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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 2/5/25 15:18, K Prateek Nayak wrote:
> Hello all,
> 
> On 2/3/2025 5:17 PM, Naman Jain wrote:
>> [..snip..]
>>
>> Adding a link to the other patch which is under review.
>> https://lore.kernel.org/lkml/20241031200431.182443-1-steve.wahl@hpe.com/
>> Above patch tries to optimize the topology sanity check, whereas this
>> patch makes it optional. We believe both patches can coexist, as even
>> with optimization, there will still be some performance overhead for
>> this check.
> 
> I would like to discuss this parallelly here. Going back to the original
> problem highlighted in [1], the topology_span_sane() came to be as a
> result of how drivers/base/arch_topology.c computed the
> cpu_coregroup_mask().
> 
> [1] https://lore.kernel.org/all/1577088979-8545-1-git-send-email- 
> prime.zeng@hisilicon.com/
> 
> Originally described problematic topology is as follows:
> 
>      **************************
>      NUMA:               0-2,  3-7
>      core_siblings:   0-3,  4-7
>      **************************
> 
> with the problematic bit in the handling being:
> 
>      const struct cpumask *cpu_coregroup_mask(int cpu)
>      {
>              const cpumask_t *core_mask = 
> cpumask_of_node(cpu_to_node(cpu));
> 
>              ...
> 
>              if (last_level_cache_is_valid(cpu)) {
>                      /* If the llc_sibling is subset of node return 
> llc_sibling */
>                      if (cpumask_subset(&cpu_topology[cpu].llc_sibling, 
> core_mask))
>                              core_mask = &cpu_topology[cpu].llc_sibling;
> 
>                      /* else the core_mask remains cpumask_of_node() */
>              }
> 
>              ...
> 
>              return core_mask;
>      }
> 
> For CPU3, the llc_sibling 0-3 is not a subset of node mask 3-7, and the
> fallback is to use 3-7. For CPUs 4-7, the llc_sibling 4-7 is a subset of
> the node mask 3-7 and the coremask is returned a 4-7.
> 
> In case of x86 (and perhaps other arch too) the arch/x86 bits ensure
> that this inconsistency never happens for !NUMA domains using the
> topology IDs. If a set of IDs match between two CPUs, the CPUs are set
> in each other's per-CPU topology mask (see link_mask() usage and
> match_*() functions in arch/x86/kernel/smpboot.c)
> 
> If the set of IDs match with one CPU, it should match with all other
> CPUs set in the cpumask for a given topology level. If it doesn't match
> with one, it will not match with any other CPUs in the cpumask either.
> The cpumasks of two CPUs can either be equal or disjoint at any given
> level. Steve's optimization reverses this to check if the the cpumask
> of set of CPUs match.
> 
> Have there been any reports on an x86 system / VM where
> topology_span_sane() was tripped? Looking at the implementation it
> does not seem possible (at least to my eyes) with one exception of
> AMD Fam 0x15 processors which set "cu_id" and match_smt() will look at
> cu_id if the core_id doesn't match between 2 CPUs. It may so happen
> that core IDs may match with one set of CPUs and cu_id may match with
> another set of CPUs if the information from CPUID is faulty.
> 
> What I'm getting to is that the arch specific topology parsing code
> can set a "SDTL_ARCH_VERIFIED" flag indicating that the arch specific
> bits have verified that the cpumasks are either equal or disjoint and
> since sched_debug() is "false" by default, topology_span_sane() can
> bail out if:
> 
>      if (!sched_debug() && (tl->flags & SDTL_ARCH_VERIFIED))
>          return;
> 

it would simpler to use sched_debug(). no?

Since it can be enabled at runtime by "echo Y > verbose", if one one 
needs to enable even after boot. Wouldn't that suffice to run 
topology_span_sane by doing a hotplug?

> In case arch specific parsing was wrong, "sched_verbose" can always
> be used to double check the topology and for the arch that require
> this sanity check, Steve's optimized version of
> topology_span_sane() can be run to be sure of the sanity.
> 
> All this justification is in case folks want to keep
> topology_span_sane() around but if no one cares, Naman and Saurabh's
> approach works as intended.
>