Re: [PATCH v9 05/12] x86/resctrl: Drop global 'rdt_mon_capable' flag

[Luck, Tony]

On Wed, Jul 08, 2026 at 03:47:51PM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 7/1/26 2:35 PM, Tony Luck wrote:
> > File system code calls resctrl_arch_mon_capable() to determine if the
> > system supports any monitoring features.
> > 
> > x86 architecture code sets a global flag to support implementation of
> > resctrl_arch_mon_capable() but the upcoming change to enumerate AET
> > (Application Energy Telemetry) features on each mount introduces a new
> > corner case.
> > 
> > Specifically when AET is the only monitoring feature, loading and
> > unloading the pmt_telemetry module between resctrl mounts may result
> > in monitoring support enabled on some mounts, but not on others.
> > 
> > Replace the global flag with a scan of resources to check if any are
> > marked mon_capable.
> > 
> > Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> > ---
> 
> ...
> 
> > diff --git a/arch/x86/include/asm/resctrl.h b/arch/x86/include/asm/resctrl.h
> > index 575f8408a9e7..a9f481b7a8ed 100644
> > --- a/arch/x86/include/asm/resctrl.h
> > +++ b/arch/x86/include/asm/resctrl.h
> 
> ...
> 
> > @@ -66,10 +65,7 @@ static inline void resctrl_arch_disable_alloc(void)
> >  	static_branch_dec_cpuslocked(&rdt_enable_key);
> >  }
> >  
> > -static inline bool resctrl_arch_mon_capable(void)
> > -{
> > -	return rdt_mon_capable;
> > -}
> > +bool resctrl_arch_mon_capable(void);
> >  
> >  static inline void resctrl_arch_enable_mon(void)
> >  {
> > diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> > index 9b9495174041..60d50ac79e7b 100644
> > --- a/arch/x86/kernel/cpu/resctrl/core.c
> > +++ b/arch/x86/kernel/cpu/resctrl/core.c
> > @@ -51,6 +51,20 @@ DEFINE_PER_CPU(struct resctrl_pqr_state, pqr_state);
> >   */
> >  bool rdt_alloc_capable;
> >  
> > +/*
> > + * Need to re-evaluate on each mount whether any mon_capable resources
> > + * are enabled.
> > + */
> 
> This series introduces a significant change in behavior of this call and I
> find that having the declaration in asm header and this significant comment in
> arch code to essentially bury very important information on this
> call's usage. Could the declaration be elevated to include/linux/resctrl.h
> where its function comments can provide the details on how resctrl fs
> uses this call and what its expectations from the architecture are? 
> Basically this series causes resctrl_arch_mon_capable() to possibly return
> different value depending on when it is called so there needs to be
> a contract between resctrl fs and architecture on when resctrl fs can safely
> call this and expect consistent results.

Ok. I'll move declaration to <linux/resctrl.h> and add comments on usage
and expectations.

> > +bool resctrl_arch_mon_capable(void)
> > +{
> > +	struct rdt_resource *r;
> > +
> > +	for_each_mon_capable_rdt_resource(r)
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> 
> Reinette

-Tony