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

[Fenghua Yu]

On Mon, Jun 01, 2020 at 06:00:29PM -0500, 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>
> ---
> - Sending it second time. Email client had some issues first time.
> - Generated the patch on top of 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git (x86/cache).
> 
>  arch/x86/kernel/cpu/resctrl/core.c     |    8 +++++++-
>  arch/x86/kernel/cpu/resctrl/internal.h |    1 +
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 12f967c6b603..6040e9ae541b 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -983,7 +983,13 @@ void resctrl_cpu_detect(struct cpuinfo_x86 *c)
>  		c->x86_cache_occ_scale = ebx;
>  		if (c->x86_vendor == X86_VENDOR_INTEL)
>  			c->x86_cache_mbm_width_offset = eax & 0xff;
> -		else
> +		else if (c->x86_vendor == X86_VENDOR_AMD) {
> +			if (eax)
> +				c->x86_cache_mbm_width_offset = eax & 0xff;

When AMD implements CPUID.0x1f.1:eax, will the offset be based on 24 or 44?
Seems it makes senses to be based on 44 because default counter width is 44.

> +			else
> +				c->x86_cache_mbm_width_offset =
> +					MBM_CNTR_WIDTH_OFFSET_AMD;

If that's the case, you don't need this "else" because the CPUID reports
offset as 0 for default width 44.

This will match the Intel code above.

Otherwise, the code is awkward.

Thanks.

-Fenghua