Messages in this thread | | | Subject | Re: [mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression | From | Rong Chen <> | Date | Fri, 27 Mar 2020 16:46:22 +0800 |
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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@intel.com> 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
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