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

From: Reinette Chatre
Date: Tue Jun 02 2020 - 13:13:15 EST

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 softwareâs
> 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@xxxxxxx>

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?