Re: [PATCH] x86/resctrl: Fix event counts regression in reused RMIDs

[Reinette Chatre]

Hi Peter,

On 12/7/2022 3:29 AM, Peter Newman wrote:
> When creating a new monitoring group, the RMID allocated for it may have
> been used by a group which was previously removed. In this case, the
> hardware counters will have non-zero values which should be deducted
> from what is reported in the new group's counts.
> 
> resctrl_arch_reset_rmid() initializes the prev_msr value for counters to
> 0, causing the initial count to be charged to the new group. Resurrect
> __rmid_read() and use it to initialize prev_msr correctly.

Thank you very much for catching this.

> 
> Fixes: 1d81d15db39c ("x86/resctrl: Move mbm_overflow_count() into resctrl_arch_rmid_read()")
> Signed-off-by: Peter Newman <peternewman@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/resctrl/monitor.c | 39 ++++++++++++++++++---------
>  1 file changed, 27 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index efe0c30d3a12..404dd9c472c7 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -146,6 +146,24 @@ static inline struct rmid_entry *__rmid_entry(u32 rmid)
>  	return entry;
>  }
> 
> +static u64 __rmid_read(u32 rmid, enum resctrl_event_id eventid)
> +{
> +	u64 val;
> +
> +	/*
> +	 * As per the SDM, when IA32_QM_EVTSEL.EvtID (bits 7:0) is configured
> +	 * with a valid event code for supported resource type and the bits
> +	 * IA32_QM_EVTSEL.RMID (bits 41:32) are configured with valid RMID,
> +	 * IA32_QM_CTR.data (bits 61:0) reports the monitored data.
> +	 * IA32_QM_CTR.Error (bit 63) and IA32_QM_CTR.Unavailable (bit 62)
> +	 * are error bits.
> +	 */
> +	wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> +	rdmsrl(MSR_IA32_QM_CTR, val);
> +
> +	return val;
> +}
> +
>  static struct arch_mbm_state *get_arch_mbm_state(struct rdt_hw_domain *hw_dom,
>  						 u32 rmid,
>  						 enum resctrl_event_id eventid)
> @@ -170,10 +188,17 @@ void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d,
>  {
>  	struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
>  	struct arch_mbm_state *am;
> +	uint64_t val;
> 
>  	am = get_arch_mbm_state(hw_dom, rmid, eventid);
> -	if (am)
> +	if (am) {
>  		memset(am, 0, sizeof(*am));
> +
> +		/* Record any initial, non-zero count value. */
> +		val = __rmid_read(rmid, eventid);
> +		if (!(val & (RMID_VAL_ERROR | RMID_VAL_UNAVAIL)))
> +			am->prev_msr = val;
> +	}
>  }
> 
>  static u64 mbm_overflow_count(u64 prev_msr, u64 cur_msr, unsigned int width)
> @@ -195,17 +220,7 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>  	if (!cpumask_test_cpu(smp_processor_id(), &d->cpu_mask))
>  		return -EINVAL;
> 
> -	/*
> -	 * As per the SDM, when IA32_QM_EVTSEL.EvtID (bits 7:0) is configured
> -	 * with a valid event code for supported resource type and the bits
> -	 * IA32_QM_EVTSEL.RMID (bits 41:32) are configured with valid RMID,
> -	 * IA32_QM_CTR.data (bits 61:0) reports the monitored data.
> -	 * IA32_QM_CTR.Error (bit 63) and IA32_QM_CTR.Unavailable (bit 62)
> -	 * are error bits.
> -	 */
> -	wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> -	rdmsrl(MSR_IA32_QM_CTR, msr_val);
> -
> +	msr_val = __rmid_read(rmid, eventid);
>  	if (msr_val & RMID_VAL_ERROR)
>  		return -EIO;
>  	if (msr_val & RMID_VAL_UNAVAIL)
> 
> base-commit: 76dcd734eca23168cb008912c0f69ff408905235
> --
> 2.39.0.rc0.267.gcb52ba06e7-goog
> 

To get back to the original behavior before the refactoring it also seems
that __mon_event_count() needs to return right after calling 
resctrl_arch_reset_rmid(). The only caller with rr->first set is when
the mon directory is created and the returned values are not used,
it is just run to get prev_msr set. This also avoids unnecessarily reading
the counters twice.

So, how about:

static int __mon_event_count(u32 rmid, struct rmid_read *rr)
{

	...
	if (rr->first) {
		resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
		return 0;
	}
	...

}

Also ... there appears to be a leftover related snippet in __mon_event_count()
that does not belong anymore and may still cause incorrect behavior:

static int __mon_event_count(u32 rmid, struct rmid_read *rr)
{
	...
	if (rr->first) {
		memset(m, 0, sizeof(struct mbm_state));
		return 0;
	}
	...
}


Reinette