[PATCH v3] tracing: add cond_resched to ftrace_replace_code()

[PATCH v3] tracing: add cond_resched to ftrace_replace_code()

[Anders Roxell]

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@goodmis.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.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..7c9f49a5a1ab 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 call 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();
+
 	do_for_each_ftrace_rec(pg, rec) {
 
 		if (rec->flags & FTRACE_FL_DISABLED)
@@ -2435,6 +2443,8 @@ void __weak ftrace_replace_code(int enable)
 			/* Stop processing */
 			return;
 		}
+		if (schedulable)
+			cond_resched();
 	} while_for_each_ftrace_rec();
 }
 
-- 
2.19.2

Re: [PATCH v3] tracing: add cond_resched to ftrace_replace_code()

[Steven Rostedt]

On Tue,  4 Dec 2018 20:40:44 +0100
Anders Roxell <anders.roxell@linaro.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().

This isn't urgent is it? That is, it doesn't need a stable tag?

-- Steve

Re: [PATCH v3] tracing: add cond_resched to ftrace_replace_code()

[Anders Roxell]

On Tue, 4 Dec 2018 at 23:33, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue,  4 Dec 2018 20:40:44 +0100
> Anders Roxell <anders.roxell@linaro.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().
>
> This isn't urgent is it?

No, it is not urgent. I'm trying to get allmodconfig to boot on arm64.

> That is, it doesn't need a stable tag?

Also I don't think its a regression, since clearly nobody has ever been
able to boot an arm64 allmodconfig kernel or anything close to that.

Cheers,
Anders