linux-kernel - Re: [PATCH v2 3/3] x86: Support local_flush_tlb_kernel

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Date:	Fri, 15 Jun 2012 10:13:11 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	David Howells <dhowells@...hat.com>, x86@...nel.org,
	Nick Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2 3/3] x86: Support local_flush_tlb_kernel_range

On 05/17/2012 09:51 AM, Peter Zijlstra wrote:

> On Thu, 2012-05-17 at 17:11 +0900, Minchan Kim wrote:
>>> +++ b/arch/x86/include/asm/tlbflush.h
>>> @@ -172,4 +172,16 @@ static inline void flush_tlb_kernel_range(unsigned long start,
>>>       flush_tlb_all();
>>>  }
>>>  
>>> +static inline void local_flush_tlb_kernel_range(unsigned long start,
>>> +             unsigned long end)
>>> +{
>>> +     if (cpu_has_invlpg) {
>>> +             while (start < end) {
>>> +                     __flush_tlb_single(start);
>>> +                     start += PAGE_SIZE;
>>> +             }
>>> +     } else
>>> +             local_flush_tlb();
>>> +}
> 
> 
> It would be much better if you wait for Alex Shi's patch to mature.
> doing the invlpg thing for ranges is not an unconditional win.


>From what I can tell Alex's patches have stalled.  The last post was v6
on 5/17 and there wasn't a single reply to them afaict.

According to Alex's investigation of this "tipping point", it seems that
a good generic value is 8.  In other words, on most x86 hardware, it is
cheaper to flush up to 8 tlb entries one by one rather than doing a
complete flush.

So we can do something like:

     if (cpu_has_invlpg && (end - start)/PAGE_SIZE <= 8) {
             while (start < end) {

Would this be acceptable?

Thanks,
Seth

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