linux-kernel - Re: [PATCH] arm_pmu: Delete incorrect cache event mapping for some armv8

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Message-ID: <20181001142914.GD9716@arm.com>
Date:   Mon, 1 Oct 2018 15:29:15 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Kulkarni, Ganapatrao" <Ganapatrao.Kulkarni@...ium.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "Nair, Jayachandran" <Jayachandran.Nair@...ium.com>,
        "Richter, Robert" <Robert.Richter@...ium.com>,
        "Lomovtsev, Vadim" <Vadim.Lomovtsev@...ium.com>,
        Jan Glauber <Jan.Glauber@...ium.com>,
        "gklkml16@...il.com" <gklkml16@...il.com>
Subject: Re: [PATCH] arm_pmu: Delete incorrect cache event mapping for some
 armv8_pmuv3 events.

Hi Ganapat,

On Mon, Oct 01, 2018 at 10:07:43AM +0000, Kulkarni, Ganapatrao wrote:
> Perf events L1-dcache-load-misses, L1-dcache-store-misses are mapped to
> armv8_pmuv3 (both DT and ACPI) event L1D_CACHE_REFILL. This is incorrect,
> since L1D_CACHE_REFILL counts both load and store misses.
> Similarly the events L1-dcache-loads, L1-dcache-stores, dTLB-load-misses
> and dTLB-loads are wrongly mapped. Hence Deleting all these cache events
> from armv8_pmuv3 cache mapping.
> 
> Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>
> ---
>  arch/arm64/kernel/perf_event.c | 8 --------
>  1 file changed, 8 deletions(-)

The "generic" events are really implemented on a best-effort basis, as
they rarely tend to map exactly to what the hardware supports. I think
they originally stemmed from the x86 CPU PMU, but that doesn't really
help us.

I had a discussion with Ingo back when we originally implemented perf
because I actually preferred not to implement the generic events at all.
However, he was strongly of the opinion that a best-effort approach was
sufficient to get casual users going with the tool, so that's what we went
with.

Will