linux-kernel - Re: [PATCH v10 2/6] arm64: idle: Tag the arm64 idle functions as _

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Message-ID: <CAE-0n53QBBfX77ibE+NdWZa70VVOKkNnhn5BNhBk04XKEFKNJw@mail.gmail.com>
Date:   Tue, 22 Aug 2023 16:14:21 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Catalin Marinas <catalin.marinas@....com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sumit Garg <sumit.garg@...aro.org>,
        Will Deacon <will@...nel.org>
Cc:     Lecopzer Chen <lecopzer.chen@...iatek.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        kgdb-bugreport@...ts.sourceforge.net, ito-yuichi@...itsu.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Chen-Yu Tsai <wens@...e.org>, Ard Biesheuvel <ardb@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Guo Ren <guoren@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 2/6] arm64: idle: Tag the arm64 idle functions as __cpuidle

Quoting Douglas Anderson (2023-08-22 14:26:57)
> As per the (somewhat recent) comment before the definition of
> `__cpuidle`, the tag is like `noinstr` but also marks a function so it
> can be identified by cpu_in_idle(). Let'a add this.

s/Let'a/Let's/

Maybe also define "this" to be "Let's add these markings to arm64
cpuidle functions".

>
> After doing this then when we dump stack traces of all processors
> using nmi_cpu_backtrace() then instead of getting useless backtraces
> we get things like:

Sorry, this sentence is really hard for me to read. Perhaps:

With this change we get useful backtraces like:

   NMI backtrace for cpu N skipped: idling at cpu_do_idle+0x94/0x98

instead of useless backtraces when dumping all processors using
nmi_cpu_backtrace().

>
>   NMI backtrace for cpu N skipped: idling at cpu_do_idle+0x94/0x98
>
> NOTE: this patch won't make cpu_in_idle() work perfectly for arm64,
> but it doesn't hurt and does catch some cases. Specifically an example
> that wasn't caught in my testing looked like this:

I wonder if it improves locality of cpu idle code as well by moving the
functions to the idle text section so any branch targets are closer.

>
>  gic_cpu_sys_reg_init+0x1f8/0x314
>  gic_cpu_pm_notifier+0x40/0x78
>  raw_notifier_call_chain+0x5c/0x134
>  cpu_pm_notify+0x38/0x64
>  cpu_pm_exit+0x20/0x2c
>  psci_enter_idle_state+0x48/0x70
>  cpuidle_enter_state+0xb8/0x260
>  cpuidle_enter+0x44/0x5c
>  do_idle+0x188/0x30c
>
> Acked-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Stephen Boyd <swboyd@...omium.org>