linux-kernel - Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD

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Date:   Tue, 2 Jun 2020 14:59:12 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Babu Moger <babu.moger@....com>, fenghua.yu@...el.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD

Hi Babu,

On 6/2/2020 10:33 AM, Babu Moger wrote:
> 
> 
> On 6/2/20 12:13 PM, Reinette Chatre wrote:
>> On 6/1/2020 4:00 PM, Babu Moger wrote:
>>> Memory bandwidth is calculated reading the monitoring counter
>>> at two intervals and calculating the delta. It is the software’s
>>> responsibility to read the count often enough to avoid having
>>> the count roll over _twice_ between reads.
>>>
>>> The current code hardcodes the bandwidth monitoring counter's width
>>> to 24 bits for AMD. This is due to default base counter width which
>>> is 24. Currently, AMD does not implement the CPUID 0xF.[ECX=1]:EAX
>>> to adjust the counter width. But, the AMD hardware supports much
>>> wider bandwidth counter with the default width of 44 bits.
>>>
>>> Kernel reads these monitoring counters every 1 second and adjusts the
>>> counter value for overflow. With 24 bits and scale value of 64 for AMD,
>>> it can only measure up to 1GB/s without overflowing. For the rates
>>> above 1GB/s this will fail to measure the bandwidth.
>>>
>>> Fix the issue setting the default width to 44 bits by adjusting the
>>> offset.
>>>
>>> AMD future products will implement the CPUID 0xF.[ECX=1]:EAX.
>>>
>>> Signed-off-by: Babu Moger <babu.moger@....com>
>>
>> There is no fixes tag but if I understand correctly this issue has been
>> present since AMD support was added to resctrl. This fix builds on top
>> of a recent feature addition and would thus not work for earlier
>> kernels. Are you planning to create a different fix for earlier kernels?
> 
> Yes. This was there from day one. I am going to back port to older kernels
> once we arrive on the final patch. Do we need fixes tag here?
> 

Yes, this needs a fixes tag. This would help the teams understand which
kernels should be fixed.

Reinette