linux-kernel - Re: [PATCH v3 15/21] x86/resctrl: Abstract __rmid

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Message-ID: <c2358bf0-1369-f8c3-6b44-e86a44c5016e@arm.com>
Date:   Wed, 30 Mar 2022 17:44:29 +0100
From:   James Morse <james.morse@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Babu Moger <Babu.Moger@....com>,
        shameerali.kolothum.thodi@...wei.com,
        Jamie Iles <jamie@...iainc.com>,
        D Scott Phillips OS <scott@...amperecomputing.com>,
        lcherian@...vell.com, bobo.shaobowang@...wei.com,
        tan.shaopeng@...itsu.com
Subject: Re: [PATCH v3 15/21] x86/resctrl: Abstract __rmid_read()

Hi Reinette,

On 16/03/2022 21:52, Reinette Chatre wrote:
> 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.

>> @@ -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.

Good point. Fixed.


>> @@ -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.

(yup, fixed)


>> -	/* 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."

Sure,


Thanks,

James