linux-kernel - Re: [PATCH 3/3] arm64: ftrace: add cond_resched() to func ftrace_make

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Date:   Fri, 30 Nov 2018 10:23:51 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     mingo@...hat.com, catalin.marinas@....com, will.deacon@....com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 3/3] arm64: ftrace: add cond_resched() to func
 ftrace_make_(call|nop)

On Fri, 30 Nov 2018 16:09:56 +0100
Anders Roxell <anders.roxell@...aro.org> wrote:

> Both of those functions end up calling ftrace_modify_code(), which is
> expensive because it changes the page tables and flush caches.
> Microseconds add up because this is called in a loop for each dyn_ftrace
> record, and this triggers the softlockup watchdog unless we let it sleep
> occasionally.
> Rework so that we call cond_resched() before going into the
> ftrace_modify_code() function.
> 
> Co-developed-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>

I'm fine with this patch, but I'm not placing an ack on this patch
just because I don't know the repercussions of such a change. I'll let
you folks take full responsibility ;-)

-- Steve
 

> ---
>  arch/arm64/kernel/ftrace.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index de1a397d2d3f..9da38da58df7 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -130,6 +130,11 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
>  	old = aarch64_insn_gen_nop();
>  	new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
>  
> +	/* This function can take a long time when sanitizers are enabled, so
> +	 * lets make sure we allow RCU processing.
> +	 */
> +	cond_resched();
> +
>  	return ftrace_modify_code(pc, old, new, true);
>  }
>  
> @@ -188,6 +193,11 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  
>  	new = aarch64_insn_gen_nop();
>  
> +	/* This function can take a long time when sanitizers are enabled, so
> +	 * lets make sure we allow RCU processing.
> +	 */
> +	cond_resched();
> +
>  	return ftrace_modify_code(pc, old, new, validate);
>  }
>