Re: [tip: sched/core] sched: Add cluster scheduler level for x86

[Peter Zijlstra]

On Thu, Oct 21, 2021 at 11:32:36PM +1300, Barry Song wrote:
> On Thu, Oct 21, 2021 at 9:43 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Wed, Oct 20, 2021 at 10:36:19PM +0200, Peter Zijlstra wrote:
> >
> > > OK, I think I see what's happening.
> > >
> > > AFAICT cacheinfo.c does *NOT* set l2c_id on AMD/Hygon hardware, this
> > > means it's set to BAD_APICID.
> > >
> > > This then results in match_l2c() to never match. And as a direct
> > > consequence set_cpu_sibling_map() will generate cpu_l2c_shared_mask with
> > > just the one CPU set.
> > >
> > > And we have the above result and things come unstuck if we assume:
> > >   SMT <= L2 <= LLC
> > >
> > > Now, the big question, how to fix this... Does AMD have means of
> > > actually setting l2c_id or should we fall back to using match_smt() for
> > > l2c_id == BAD_APICID ?
> >
> > The latter looks something like the below and ought to make EPYC at
> > least function as it did before.
> >
> >
> > ---
> >  arch/x86/kernel/smpboot.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index 849159797101..c2671b2333d1 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -472,7 +472,7 @@ static bool match_l2c(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
> >
> >         /* Do not match if we do not have a valid APICID for cpu: */
> >         if (per_cpu(cpu_l2c_id, cpu1) == BAD_APICID)
> > -               return false;
> > +               return match_smt(c, o); /* assume at least SMT shares L2 */
> 
> Rather than making a fake cluster_cpus and cluster_cpus_list which
> will expose to userspace
> through /sys/devices/cpus/cpux/topology, could we just fix the
> sched_domain mask by the
> below?

I don't think it's fake; SMT fundamentally has to share all cache
levels. And having the sched domains differ in setup from the reported
(nonsensical) topology also isn't appealing.