Re: [PATCH v3] sched/topology: Enable topology_span_sane check only for debug builds

Naman Jain
2025-02-06T09:48:01Z

On 2/6/2025 2:40 PM, K Prateek Nayak wrote:
> Hello Peter,
>
> On 2/5/2025 3:46 PM, Peter Zijlstra wrote:
>> On Wed, Feb 05, 2025 at 03:43:54PM +0530, K Prateek Nayak wrote:
>>> Hello Peter,
>>>
>>> Thank you for the background!
>>>
>>> On 2/5/2025 3:25 PM, Peter Zijlstra wrote:
>>>> On Wed, Feb 05, 2025 at 03:18:24PM +0530, K Prateek Nayak wrote:
>>>>
>>>>> Have there been any reports on an x86 system / VM where
>>>>> topology_span_sane() was tripped?
>>>>
>>>> At the very least Intel SNC 'feature' tripped it at some point. They
>>>> figured it made sense to have the LLC span two nodes.
>
> I'm 99% sure that this might have been the topology_sane() check on
> the x86 side and not the topology_span_sane() check in
> kernel/sched/topology.c
>
> I believe one of the original changes that did the plumbing for SNC was
> commit 2c88d45edbb8 ("x86, sched: Treat Intel SNC topology as default,
> COD as exception") from Alison where they mentions that they saw the
> following splat when running with SNC:
>
>     sched: CPU #3's llc-sibling CPU #0 is not on the same node! [node:
> 1 != 0]. Ignoring dependency.
>
> This comes from the topology_sane() check in arch/x86/boot/smpboot.c
> and match_llc() on x86 side was modified to work around that.
>
>>>>
>>>> But I think there were some really dodgy VMs too.
>
> For VMs too, it is easy to trip topology_sane() check on x86 side. With
> QEMU, I can run:
>
>     qemu-system-x86_64 -enable-kvm -cpu host \
>     -smp cpus=32,sockets=2,cores=8,threads=2 \
>     ...
>     -numa node,cpus=0-7,cpus=16-23,memdev=m0,nodeid=0 \
>     -numa node,cpus=8-15,cpus=24-31,memdev=m1,nodeid=1 \
>     ...
>
> and I get:
>
>     sched: CPU #8's llc-sibling CPU #0 is not on the same node! [node:
> 1 != 0]. Ignoring dependency.
>
> This is because consecutive CPUs (0-1,2-3,...) are SMT siblings and
> CPUs 0-15 are on the same socket as a result of how QEMU presents
> MADT to the guest but then I go ahead and mess things up by saying
> CPUs 0-7,16-23 are on one NUMA node, and the rest are on the other.
>
> I still haven't managed to trip topology_span_sane() tho.
>
>>>>
>>>> But yeah, its not been often. But basically dodgy BIOS/VM data can mess
>>>> up things badly enough for it to trip.
>>>
>>> Has it ever happened without tripping the topology_sane() check first
>>> on the x86 side?
>
> What topology_span_sane() does is, it iterates over all the CPUs at a
> given topology level and makes sure that the cpumask for a CPU at
> that domain is same as the cpumask of every other CPU set on that mask
> for that topology level.
>
> If two CPUs are set on a mask, they should have the same mask. If CPUs
> are not set on each other's mask, the masks should be disjoint.
>
> On x86, the way set_cpu_sibling_map() works, CPUs are set on each other's
> shared masks iff match_*() returns true:
>
> o For SMT, this means:
>
>   - If X86_FEATURE_TOPOEXT is set:
>     - pkg_id must match.
>     - die_id must match.
>     - amd_node_id must match.
>     - llc_id must match.
>     - Either core_id or cu_id must match. (*)
>     - NUMA nodes must match.
>
>   - If !X86_FEATURE_TOPOEXT:
>     - pkg_id must match.
>     - die_id must match.
>     - core_id must match.
>     - NUMA nodes must match.
>
> o For CLUSTER this means:
>
>   - If l2c_id is not populated (== BAD_APICID)
>     - Same conditions as SMT.
>
>   - If l2c_id is populated (!= BAD_APICID)
>     - l2c_id must match.
>     - NUMA nodes must match.
>
> o For MC it means:
>
>   - llc_id must be populated (!= BAD_APICID) and must match.
>   - If INTEL_SNC: pkg_id must match.
>   - If !INTEL_SNC: NUMA nodes must match.
>
> o For PKG domain:
>   - Inserted only if !x86_has_numa_in_package.
>   - CPUs should be in same NUMA node.
>
> All in all, other that the one (*) decision point, everything else has
> to strictly match for CPUs to be set in each other's CPU mask. And if
> they match with one CPU, they should match will all other CPUs in mask
> and it they mismatch with one, they should mismatch with all leading
> to link_mask() never being called.
>
> This is why I think that the topology_span_sane() check is redundant
> when the x86 bits have already ensured masks cannot overlap in all
> cases except for potentially in the (*) case.
>
> So circling back to my original question around "SDTL_ARCH_VERIFIED",
> would folks be okay to an early bailout from topology_span_sane() on:
>
>     if (!sched_debug() && (tl->flags & SDTL_ARCH_VERIFIED))
>         return;
>
> and more importantly, do folks care enough about topology_span_sane()
> to have it run on other architectures and not just have it guarded
> behind just "sched_debug()" which starts off as false by default?
>
> (Sorry for the long answer explaining my thought process.)
>

Thanks for sharing your valuable insights.
I am sorry, I could not find SDTL_ARCH_VERIFIED in linux-next tip. Am I
missing something?

Regards,
Naman