linux-kernel - [RFC PATCH] pstore: Switch pmsg_lock to an rt

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Message-ID: <20221205203253.3923812-1-jstultz@google.com>
Date:   Mon,  5 Dec 2022 20:32:53 +0000
From:   John Stultz <jstultz@...gle.com>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     John Stultz <jstultz@...gle.com>, Wei Wang <wvw@...gle.com>,
        Midas Chien <midaschieh@...gle.com>,
        "Connor O'Brien" <connoro@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>, kernel-team@...roid.com
Subject: [RFC PATCH] pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion

Wei Wang reported seeing priority inversion caused latencies
caused by contention on pmsg_lock, and suggested it be switched
to a rt_mutex.

I was initially hesitant this would help, as the tasks in that
trace all seemed to be SCHED_NORMAL, so the benefit would be
limited to only nice boosting.

However, another similar issue was raised where the priority
inversion was seen did involve a blocked RT task so it is clear
this would be helpful in that case.

Feedback would be appreciate!

Cc: Wei Wang <wvw@...gle.com>
Cc: Midas Chien<midaschieh@...gle.com>
Cc: Connor O'Brien <connoro@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Anton Vorontsov <anton@...msg.org>
Cc: Colin Cross <ccross@...roid.com>
Cc: Tony Luck <tony.luck@...el.com>
Cc: kernel-team@...roid.com
Reported-by: Wei Wang <wvw@...gle.com>
Signed-off-by: John Stultz <jstultz@...gle.com>
---
 fs/pstore/pmsg.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/pstore/pmsg.c b/fs/pstore/pmsg.c
index d8542ec2f38c..18cf94b597e0 100644
--- a/fs/pstore/pmsg.c
+++ b/fs/pstore/pmsg.c
@@ -7,9 +7,10 @@
 #include <linux/device.h>
 #include <linux/fs.h>
 #include <linux/uaccess.h>
+#include <linux/rtmutex.h>
 #include "internal.h"
 
-static DEFINE_MUTEX(pmsg_lock);
+static DEFINE_RT_MUTEX(pmsg_lock);
 
 static ssize_t write_pmsg(struct file *file, const char __user *buf,
 			  size_t count, loff_t *ppos)
@@ -28,9 +29,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf,
 	if (!access_ok(buf, count))
 		return -EFAULT;
 
-	mutex_lock(&pmsg_lock);
+	rt_mutex_lock(&pmsg_lock);
 	ret = psinfo->write_user(&record, buf);
-	mutex_unlock(&pmsg_lock);
+	rt_mutex_unlock(&pmsg_lock);
 	return ret ? ret : count;
 }
 
-- 
2.39.0.rc0.267.gcb52ba06e7-goog