netdev - Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and rcu_idle

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Date:   Sun, 15 May 2022 21:25:35 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH bpf-next 1/2] cpuidle/rcu: Making arch_cpu_idle and
 rcu_idle_exit noinstr

On Sun, May 15, 2022 at 10:36:52PM +0200, Jiri Olsa wrote:
> Making arch_cpu_idle and rcu_idle_exit noinstr. Both functions run
> in rcu 'not watching' context and if there's tracer attached to
> them, which uses rcu (e.g. kprobe multi interface) it will hit RCU
> warning like:
> 
>   [    3.017540] WARNING: suspicious RCU usage
>   ...
>   [    3.018363]  kprobe_multi_link_handler+0x68/0x1c0
>   [    3.018364]  ? kprobe_multi_link_handler+0x3e/0x1c0
>   [    3.018366]  ? arch_cpu_idle_dead+0x10/0x10
>   [    3.018367]  ? arch_cpu_idle_dead+0x10/0x10
>   [    3.018371]  fprobe_handler.part.0+0xab/0x150
>   [    3.018374]  0xffffffffa00080c8
>   [    3.018393]  ? arch_cpu_idle+0x5/0x10
>   [    3.018398]  arch_cpu_idle+0x5/0x10
>   [    3.018399]  default_idle_call+0x59/0x90
>   [    3.018401]  do_idle+0x1c3/0x1d0
> 
> The call path is following:
> 
> default_idle_call
>   rcu_idle_enter
>   arch_cpu_idle
>   rcu_idle_exit
> 
> The arch_cpu_idle and rcu_idle_exit are the only ones from above
> path that are traceble and cause this problem on my setup.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>

>From an RCU viewpoint:

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

[ I considered asking for an instrumentation_on() in rcu_idle_exit(),
but there is no point given that local_irq_restore() isn't something
you instrument anyway. ]

> ---
>  arch/x86/kernel/process.c | 2 +-
>  kernel/rcu/tree.c         | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index b370767f5b19..1345cb0124a6 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -720,7 +720,7 @@ void arch_cpu_idle_dead(void)
>  /*
>   * Called from the generic idle code.
>   */
> -void arch_cpu_idle(void)
> +void noinstr arch_cpu_idle(void)
>  {
>  	x86_idle();
>  }
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a4b8189455d5..20d529722f51 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -896,7 +896,7 @@ static void noinstr rcu_eqs_exit(bool user)
>   * If you add or remove a call to rcu_idle_exit(), be sure to test with
>   * CONFIG_RCU_EQS_DEBUG=y.
>   */
> -void rcu_idle_exit(void)
> +void noinstr rcu_idle_exit(void)
>  {
>  	unsigned long flags;
>  
> -- 
> 2.35.3
>