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Subject: Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD
To:     Babu Moger <babu.moger@amd.com>, fenghua.yu@intel.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, linux-kernel@vger.kernel.org
References: <159105232628.48268.7763865625735367523.stgit@naples-babu.amd.com>
From:   Reinette Chatre <reinette.chatre@intel.com>
Message-ID: <63f23edd-39d3-0577-6e00-21066ddfe57f@intel.com>
Date:   Tue, 2 Jun 2020 10:13:10 -0700
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Hi Babu,

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@amd.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?

Reinette