linux-kernel - Re: [PATCH] perf/x86/amd: Make HW_CACHE_REFERENCES and HW_CACHE

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Date:	Mon, 15 Aug 2016 16:13:16 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] perf/x86/amd: Make HW_CACHE_REFERENCES and
 HW_CACHE_MISSES measure L2

On Thu, 11 Aug, at 06:41:50PM, Borislav Petkov wrote:
> Drop stable from CC.
> 
> On Thu, Aug 11, 2016 at 04:21:42PM +0100, Matt Fleming wrote:
> > While the Intel PMU monitors the LLC when perf enables the
> > HW_CACHE_REFERENCES and HW_CACHE_MISSES events, these events monitor
> > L1 instruction cache fetches (0x0080) and instruction cache misses
> > (0x0081) on the AMD PMU.
> > 
> > This is extremely confusing when monitoring the same workload across
> > Intel and AMD machines, since parameters like,
> > 
> >   $ perf stat -e cache-references,cache-misses
> > 
> > measure completely different things.
> > 
> > Instead, make the AMD PMU measure instruction/data cache fill requests
> > to the L2 and instruction/data cache misses in the L2 when
> > HW_CACHE_REFERENCES and HW_CACHE_MISSES are enabled, respectively.
> > That way the events measure unified caches on both platforms.
> 
> I guess that's closer.
> 
> Even though LLC is not always L2 on AMD (some have L3). Btw,
> what are the exact events for PERF_COUNT_HW_CACHE_REFERENCES and
> PERF_COUNT_HW_CACHE_MISSES called on Intel?
 
They're referred to as "LLC Reference" and "LLC Misses" in the Intel
SDM Table 18-1 and "Longest latency cache references/misses" in Table
19-1.

> I could try to find better/more fitting event selectors on AMD...
 
If you've got any other suggestions, I'm all ears. Note that one thing
I wasn't sure about was whether we want to include TLB events hitting
the L2. I left them out of this patch, but it might make sense to add
them so that HW_CACHE_{REFERENCES,MISSES} is actually distinguishable
from LLC-{loads,misses}.

> > Signed-off-by: Matt Fleming <matt@...eblueprint.co.uk>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: <stable@...r.kernel.org>
> > ---
> >  arch/x86/events/amd/core.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> > index e07a22bb9308..8fd8bf79f32b 100644
> > --- a/arch/x86/events/amd/core.c
> > +++ b/arch/x86/events/amd/core.c
> > @@ -119,8 +119,8 @@ static const u64 amd_perfmon_event_map[PERF_COUNT_HW_MAX] =
> >  {
> >    [PERF_COUNT_HW_CPU_CYCLES]			= 0x0076,
> >    [PERF_COUNT_HW_INSTRUCTIONS]			= 0x00c0,
> > -  [PERF_COUNT_HW_CACHE_REFERENCES]		= 0x0080,
> > -  [PERF_COUNT_HW_CACHE_MISSES]			= 0x0081,
> > +  [PERF_COUNT_HW_CACHE_REFERENCES]		= 0x037d,
> > +  [PERF_COUNT_HW_CACHE_MISSES]			= 0x037e,
> >    [PERF_COUNT_HW_BRANCH_INSTRUCTIONS]		= 0x00c2,
> >    [PERF_COUNT_HW_BRANCH_MISSES]			= 0x00c3,
> >    [PERF_COUNT_HW_STALLED_CYCLES_FRONTEND]	= 0x00d0, /* "Decoder empty" event */
> 
> Btw, there's also amd_event_mapping in arch/x86/kvm/pmu_amd.c which has
> duplicated amd_perfmon_event_map. Would need adjusting too.

Urgh, right. I totally missed that. I'll update.