linux-kernel - Re: [PATCH] Partially revert "perf/arm-cmn: Optimise DTC counter accesses"

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Message-Id: <167474133878.1608677.3170895609021328329.b4-ty@kernel.org>
Date:   Thu, 26 Jan 2023 15:42:25 +0000
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     catalin.marinas@....com, kernel-team@...roid.com,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        ilkka@...amperecomputing.com, linux-arm-kernel@...ts.infradead.org,
        mark.rutland@....com
Subject: Re: [PATCH] Partially revert "perf/arm-cmn: Optimise DTC counter accesses"

On Mon, 23 Jan 2023 18:30:38 +0000, Robin Murphy wrote:
> It turns out the optimisation implemented by commit 4f2c3872dde5 is
> totally broken, since all the places that consume hw->dtcs_used for
> events other than cycle count are still not expecting it to be sparsely
> populated, and fail to read all the relevant DTC counters correctly if
> so.
> 
> If implemented correctly, the optimisation potentially saves up to 3
> register reads per event update, which is reasonably significant for
> events targeting a single node, but still not worth a massive amount of
> additional code complexity overall. Getting it right within the current
> design looks a fair bit more involved than it was ever intended to be,
> so let's just make a functional revert which restores the old behaviour
> while still backporting easily.
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[1/1] Partially revert "perf/arm-cmn: Optimise DTC counter accesses"
      https://git.kernel.org/arm64/c/a428eb4b99ab

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev