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Subject[RFC][PATCHv5 13/13] printk: move offloading logic to per-cpu
Date
We have a global offloading state and make the offloading
decision based on printing task pointer and timestamp. If
we keep seeing the same task performing printing for too
long (`atomic_print_limit') we request offloading. Similarly
when we see that printing is now performed by another task,
we reset the timestamp counter.

This, however, will not work in the following case:

===============================================================================

CPU0 CPU1
//taskA //taskB
preempt_disable() preempt_disable()

printk()
console_trylock()
console_unlock()
printing_task = taskA
up()
printk()
console_trylock()
console_unlock()
printing_task = taskB
^^^ reset offloading control
up()
printk()
console_trylock()
console_unlock()
printing_task = taskA
^^^ reset offloading control
up()
printk()
console_trylock()
console_unlock()
printing_task = taskB
^^^ reset offloading control
up()

/*
* X seconds later
*/

printk()
console_trylock()
console_unlock()
printing_task = taskA
^^^ reset offloading control
up()
printk()
console_trylock()
console_unlock()
printing_task = taskB
^^^ reset offloading control
up()

lockup! lockup!

===============================================================================

So this printk ping-pong confuses our offloading control logic.
Move it to per-CPU area and have a separate offloading control
on every CPU.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
kernel/printk/printk.c | 36 ++++++++++++++++++++----------------
1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ba82152ce5d9..f9799616e9fc 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -558,13 +558,14 @@ static inline void adj_atomic_print_limit(void)
#endif
}

-static inline unsigned long emergency_timeout(unsigned long ts)
+static inline unsigned long emergency_timeout(unsigned long now,
+ unsigned long ts)
{
#ifdef CONFIG_LOCKUP_DETECTOR
if (watchdog_thresh)
- return ts + 2 * watchdog_thresh;
+ return time_after_eq(now, ts + 2 * watchdog_thresh);
#endif
- return ts + 10 * atomic_print_limit;
+ return time_after_eq(now, ts + 10 * atomic_print_limit);
}

/*
@@ -574,13 +575,13 @@ static inline unsigned long emergency_timeout(unsigned long ts)
* amount of time a process can print from console_unlock().
*
* This function must be called from 'printk_safe' context under
- * console_sem lock.
+ * console_sem lock with preemption disabled.
*/
static inline bool console_offload_printing(void)
{
- static struct task_struct *printing_task;
- static unsigned long printing_start_ts;
- static unsigned long saved_csw;
+ static DEFINE_PER_CPU(struct task_struct, *printing_task);
+ static DEFINE_PER_CPU(unsigned long, printing_start_ts);
+ static DEFINE_PER_CPU(unsigned long, saved_csw);
unsigned long now = local_clock() >> 30LL; /* seconds */

if (printk_kthread_should_stop())
@@ -600,16 +601,17 @@ static inline bool console_offload_printing(void)
goto offload;

/* A new task - reset the counters. */
- if (printing_task != current) {
- printing_start_ts = local_clock() >> 30LL;
- saved_csw = current->nvcsw + current->nivcsw;
- printing_task = current;
+ if (this_cpu_read(printing_task) != current) {
+ this_cpu_write(printing_start_ts, local_clock() >> 30LL);
+ this_cpu_write(saved_csw, current->nvcsw + current->nivcsw);
+ this_cpu_write(printing_task, current);
return false;
}

adj_atomic_print_limit();

- if (!time_after_eq(now, printing_start_ts + atomic_print_limit))
+ if (!time_after_eq(now, this_cpu_read(printing_start_ts) +
+ atomic_print_limit))
return false;

if (current == printk_kthread) {
@@ -626,7 +628,7 @@ static inline bool console_offload_printing(void)
* back to console_unlock(), it will have another full
* `atomic_print_limit' time slice.
*/
- printing_start_ts = local_clock() >> 30LL;
+ this_cpu_write(printing_start_ts, local_clock() >> 30LL);
return true;
}

@@ -634,10 +636,12 @@ static inline bool console_offload_printing(void)
* A trivial emergency enforcement - give up on printk_kthread if
* we can't wake it up.
*/
- if (time_after_eq(now, emergency_timeout(printing_start_ts)) &&
- saved_csw == (current->nvcsw + current->nivcsw)) {
+ if (this_cpu_read(saved_csw) == (current->nvcsw + current->nivcsw)
+ && emergency_timeout(now, this_cpu_read(printing_start_ts))) {
+
printk_enforce_emergency = true;
- pr_crit("Declaring printk emergency mode.\n");
+ pr_crit("CPU%d declared a printk emergency mode.\n",
+ smp_processor_id());
return true;
}

--
2.14.1
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