linux-kernel - Re: [PATCH 2/2] arm64: ftrace: Set FTRACE_SCHEDULABLE before ftrace_modify_all

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Date:   Thu, 6 Dec 2018 13:20:07 +0000
From:   Will Deacon <will.deacon@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        keescook@...omium.org, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org,
        Anders Roxell <anders.roxell@...aro.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] arm64: ftrace: Set FTRACE_SCHEDULABLE before
 ftrace_modify_all_code()

On Wed, Dec 05, 2018 at 12:48:54PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> It has been reported that ftrace_replace_code() which is called by
> ftrace_modify_all_code() can cause a soft lockup warning for an
> allmodconfig kernel. This is because all the debug options enabled
> causes the loop in ftrace_replace_code() (which loops over all the
> functions being enabled where there can be 10s of thousands), is too
> slow, and never schedules out.
> 
> To solve this, setting FTRACE_SCHEDULABLE to the command passed into
> ftrace_replace_code() will make it call cond_resched() in the loop,
> which prevents the soft lockup warning from triggering.
> 
> Link: http://lkml.kernel.org/r/20181204192903.8193-1-anders.roxell@linaro.org
> 
> Reported-by: Anders Roxell <anders.roxell@...aro.org>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/arm64/kernel/ftrace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 57e962290df3..9a8de0a79f97 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -193,6 +193,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  
>  void arch_ftrace_update_code(int command)
>  {
> +	command |= FTRACE_SCHEDULABLE;
>  	ftrace_modify_all_code(command);
>  }

Bikeshed: I'd probably go for FTRACE_MAY_SLEEP, but I'm not going to die
on that hill so...

Acked-by: Will Deacon <will.deacon@....com>

Thanks, Steve!

Will