linux-kernel - Re: [PATCH 0/1] arm64: defconfig: Add Coresight as module

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Message-ID: <Yyw+gHXLG/fXoQbT@arm.com>
Date:   Thu, 22 Sep 2022 11:52:48 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     James Clark <james.clark@....com>
Cc:     coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
        suzuki.poulose@....com, mathieu.poirier@...aro.org,
        mike.leach@...aro.org, leo.yan@...aro.org, broonie@...nel.org,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 0/1] arm64: defconfig: Add Coresight as module

On Thu, Sep 22, 2022 at 10:34:45AM +0100, James Clark wrote:
> On 21/09/2022 16:08, Catalin Marinas wrote:
> > 2. Always on CONFIG_PID_IN_CONTEXTIDR (we might as well remove the
> >    Kconfig entry). This would write the root pid namespace value
> >    (task_pid_nr()).
> 
> If we're not worried about the overhead after all, this would be the
> easiest solution. And then SPE or Coresight already decide whether they
> want to use the value or not, so no further changes are needed.
> 
> From Leo's patch there is a table that shows a 1% overhead with it
> enabled permanently, and I've heard a figure like that mentioned before.
> So I could also resurrect that patch to use static keys? Although it's a
> bit more complicated, that would be my preference. And then we can have
> that mode always on.

I don't think we should bother with static keys, just always enable it
but try to reduce/group the ISBs from all the functions called on the
__switch_to() path. We may actually get a speed-up.

-- 
Catalin