Re: [PATCH 2/2] sched/topology: Expose numa_mask set/clear functions to arch

From: Peter Zijlstra
Date: Fri Aug 31 2018 - 07:13:13 EST

On Fri, Aug 31, 2018 at 03:27:24AM -0700, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2018-08-29 10:02:19]:

> Powerpc lpars running on Phyp have 2 modes. Dedicated and shared.
> Dedicated lpars are similar to kvm guest with vcpupin.

Like i know what that means... I'm not big on virt. I suppose you're
saying it has a fixed virt to phys mapping.

> Shared lpars are similar to kvm guest without any pinning. When running
> shared lpar mode, Phyp allows overcommitting. Now if more lpars are
> created/destroyed, Phyp will internally move / consolidate the cores. The
> objective is similar to what autonuma tries achieves on the host but with a
> different approach (consolidating to optimal nodes to achieve the best
> possible output). This would mean that the actual underlying cpus/node
> mapping has changed.

AFAIK Linux can _not_ handle cpu:node relations changing. And I'm pretty
sure I told you that before.

> Phyp will propogate upwards an event to the lpar. The
> lpar / os can choose to ignore or act on the same.
> We have found that acting on the event will provide upto 40% improvement
> over ignoring the event. Acting on the event would mean moving the cpu from
> one node to the other, and topology_work_fn exactly does that.

How? Last time I checked there was a ton of code that relies on
cpu_to_node() not changing during the runtime of the kernel.

Stuff like the per-cpu memory allocations are done using the boot time
cpu_to_node() map for instance. Similarly, kthread creation uses the
cpu_to_node() map at the time of creation.

A lot of stuff is not re-evaluated. If you're dynamically changing the
node map, you're in for a world of hurt.

> In the case where we didn't have the NUMA sched domain, we would build the
> independent (aka overlap) sched_groups. With NUMA sched domain
> introduction, we try to reuse sched_groups (aka non-overlay). This results
> in the above, which I thought I tried to explain in

That email was a ton of confusion; you show an error and you don't
explain how you get there.

> In the typical case above, lets take 2 node, 8 core each having SMT 8
> threads. Initially all the 8 cores might come from node 0. Hence
> sched_domains_numa_masks[NODE][node1] and
> sched_domains_numa_mask[NUMA][node1] is set at sched_init_numa will have
> blank cpumasks.
> Let say Phyp decides to move some of the load to another node, node 1, which
> till now has 0 cpus. Hence we will see
> "BUG: arch topology borken \n the DIE domain not a subset of the NODE
> domain" which is probably okay. This problem is even present even before
> NODE domain was created and systems still booted and ran.

No that is _NOT_ OKAY. The fact that it boots and runs just means we
cope with it, but it violates a base assumption when building domains.

> However with the introduction of NODE sched_domain,
> init_sched_groups_capacity() gets called for non-overlay sched_domains which
> gets us into even worse problems. Here we will end up in a situation where
> sgA->sgB->sgC-sgD->sgA gets converted into sgA->sgB->sgC->sgB which ends up
> creating cpu stalls.
> So the request is to expose the sched_domains_numa_masks_set /
> sched_domains_numa_masks_clear to arch, so that on topology update i.e event
> from phyp, arch set the mask correctly. The scheduler seems to take care of
> everything else.

NAK, not until you've fixed every cpu_to_node() user in the kernel to
deal with that mask changing.

This is absolutely insane.