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
