Re: [PATCH v1 0/8] x86, sched, mm: Speed up CPU hotplug on large systems

["Luck, Tony" <tony.luck@intel.com>]

On Thu, Oct 15, 2026 at 11:33:23AM -0400, Pasha Tatashin wrote:
>    - Patch 3 bounds cmci_rediscover() on CPU teardown. Currently,
>      cmci_rediscover() broadcasts an IPI via on_each_cpu() to all online
>      CPUs, serializing them on cmci_discover_lock while reading up to 32
>      MCA MSRs. Because any disabled MCA bank is shared at most with an
>      SMT sibling, L2 cluster peer, or die/package peer, probing a single
>      online topology peer via smp_call_function_single() (or returning
>      immediately when no shared banks were orphaned) reduces 479-CPU
>      offline time on Intel from 11.42 s to 9.63 s (-15.7%) and total time
>      from 22.50 s to 20.74 s (-7.8%).

I took this series for a spin on a timy 2-socket Icelake (36 cores, 72
threads per socket). My test was to just offfline/online the 72 logical
CPUs on socket 1, and I saw some good speedups:

	offline: 4.833s -> 3.17s
	online: 6.157s -> 3.796s

I noticed that your cmci_rediscover() improvements mean even better
offline performance if you work from high to low CPU numbers as the
shared banks are assigned to the lowest numbered CPU within the sharing
topology domain. So working from high to low avoids creating as many
orphan banks that then need to find new owners.

	reverse order offline: 2.984s

I haven't looked into the code yet. But the general direction seems
good.

-Tony