tracing: add cond_resched to ftrace_replace_code()

PATCH In review Anders Roxell 2 participants 3 messages Dec 4, 2018 – Dec 5, 2018

Subsystems (2)

Tracing ftrace

lore

Share & tools

lore

Anders Roxell

author

2018-12-04 19:40 UTC #0 lore raw

PATCH v3 1/1 tracing: add cond_resched to ftrace_replace_code()

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().

Signed-off-by Anders Roxell <[email protected]>

---
 kernel/trace/ftrace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

kernel/trace/ftrace.c +10 -0

index c375e33239f7..7c9f49a5a1ab 100644
@@ -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();
 }

Signature · 1 line

2.19.2

Steven Rostedt

maintainer

2018-12-04 22:33 UTC #1 lore raw

Quoted 7 lines · On Tue, 4 Dec 2018 20:40:44 +0100 Anders Roxell <[email protected]>…

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.

Quoted 5 lines

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

Anders Roxell

author

2018-12-05 10:50 UTC #2 lore raw

Quoted 11 lines · Steven Rostedt

Quoted 7 lines · On Tue, 4 Dec 2018 20:40:44 +0100 Anders Roxell <[email protected]>…
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.

Quoted 7 lines

Quoted 5 lines
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.

Quoted 1 line

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