* Borislav Petkov <b...@suse.de> wrote:
> On Mon, Nov 07, 2016 at 03:07:46PM +0100, Ingo Molnar wrote:
> > - cache domains might be seriously mixed up, resulting in serious drop in
> > performance.
> >
> > - or domains might be partitioned 'wrong' but not catastrophically
> > wrong, resulting in a minor performance drop (if at all)
>
> Something between the two.
>
> Here's some debugging output from set_cpu_sibling_map():
>
> [ 0.202033] smpboot: set_cpu_sibling_map: cpu: 0, has_smt: 0, has_mp: 1
> [ 0.202043] smpboot: set_cpu_sibling_map: first loop, llc(this): 65528, o:
> 0, llc(o): 65528
> [ 0.202058] smpboot: set_cpu_sibling_map: first loop, link mask smt
>
> so we link it into the SMT mask even if has_smt is off.
>
> [ 0.202067] smpboot: set_cpu_sibling_map: first loop, link mask llc
> [ 0.202077] smpboot: set_cpu_sibling_map: second loop, llc(this): 65528,
> o: 0, llc(o): 65528
> [ 0.202091] smpboot: set_cpu_sibling_map: second loop, link mask die
>
> I've attached the debug diff.
>
> And since those llc(o), i.e. the cpu_llc_id of the *other* CPU in the
> loops in set_cpu_sibling_map() underflows, we're generating the funniest
> thread_siblings masks and then when I run 8 threads of nbench, they get
> spread around the LLC domains in a very strange pattern which doesn't
> give you the normal scheduling spread one would expect for performance.
>
> And this is just one workload - I can't imagine what else might be
> influenced by this funkiness.
>
> Oh and other things like EDAC use cpu_llc_id so they will be b0rked too.
So the point I tried to make is that to people doing -stable backporting
decisions
this description you just gave is much more valuable than the previous
changelog.
> So we absolutely need to fix that cpu_llc_id thing.
Absolutely!
Thanks,
Ingo
