linux-kernel - Re: [PATCH v2] perf/x86/amd: Make HW_CACHE_REFERENCES and HW_CACHE

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Date:   Wed, 24 Aug 2016 20:27:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] perf/x86/amd: Make HW_CACHE_REFERENCES and
 HW_CACHE_MISSES measure L2

On Wed, Aug 24, 2016 at 04:55:14PM +0200, Borislav Petkov wrote:
> On Wed, Aug 24, 2016 at 02:12:08PM +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 and TLB fill
> > requests to the L2 and instruction/data cache and TLB 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'm still not really sure about this: we can't really compare L3 to L2
> access patterns - it is almost as comparing apples to oranges. Can we
> use the Intel L2 events instead?

They're not meant to be comparable between machines. I wouldn't even
compare the LLC numbers between two different Intel parts.

These events are meant to profile a workload on the machine you run them
on. Big cache-miss/ref ratios indicate you loose performance because of
the memory subsystem and or data structure layout.

And afaict AMD parts, even those that have L3, cannot provide L3 numbers
on a per task basis, so these L2 numbers are the best we have.