linux-kernel - [PATCH] x86/alternatives: Add cond_resched() to text_poke_bp

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Message-ID: <20230528084652.5f3b48f0@rorschach.local.home>
Date:   Sun, 28 May 2023 08:46:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] x86/alternatives: Add cond_resched() to
 text_poke_bp_batch()

From: "Steven Rostedt (Google)" <rostedt@...dmis.org>

Debugging in the kernel has started slowing down the kernel by a
noticeable amount. The ftrace start up tests are triggering the softlockup
watchdog on some boxes. This is caused by the start up tests that enable
function and function graph tracing several times. Sprinkling
cond_resched() just in the start up test code was not enough to stop the
softlockup from triggering. It would sometimes trigger in the
text_poke_bp_batch() code.

The text_poke_bp_batch() is run in schedulable context. Add
cond_resched() between each phase (adding the int3, updating the code, and
removing the int3). This keeps the softlockup from triggering in the start
up tests.

Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 arch/x86/kernel/alternative.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index f615e0cb6d93..e024eddd457f 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1953,6 +1953,14 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 	 */
 	atomic_set_release(&bp_desc.refs, 1);
 
+	/*
+	 * Function tracing can enable thousands of places that need to be
+	 * updated. This can take quite some time, and with full kernel debugging
+	 * enabled, this could cause the softlockup watchdog to trigger.
+	 * Add cond_resched() calls to each phase.
+	 */
+	cond_resched();
+
 	/*
 	 * Corresponding read barrier in int3 notifier for making sure the
 	 * nr_entries and handler are correctly ordered wrt. patching.
@@ -2030,6 +2038,7 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 		 * better safe than sorry (plus there's not only Intel).
 		 */
 		text_poke_sync();
+		cond_resched();
 	}
 
 	/*
@@ -2049,8 +2058,10 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
 		do_sync++;
 	}
 
-	if (do_sync)
+	if (do_sync) {
 		text_poke_sync();
+		cond_resched();
+	}
 
 	/*
 	 * Remove and wait for refs to be zero.
-- 
2.39.2