Re: [PATCH v3 15/21] x86/resctrl: Abstract __rmid_read()

[Reinette Chatre <reinette.chatre@intel.com>]

Hi James,

On 2/17/2022 10:21 AM, James Morse wrote:
> __rmid_read() selects the specified eventid and returns the counter
> value from the MSR. The error handling is architecture specific, and
> handled by the callers, rdtgroup_mondata_show() and __mon_event_count().
> 
> Error handling should be handled by architecture specific code, as
> a different architecture may have different requirements. MPAM's
> counters can report that they are 'not ready', requiring a second
> read after a short delay. This should be hidden from resctrl.
> 
> Make __rmid_read() the architecture specific function for reading
> a counter. Rename it resctrl_arch_rmid_read() and move the error
> handling into it.
> 
> Signed-off-by: James Morse <james.morse@arm.com>

...

> @@ -167,9 +167,9 @@ void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d,
>  		memset(am, 0, sizeof(*am));
>  }
>  
> -static u64 __rmid_read(u32 rmid, enum resctrl_event_id eventid)
> +int resctrl_arch_rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val)
>  {
> -	u64 val;
> +	u64 msr_val;
>  
>  	/*
>  	 * As per the SDM, when IA32_QM_EVTSEL.EvtID (bits 7:0) is configured
> @@ -180,14 +180,24 @@ static u64 __rmid_read(u32 rmid, enum resctrl_event_id eventid)
>  	 * are error bits.
>  	 */
>  	wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> -	rdmsrl(MSR_IA32_QM_CTR, val);
> +	rdmsrl(MSR_IA32_QM_CTR, msr_val);
>  
> -	return val;
> +	if (msr_val & RMID_VAL_ERROR)
> +		return -EIO;
> +	if (msr_val & RMID_VAL_UNAVAIL)
> +		return -EINVAL;
> +
> +	*val = msr_val;
> +
> +	return 0;
>  }

From above we see that resctrl_arch_rmid_read() returns an int that could be
-EIO or -EINVAL ...

...

> @@ -319,15 +331,15 @@ static u64 __mon_event_count(u32 rmid, struct rmid_read *rr)
>  {
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(rr->r);
>  	struct mbm_state *m;
> -	u64 chunks, tval;
> +	u64 chunks, tval = 0;
>  
>  	if (rr->first)
>  		resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
>  
> -	tval = __rmid_read(rmid, rr->evtid);
> -	if (tval & (RMID_VAL_ERROR | RMID_VAL_UNAVAIL)) {
> -		return tval;
> -	}
> +	rr->err = resctrl_arch_rmid_read(rmid, rr->evtid, &tval);
> +	if (rr->err)
> +		return rr->err;
> +

Setting rr->err, an int, to the return of resctrl_arch_rmid_read() is ok and
can handle the negative error codes, but returning it here means that
__mon_event_count()'s return type should be changed,
it is currently u64.

>  	switch (rr->evtid) {
>  	case QOS_L3_OCCUP_EVENT_ID:
>  		rr->val += tval;
> @@ -343,7 +355,7 @@ static u64 __mon_event_count(u32 rmid, struct rmid_read *rr)
>  		 * Code would never reach here because an invalid
>  		 * event id would fail the __rmid_read.
>  		 */
> -		return RMID_VAL_ERROR;
> +		return -EINVAL;

Same here ... __mon_event_count() is u64 still ...

>  	}
>  
>  	if (rr->first) {
> @@ -419,9 +431,14 @@ void mon_event_count(void *info)
>  		}
>  	}
>  

Also take care here ... ret_val in mon_event_count() is still u64 while
__mon_event_count() attempts to return negative errors.

> -	/* Report error if none of rmid_reads are successful */
> -	if (ret_val)
> -		rr->val = ret_val;
> +	/*
> +	 * __mon_event_count() calls for newly created monitor groups may
> +	 * report -EINVAL/Unavailable if the monitor hasn't seen any traffic.
> +	 * If the first call for the control group succeed, discard any error
> +	 * set by reads of monitor groups.
> +	 */

Additionally, if the first call fails, but a following read of monitor group
succeeds then the first call's error is discarded.

How about if the last sentence is replaced with:
"Discard error if any of the monitor event reads succeeded."

> +	if (ret_val == 0)
> +		rr->err = 0;
>  }
>  
>  /*
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 0b48239f5920..70112dbfa128 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -219,6 +219,7 @@ u32 resctrl_arch_get_config(struct rdt_resource *r, struct rdt_domain *d,
>  			    u32 closid, enum resctrl_conf_type type);
>  int resctrl_online_domain(struct rdt_resource *r, struct rdt_domain *d);
>  void resctrl_offline_domain(struct rdt_resource *r, struct rdt_domain *d);
> +int resctrl_arch_rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *res);
>  
>  /**
>   * resctrl_arch_reset_rmid() - Reset any private state associated with rmid


Reinette