Re: [PATCH v2] sched/topology: Add a cpus_read_lock to rebuild_sched_domains()

[Shrikanth Hegde]

On 8/19/26 11:24 PM, Shrikanth Hegde wrote:
> Hi Sebastian.
>
> On 8/13/26 1:08 PM, Sebastian Andrzej Siewior wrote:
> > A read from /proc/sys/kernel/sched_rt_runtime_us leads to backtrace due
> > to missing cpu_hotplug_lock with CONFIG_CPUSETS=n. The callchain is
> > sched_rt_handler() -> partition_sched_domains() -> sched_cache_set() ->
> > static_key_enable_cpuslocked(&sched_cache_present).
> >
> > sched_cache_set() itself is also invoked from sched_init_domains() which
> > is early during the boot, holding just the sched_domains_mutex_lock().
> > Here is no warning because it happens before user space is running (and
> > hotplug operations are not possible).
> >
> > There is also sched_cache_active_set() which acquires the hotplug lock
> > before invoking any of the _cpuslocked() functions.
> >
> > This is only a problem with CONFIG_CPUSETS=n because in the =y case the
> > other implementation of rebuild_sched_domains acquires the CPU-hotplug
> > lock.
> >
> > Acquire CPU hotplug lock before in rebuild_sched_domains(), before
> > partition_sched_domains() is invoked for the CONFIG_CPUSETS=n case.
> >
> > Fixes: a7660ce1590fc ("sched/cache: Fix has_multi_llcs iff at least one partition has multiple LLCs")
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> > ---
> > v1…v2: https://lore.kernel.org/all/20260812095800.gl06ANul@xxxxxxxxxxxxx/
> >    - Move the lock from partition_sched_domains() to
> >      rebuild_sched_domains() in the CONFIG_CPUSETS=n since this is the
> >      only affected case. Noticed by Yu C Chen and Tim Chen.
> >
> >   include/linux/cpuset.h | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> > index 65d76a38974ba..bf3999daa080a 100644
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -273,6 +273,7 @@ static inline void dl_rebuild_rd_accounting(void)
> >   static inline void rebuild_sched_domains(void)
> >   {
> > +    guard(cpus_read_lock)();
> >       partition_sched_domains(1, NULL, NULL);
> >   }
>
> Now, both rebuild_sched_domains have the same code. I guess it is worth
> moving it out of CONFIG_CPUSETS.

Ignore my comments. I clearly need to get rest.

> kernel/cgroup/cpuset.c:
> void rebuild_sched_domains(void)
> {
>          cpus_read_lock();
>          rebuild_sched_domains_cpuslocked();
>          cpus_read_unlock();
> }

Sorry for the noise.
Looks good to me.

Reviewed-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>