linux-kernel - Re: [PATCH v3 21/21] x86/resctrl: Make resctrl_arch_rmid

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Date:   Mon, 4 Apr 2022 17:36:02 +0100
From:   James Morse <james.morse@....com>
To:     Rob Herring <robh@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...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 21/21] x86/resctrl: Make resctrl_arch_rmid_read()
 return values in bytes

Hi Rob,

On 3/23/22 21:17, Rob Herring wrote:
> On Thu, Feb 17, 2022 at 06:21:10PM +0000, James Morse wrote:
>> resctrl_arch_rmid_read() returns a value in chunks, as read from the
>> hardware. This needs scaling to bytes by mon_scale, as provided by
>> the architecture code.
>>
>> Now that resctrl_arch_rmid_read() performs the overflow and corrections
>> itself, it may as well return a value in bytes directly. This allows
>> the accesses to the architecture specific 'hw' structure to be removed.
>>
>> Move the mon_scale conversion into resctrl_arch_rmid_read().
>> mbm_bw_count() is updated to calculate bandwidth from bytes.

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index 3a6555f49720..437e7db77f93 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>> @@ -211,10 +211,11 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
>>   	if (am) {
>>   		am->chunks += mbm_overflow_count(am->prev_msr, msr_val,
>>   						 hw_res->mbm_width);
>> -		*val = get_corrected_mbm_count(rmid, am->chunks);
>> +		chunks = get_corrected_mbm_count(rmid, am->chunks);
>> +		*val = chunks * hw_res->mon_scale;
> 
> This can be moved out of the if/else if you make the following change:
> 
>>   		am->prev_msr = msr_val;
>>   	} else {
>> -		*val = msr_val;
>> +		*val = msr_val * hw_res->mon_scale;
> 
> chunks = msr_val;
> 
>>   	}

Sure, I guess it makes sense to only set *val in one place.

>>   
>>   	return 0;
>> @@ -400,15 +397,14 @@ static u64 __mon_event_count(u32 rmid, struct rmid_read *rr)
>>    */
>>   static void mbm_bw_count(u32 rmid, struct rmid_read *rr)
>>   {
>> -	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(rr->r);
>>   	struct mbm_state *m = &rr->d->mbm_local[rmid];
>> -	u64 cur_bw, chunks, cur_chunks;
>> +	u64 cur_bw, bytes, cur_bytes;
>>   
>> -	cur_chunks = rr->val;
>> -	chunks = cur_chunks - m->prev_bw_chunks;
>> -	m->prev_bw_chunks = cur_chunks;
>> +	cur_bytes = rr->val;
>> +	bytes = cur_bytes - m->prev_bw_bytes;
>> +	m->prev_bw_bytes = cur_bytes;
>>   
>> -	cur_bw = (chunks * hw_res->mon_scale) >> 20;
>> +	cur_bw = bytes >> 20;
> 
> 'bytes / SZ_1M' would be more readable and any decent compiler should
> optimize a power of 2 divide. But maybe that's a separate change as
> there are other cases of bytes to MB.

I agree its more readable. As I've touched this one, I'll change it.


Thanks,

James