com, fenghua.yu@...el.com, borislav.petkov@....com,
	yinghai@...nel.org, cpw@....com, steiner@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86/flush_tlb: try flush_tlb_single one by one in
 flush_tlb_range

On 04/28/2012 04:50 AM, Alex Shi wrote:
> x86 has no flush_tlb_range support in instruction level. Currently the
> flush_tlb_range just implemented by flushing all page table. That is not
> the best solution for all scenarios. In fact, if we just use 'invlpg' to
> flush few lines from TLB, we can get the performance gain from later
> remain TLB lines accessing.
>
> But the 'invlpg' instruction costs much of time. Its execution time can
> compete with cr3 rewriting, and even a bit more on SNB CPU.
>
> So, on a 512 4KB TLB entries CPU, the balance points is at:
> 512 * 100ns(assumed TLB refill cost) =
> x(TLB flush entries) * 140ns(assumed invlpg cost)
>
> Here, x is about 360, that is about 5/8 of 512 entries.
>
> But with the mysterious CPU pre-fetcher and page miss handler Unit, the
> assumed TLB refill cost is far lower then 100ns in sequential access. And
> 2 HT siblings in one core makes the memory access more faster if they are
> accessing the same memory. So, in the patch, I just do the change when
> the target entries is less than 1/16 of whole active tlb entries.
> Actually, I have no data support for the percentage '1/16', so any
> suggestions are welcomed.

The numbers speak for themselves, 1/16th seems to work
fine on current generation CPUs.

> +
> +#define FLUSHALL_BAR	16

However, since this is a somewhat arbitrary number, it
would be good to accompany this #define with a multi-line
comment explaining your reasoning for choosing this number.

That will make it easy to re-evaluate in the future, if neeeded.
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