linux-kernel - [PATCH v2] tracing: add cond_resched to ftrace_replace

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Date:   Tue,  4 Dec 2018 20:29:03 +0100
From:   Anders Roxell <anders.roxell@...aro.org>
To:     rostedt@...dmis.org, mingo@...hat.com, catalin.marinas@....com,
        will.deacon@....com
Cc:     keescook@...omium.org, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Anders Roxell <anders.roxell@...aro.org>
Subject: [PATCH v2] 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().

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..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();
+
 	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