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

[Babu Moger]

Hi Fenghua,

> -----Original Message-----
> From: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Sent: Monday, June 1, 2020 6:23 PM
> To: Moger, Babu <Babu.Moger@xxxxxxx>
> Cc: reinette.chatre@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
> 
> 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.

It will be based on 24 just like Intel. So, it will be 24 + offset

> 
> > +			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.

Yes. It is bit awkward. Other way is to add check in
rdt_get_mon_l3_config. I thought this way is better.
Thanks