printk/sched: Introduce special printk_sched() for those awkward moments

There's a few awkward printk()s inside of scheduler guts that people
prefer to keep but really are rather deadlock prone. Fudge around it
by storing the text in a per-cpu buffer and poll it using the existing
printk_tick() handler.

This will drop output when its more frequent than once a tick, however
only the affinity thing could possible go that fast and for that just
one should suffice to notify the admin he's done something silly..

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-wua3lmkt3dg8nfts66o6brne@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 37 insertions, 3 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 13c0a1143f49..7ca7ba591e21 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1208,13 +1208,47 @@ int is_console_locked(void)
         return console_locked;
 }
 
+/*
+ * Delayed printk facility, for scheduler-internal messages:
+ */
+#define PRINTK_BUF_SIZE         512
+
+#define PRINTK_PENDING_WAKEUP   0x01
+#define PRINTK_PENDING_SCHED    0x02
+
 static DEFINE_PER_CPU(int, printk_pending);
+static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
+
+int printk_sched(const char *fmt, ...)
+{
+        unsigned long flags;
+        va_list args;
+        char *buf;
+        int r;
+
+        local_irq_save(flags);
+        buf = __get_cpu_var(printk_sched_buf);
+
+        va_start(args, fmt);
+        r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args);
+        va_end(args);
+
+        __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
+        local_irq_restore(flags);
+
+        return r;
+}
 
 void printk_tick(void)
 {
         if (__this_cpu_read(printk_pending)) {
-                __this_cpu_write(printk_pending, 0);
-                wake_up_interruptible(&log_wait);
+                int pending = __this_cpu_xchg(printk_pending, 0);
+                if (pending & PRINTK_PENDING_SCHED) {
+                        char *buf = __get_cpu_var(printk_sched_buf);
+                        printk(KERN_WARNING "[sched_delayed] %s", buf);
+                }
+                if (pending & PRINTK_PENDING_WAKEUP)
+                        wake_up_interruptible(&log_wait);
         }
 }
 
@@ -1228,7 +1262,7 @@ int printk_needs_cpu(int cpu)
 void wake_up_klogd(void)
 {
         if (waitqueue_active(&log_wait))
-                this_cpu_write(printk_pending, 1);
+                this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
 }
 
 /**