Re: [PATCH v2] tracing: add cond_resched to ftrace_replace_code()

From: Will Deacon
Date: Wed Dec 05 2018 - 04:54:37 EST


Hi Anders, Steve,

On Tue, Dec 04, 2018 at 08:29:03PM +0100, Anders Roxell 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@xxxxxxxxxxx>
> Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
> ---
> kernel/trace/ftrace.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index c375e33239f7..7080eb464983 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2419,11 +2419,19 @@ void __weak ftrace_replace_code(int enable)
> {
> struct dyn_ftrace *rec;
> struct ftrace_page *pg;
> + bool schedulable;
> int failed;
>
> if (unlikely(ftrace_disabled))
> return;
>
> + /*
> + * Some archs calls this function with interrupts or preemption
> + * disabled. However, for other archs that can preempt, this can cause
> + * an tremendous unneeded latency.
> + */
> + schedulable = !irqs_disabled() && !preempt_count();

Is there a reason not to use preemptible() here?

Will