Re: [RFC PATCH] sched/topology: Introduce NUMA identity node sched domain

[Suravee Suthikulpanit]

On 8/10/17 23:41, Peter Zijlstra wrote:
> On Thu, Aug 10, 2017 at 10:20:52AM -0500, Suravee Suthikulpanit wrote:
> > On AMD Family17h-based (EPYC) system, a NUMA node can contain
> > upto 8 cores (16 threads) with the following topology.
> >
> >              ----------------------------
> >          C0  | T0 T1 |    ||    | T0 T1 | C4
> >              --------|    ||    |--------
> >          C1  | T0 T1 | L3 || L3 | T0 T1 | C5
> >              --------|    ||    |--------
> >          C2  | T0 T1 | #0 || #1 | T0 T1 | C6
> >              --------|    ||    |--------
> >          C3  | T0 T1 |    ||    | T0 T1 | C7
> >              ----------------------------
> >
> > Here, there are 2 last-level (L3) caches per NUMA node. A socket can
> > contain upto 4 NUMA nodes, and a system can support upto 2 sockets.
> > With full system configuration, current scheduler creates 4 sched
> > domains:
> >
> >   domain0 SMT       (span a core)
> >   domain1 MC        (span a last-level-cache)
>
> Right, so traditionally we'd have the DIE level do that, but because
> x86_has_numa_in_package we don't generate that, right?

That's correct.

> >   domain2 NUMA      (span a socket: 4 nodes)
> >   domain3 NUMA      (span a system: 8 nodes)
> >
> > Note that there is no domain to represent cpus spaning a NUMA node.
> > With this hierachy of sched domains, the scheduler does not balance
> > properly in the following cases:
> >
> > Case1:
> > When running 8 tasks, a properly balanced system should
> > schedule a task per NUMA node. This is not the case for
> > the current scheduler.
> >
> > Case2:
> > When running 'taskset -c 0-7 <a_program_with_8_independent_threads>',
> > a properly balanced system should schedule 8 threads on 8 cpus
> > (e.g. T0 of C0-C7).  However, current scheduler would schedule
> > some threads on the same cpu, while others are idle.
>
> Sure.. could you amend with a few actual performance numbers?

Sure.

> > [...]
> > @@ -1445,9 +1448,24 @@ void sched_init_numa(void)
> >  		tl[i] = sched_domain_topology[i];
> >
> >  	/*
> > +	 * Ignore the NUMA identity level if it has the same cpumask
> > +	 * as previous level. This is the case for:
> > +	 *   - System with last-level-cache (MC) sched domain span a NUMA node.
> > +	 *   - System with DIE sched domain span a NUMA node.
> > +	 *
> > +	 * Assume all NUMA nodes are identical, so only check node 0.
> > +	 */
> > +	if (!cpumask_equal(sched_domains_numa_masks[0][0], tl[i-1].mask(0)))
> > +		tl[i++] = (struct sched_domain_topology_level){
> > +			.mask = sd_numa_mask,
> > +			.numa_level = 0,
> > +			SD_INIT_NAME(NUMA_IDEN)
>
> Shall we make that:
>
> 			SD_INIT_NAME(NODE),
>
> instead?

Sounds good.

> > +		};
>
> This misses a set of '{}'. While C doesn't require it, out coding style
> warrants blocks around any multi-line statement.
>
> So what you've forgotten to mention is that for those systems where the
> LLC == NODE this now superfluous level gets removed by the degenerate
> code. Have you verified that does the right thing?

Let me check with that one and get back.

Thanks,
Suravee