Re: [mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression

[Rong Chen]

On 3/27/20 12:57 AM, Linus Torvalds wrote:
> On Wed, Mar 25, 2020 at 10:57 PM kernel test robot
> <rong.a.chen@xxxxxxxxx> wrote:
> > FYI, we noticed a -30.5% regression of stress-ng.switch.ops_per_sec due to commit:
> >
> > commit: fd4d9c7d0c71866ec0c2825189ebd2ce35bd95b8 ("mm: slub: add missing TID bump in kmem_cache_alloc_bulk()")
> This looks odd.
>
> I would not expect the update of c->tid to have that noticeable an
> impact, even on a big machine that might be close to some scaling
> limit.
>
> It doesn't add any expensive atomic ops, and while it _could_ make a
> percpu cacheline dirty, I think that cacheline should already be dirty
> anyway under any load where this is noticeable. Plus this should be a
> relatively cold path anyway.
>
> So mind humoring me, and double-check that regression?
>
> Of course, it might be another "just magic cache placement" detail
> where code moved enough to make a difference.
>
> Or maybe it really ends up causing new tid mismatches and we end up
> failing the fast path in slub as a result. But looking at the stats
> that changed in your message doesn't make me go "yeah, that looks like
> a slub difference".
>
> So before we look more at this, I'd like to make sure that the
> regression is actually real, and not noise.
>
> Please?
>
>               Linus

Hi Linus,

We rebuilt the kernels and tested more times, but the data is constant,
we are still checking this case.

Best Regards,
Rong Chen