Re: [PATCH 3/3] arm64: topology: Reduce the skew for AMU feedback counter sampling

[Beata Michalska <beata.michalska@arm.com>]

Hi Christian,
On Wed, Nov 11, 2026 at 01:56:49PM +0000, Christian Loehle wrote:
> On 11/11/26 12:39, Beata Michalska wrote:
> > With the FFH support for CPPC, the AMU feedback counters, core and const
> > counter, are being sampled to measure CPU performance. Checking for errata
> > (which can affect both counters) between the reads delays the second
> > sample and can skew the result.
> > 
> > Take both samples in succession, then discard values from affected
> > counters. This narrows the sampling interval used for CPPC feedback.
> > 
> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > ---
> >  arch/arm64/kernel/topology.c | 76 +++++++++++++++++++++++-------------
> >  1 file changed, 49 insertions(+), 27 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> > index 66ac67bd025e..c968fa5b0c88 100644
> > --- a/arch/arm64/kernel/topology.c
> > +++ b/arch/arm64/kernel/topology.c
> > @@ -381,29 +381,45 @@ enum cpc_ffh_ctr_id {
> >  	CPC_FFH_CTR_CONST = 0x1,
> >  };
> >  
> > -static void cpu_read_corecnt(void *val)
> > +static inline void cpu_read_corecnt(void *val)
> >  {
> > -	/*
> > -	 * A value of 0 can be returned if the current CPU does not support AMUs
> > -	 * or if the counter is disabled for this CPU. A return value of 0 at
> > -	 * counter read is properly handled as an error case by the users of the
> > -	 * counter.
> > -	 */
> > -	*(u64 *)val = this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CORECNT) ?
> > -		0UL : read_corecnt();
> > +	*(u64 *)val = read_corecnt();
> >  }
> >  
> > -static void cpu_read_constcnt(void *val)
> > +static void cpu_try_read_corecnt(void *val)
> >  {
> > -	/*
> > -	 * Return 0 if the current CPU is affected by a HW erratum.
> > -	 * A value of 0 is also returned if the current CPU does not
> > -	 * support AMUs or if the counter is disabled. A return
> > -	 * value of 0 at counter read is properly handled as an error
> > -	 * case by the users of the counter.
> > -	 */
> > -	*(u64 *)val = this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT) ?
> > -		      0UL : read_constcnt();
> > +	if (!this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CORECNT)) {
> > +		cpu_read_corecnt(val);
> > +	} else {
> > +		/*
> > +		 * A value of 0 can be returned if the current CPU does not support AMUs
> > +		 * or if the counter is disabled for this CPU. A return value of 0 at
> > +		 * counter read is properly handled as an error case by the users of the
> > +		 * counter.
> > +		 */
> > +		*(u64 *)val = 0UL;
> > +	}
> > +}
> > +
> > +static inline void cpu_read_constcnt(void *val)
> > +{
> > +	*(u64 *)val = read_constcnt();
> > +}
> > +
> > +static void cpu_try_read_constcnt(void *val)
> > +{
> > +	if (!this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT)) {
> > +		cpu_read_constcnt(val);
> > +	} else {
> > +		/*
> > +		 * Return 0 if the current CPU is affected by a HW erratum.
> > +		 * A value of 0 is also returned if the current CPU does not
> > +		 * support AMUs or if the counter is disabled. A return
> > +		 * value of 0 at counter read is properly handled as an error
> > +		 * case by the users of the counter.
> > +		 */
> > +		*(u64 *)val = 0UL;
> > +	}
> >  }
> >  
> >  static inline
> > @@ -461,14 +477,20 @@ static void amu_read_core_const_ctrs(void *val)
> >  {
> >  	struct amu_ffh_ctrs *ctrs = val;
> >  
> > +	cpu_read_corecnt(&ctrs->corecnt);
> > +	cpu_read_constcnt(&ctrs->constcnt);
> > +
> >  	/*
> > -	 * cpu_read_constcnt() incurs slight latency due to the
> > -	 * ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT check.
> > -	 * Read it first to minimize the sampling skew between the const
> > -	 * and core counters.
> > +	 * Return 0 if the current CPU is affected by a HW erratum.
> > +	 * A value of 0 is also returned if the current CPU does not
> > +	 * support AMUs or if the counter is disabled. A return
> > +	 * value of 0 at counter read is properly handled as an error
> > +	 * case by the users of the counter.
> 
> Why do we read them first then on 
> ARM64_WORKAROUND_BROKEN_AMU_CORECNT or ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT
> respectively?
>
That's what this patch is all about - isn't it ?
It allows direct read of both counters (those being read first),
and only once that's done, it verifies whether the workaround should be applied
or not. For cores that do not have either it eliminates the check between
reading the counters.
Or you had smth else in mind I did not quite manage to grasp ?

---
BR
Beata
> >  	 */
> > -	cpu_read_constcnt(&ctrs->constcnt);
> > -	cpu_read_corecnt(&ctrs->corecnt);
> > +	if (this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CORECNT))
> > +		ctrs->corecnt = 0UL;
> > +	if (this_cpu_has_cap(ARM64_WORKAROUND_BROKEN_AMU_CONSTCNT))
> > +		ctrs->constcnt = 0UL;
> >  }
> >  
> >  static bool cpc_ffh_reg_valid(const struct cpc_reg *reg)
> > @@ -543,10 +565,10 @@ int cpc_read_ffh(int cpu, struct cpc_reg *reg, u64 *val)
> >  
> >  	switch ((u64)reg->address) {
> >  	case CPC_FFH_CTR_CORE:
> > -		ret = counters_read_on_cpu(cpu, cpu_read_corecnt, val);
> > +		ret = counters_read_on_cpu(cpu, cpu_try_read_corecnt, val);
> >  		break;
> >  	case CPC_FFH_CTR_CONST:
> > -		ret = counters_read_on_cpu(cpu, cpu_read_constcnt, val);
> > +		ret = counters_read_on_cpu(cpu, cpu_try_read_constcnt, val);
> >  		break;
> >  	}
> >  
>