linux-kernel - Re: [PATCH] asm-generic/io.h: Skip trace helpers if rwmmio events are disabled

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Message-Id: <f74d9899-6aba-4c8e-87b1-cd6ecc7772e6@app.fastmail.com>
Date: Sat, 26 Jul 2025 18:22:27 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Varad Gautam" <varadgautam@...gle.com>,
 Linux-Arch <linux-arch@...r.kernel.org>
Cc: "Sai Prakash Ranjan" <quic_saipraka@...cinc.com>,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] asm-generic/io.h: Skip trace helpers if rwmmio events are disabled

On Thu, Jul 24, 2025, at 13:49, Varad Gautam wrote:
> On Wed, May 28, 2025 at 5:28 PM Varad Gautam <varadgautam@...gle.com> wrote:
>>
>> On Mon, Apr 28, 2025 at 9:41 PM Varad Gautam <varadgautam@...gle.com> wrote:
>> >
>> > On Mon, Apr 7, 2025 at 6:13 PM Varad Gautam <varadgautam@...gle.com> wrote:
>> > >
>> > > On Sun, Mar 30, 2025 at 6:42 PM Varad Gautam <varadgautam@...gle.com> wrote:
>> > > >
>> > > > With `CONFIG_TRACE_MMIO_ACCESS=y`, the `{read,write}{b,w,l,q}{_relaxed}()`
>> > > > mmio accessors unconditionally call `log_{post_}{read,write}_mmio()`
>> > > > helpers, which in turn call the ftrace ops for `rwmmio` trace events
>> > > >
>> > > > This adds a performance penalty per mmio accessor call, even when
>> > > > `rwmmio` events are disabled at runtime (~80% overhead on local
>> > > > measurement).
>> > > >
>> > > > Guard these with `tracepoint_enabled()`.
>> > > >
>> > > > Signed-off-by: Varad Gautam <varadgautam@...gle.com>
>> > > > Fixes: 210031971cdd ("asm-generic/io: Add logging support for MMIO accessors")
>> > > > Cc: <stable@...r.kernel.org>
>> > >
>> > > Ping.
>> > >
>> >
>> > Ping.
>> >
>>
>> Ping. Arnd, can this be picked up into the asm-generic tree?
>>
>
> Ping.

I'm sorry I keep missing this one. It's really too late again for
the merge window, so it won't be in 6.17 either, but I've applied
it locally in my asm-generic branch that I'm planning for 6.18
so I hope I won't miss it again.

I currently have nothing queued up for 6.17 at all, but I already
have some of my own patches that I plan to submit for review after
the merge window and merge through the asm-generic tree.

     Arnd