Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Anders Roxell <anders.roxell@linaro.org>
To:     rostedt@goodmis.org, mingo@redhat.com, catalin.marinas@arm.com,
        will.deacon@arm.com
Cc:     keescook@chromium.org, arnd@arndb.de, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Anders Roxell <anders.roxell@linaro.org>
Subject: [PATCH] tracing: add cond_resched to ftrace_replace_code()
Date:   Tue,  4 Dec 2018 20:12:28 +0100
Message-Id: <20181204191229.6851-1-anders.roxell@linaro.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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..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.
+		 */
+		if (schedulable)
+			cond_resched();
 	} while_for_each_ftrace_rec();
 }
 
-- 
2.19.2