Re: [PATCH 5/8] x86/clear_page: add clear_page_uncached()

[Ankur Arora <ankur.a.arora@oracle.com>]

On 2020-10-14 2:12 p.m., Borislav Petkov wrote:
> On Wed, Oct 14, 2020 at 02:07:30PM -0700, Andy Lutomirski wrote:
>> I assume it’s for a little optimization of clearing more than one
>> page per SFENCE.
>>
>> In any event, based on the benchmark data upthread, we only want to do
>> NT clears when they’re rather large, so this shouldn’t be just an
>> alternative. I assume this is because a page or two will fit in cache
>> and, for most uses that allocate zeroed pages, we prefer cache-hot
>> pages. When clearing 1G, on the other hand, cache-hot is impossible
>> and we prefer the improved bandwidth and less cache trashing of NT
>> clears.
> 
> Yeah, use case makes sense but people won't know what to use. At the
> time I was experimenting with this crap, I remember Linus saying that
> that selection should be made based on the size of the area cleared, so
> users should not have to know the difference.
I don't disagree but I think the selection of cached/uncached route should
be made where we have enough context available to be able to choose to do
this.

This could be for example, done in mm_populate() or gup where if say the
extent is larger than LLC-size, it takes the uncached path.

> 
> Which perhaps is the only sane use case I see for this.
> 
>> Perhaps SFENCE is so fast that this is a silly optimization, though,
>> and we don’t lose anything measurable by SFENCEing once per page.
> 
> Yes, I'd like to see real use cases showing improvement from this, not
> just microbenchmarks.
Sure will add.

Thanks
Ankur

>