linux-kernel - [RFC][PATCHv5 12/13] printk: do not cond

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Date:   Tue, 15 Aug 2017 11:56:24 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: [RFC][PATCHv5 12/13] printk: do not cond_resched() when we can offload

console_unlock() may sleep with console_sem locked, which is a bit
counter intuitive: we neither print pending logbuf messages to the
serial console, nor let anyone else to do it for us.

With printing offloading enabled, however, we can disable preemption,
because we know for sure how long we can stay in console_unlock() and
that eventually we will offload to another task.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 kernel/printk/printk.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0ffbaa33280f..ba82152ce5d9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2432,6 +2432,14 @@ void console_unlock(void)
 	 * and cleared after the the "again" goto label.
 	 */
 	do_cond_resched = console_may_schedule;
+	/*
+	 * Forbid scheduling under the console_sem lock when offloading
+	 * is enabled. Scheduling will just slow down the print out in
+	 * this case.
+	 */
+	if (printk_offloading_enabled() && is_printk_offloading_safe())
+		do_cond_resched = 0;
+
 again:
 	console_may_schedule = 0;
 
@@ -2447,6 +2455,7 @@ void console_unlock(void)
 		return;
 	}
 
+	preempt_disable();
 	for (;;) {
 		struct printk_log *msg;
 		size_t ext_len = 0;
@@ -2507,8 +2516,11 @@ void console_unlock(void)
 		start_critical_timings();
 		printk_safe_exit_irqrestore(flags);
 
-		if (do_cond_resched)
+		if (do_cond_resched) {
+			preempt_enable();
 			cond_resched();
+			preempt_disable();
+		}
 	}
 	console_locked = 0;
 
@@ -2518,6 +2530,7 @@ void console_unlock(void)
 
 	raw_spin_unlock(&logbuf_lock);
 
+	preempt_enable();
 	up_console_sem();
 
 	/*
-- 
2.14.1