Re: [PATCH v4 4/7] x86/tlb: fall back to flush all when meet a THP large page

[Alex Shi <alex.shi@intel.com>]

On 05/10/2012 06:40 PM, Borislav Petkov wrote:

> On Thu, May 10, 2012 at 11:29:05AM +0200, Peter Zijlstra wrote:
>> On Thu, 2012-05-10 at 13:00 +0800, Alex Shi wrote:
>>> We don't need to flush large pages by PAGE_SIZE step, that just waste
>>> time. and actually, large page don't need 'invlpg' optimizing according
>>> to our macro benchmark. So, just flush whole TLB is enough for them.
>>>
>>> The following result is tested on a 2CPU * 4cores * 2HT NHM EP machine,
>>> with THP 'always' setting.
>>
>> What does it do when you disable THP? That has_large_page() thing is a
>> massive amount of pointer chasing..
> 
> Yeah, this looks like a bit of a overhead. Don't we have some per-mm
> accounting of whether that mm struct has hugepages in mm/huge_memory.c,
> i.e. something like what collapse_huge_page() does, for example, at the
> end by incrementing khugepaged_pages_collapsed but in a per-mm variable?
> 
> And make this part of the THP code so we get it for free here.
> 
> Is Andrea on the CC list... hm, no, CCed.


Andrea has said there is no easy way to know if there is a large page in
mm or vma.

Actually, has_large_page just called only once, that due to the
act_entries limit. But your opinion is worth to consider, the only one
calling can be avoid if the 'start' address is not align on HPAGE_SIZE.



> 
> Andrea?
>