linux-kernel - Re: [PATCH] tracing: add cond_resched to ftrace_replace

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Date:   Tue, 4 Dec 2018 14:19:08 -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, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] tracing: add cond_resched to ftrace_replace_code()

On Tue,  4 Dec 2018 20:12:28 +0100
Anders Roxell <anders.roxell@...aro.org> wrote:

> When running in qemu on an kernel built with allmodconfig and debug
> options (in particular kcov and ubsan) enabled, ftrace_replace_code
> function call take minutes. The ftrace selftest calls
> ftrace_replace_code to look >40000 through
> ftrace_make_call/ftrace_make_nop, and these end up calling
> __aarch64_insn_write/aarch64_insn_patch_text_nosync.
> 
> 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() if !irqs_disabled() && !preempt_count().
> 
> Suggested-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>  kernel/trace/ftrace.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index c375e33239f7..582e3441e318 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2419,11 +2419,14 @@ void __weak ftrace_replace_code(int enable)
>  {
>  	struct dyn_ftrace *rec;
>  	struct ftrace_page *pg;
> +	bool schedulable;
>  	int failed;
>  
>  	if (unlikely(ftrace_disabled))
>  		return;
>  
> +	schedulable = !irqs_disabled() && !preempt_count();
> +
>  	do_for_each_ftrace_rec(pg, rec) {
>  
>  		if (rec->flags & FTRACE_FL_DISABLED)
> @@ -2435,6 +2438,13 @@ void __weak ftrace_replace_code(int enable)
>  			/* Stop processing */
>  			return;
>  		}
> +		/*
> +		 * Some archs calls this function with interrupts or preemption
> +		 * disabled. Howeve, other archs don't and this can cause a
> +		 * tremendous unneeded latency.
> +		 */

Actually, could you move the comment up where schedulable gets set?

Thanks!

-- Steve

> +		if (schedulable)
> +			cond_resched();
>  	} while_for_each_ftrace_rec();
>  }
>