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

[Babu Moger]

> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Sent: Tuesday, June 2, 2020 4:51 PM
> To: Moger, Babu <Babu.Moger@xxxxxxx>; fenghua.yu@xxxxxxxxx;
> tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; bp@xxxxxxxxx; x86@xxxxxxxxxx;
> hpa@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD
> 
> 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>
> > ---
> > - 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)
> 
> This test checks if _any_ bit is set in eax ...
> 
> > +				c->x86_cache_mbm_width_offset = eax & 0xff;
> 
> ... with the assumption that the first eight bits contain a value.
> 
> Even so, now that Intel and AMD will be using eax in the same way,
> perhaps it can be done simpler by always using eax to obtain the offset
> (and thus avoid the code duplication) and on AMD initialize the default
> if it cannot be obtained from eax?
> 
> What I mean is something like:
> 
> @@ -1024,10 +1024,12 @@ void resctrl_cpu_detect(struct cpuinfo_x86 *c)
> 
>  		c->x86_cache_max_rmid  = ecx;
>  		c->x86_cache_occ_scale = ebx;
> -		if (c->x86_vendor == X86_VENDOR_INTEL)
> -			c->x86_cache_mbm_width_offset = eax & 0xff;
> -		else
> -			c->x86_cache_mbm_width_offset = -1;
> +		c->x86_cache_mbm_width_offset = eax & 0xff;
> +		if (c->x86_vendor == X86_VENDOR_AMD &&
> +		    c->x86_cache_mbm_width_offset == 0) {
> +			c->x86_cache_mbm_width_offset =
> +				MBM_CNTR_WIDTH_OFFSET_AMD;
> +		}
>  	}
>  }
> 
> What do you think?

That looks good. But we still need to keep the
default(c->x86_cache_mbm_width_offset = -1;) for non-AMD and non-Intel.
How about this?

diff --git a/arch/x86/kernel/cpu/resctrl/core.c
b/arch/x86/kernel/cpu/resctrl/core.c
index 12f967c6b603..7269bd896ba9 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -983,6 +983,9 @@ 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 if (c->x86_vendor == X86_VENDOR_AMD)
+                       c->x86_cache_mbm_width_offset = eax ? eax & 0xff :
+
MBM_CNTR_WIDTH_OFFSET_AMD;
                else
                        c->x86_cache_mbm_width_offset = -1;
        }