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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Thu, 2026-08-13 at 17:43 +0800, Ridong Chen wrote:
> 
> On 8/13/2026 3:48 PM, Sebastian Andrzej Siewior wrote:
> > + CPUSET maintainer which I missed because re-used the CCs from v1.
> > 
> > On 2026-08-13 09:38:56 [+0200], To linux-kernel@vger.kernel.org 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@linutronix.de>
> > > ---
> > > v1…v2: https://lore.kernel.org/all/20260812095800.gl06ANul@linutronix.de/
> > >    - 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);
> > >   }
> > >   
> > > -- 
> > > 2.55.0
> > > 
> 
> LGTM. Thanks.
> 
> Reivewed-by: Ridong Chen <ridong.chen@linux.dev>

Thanks for the  patch.

Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>