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SubjectRe: [Query] Preemption (hogging) of the work handler
Cc Petr Mladek.

On (07/12/16 16:19), Viresh Kumar wrote:
[..]
> Okay, we have tracked this BUG and its really interesting.

good find!

> I hacked the platform's serial driver to implement a putchar() routine
> that simply writes to the FIFO in polling mode, that helped us in
> tracing on where we are going wrong.
>
> The problem is that we are running asynchronous printks and we call
> wake_up_process() from the last running CPU which has disabled
> interrupts. That takes us to: try_to_wake_up().
>
> In our case the CPU gets deadlocked on this line in try_to_wake_up().
>
> raw_spin_lock_irqsave(&p->pi_lock, flags);

yeah, printk() can't handle these types of recursion. it can prevent
printk() calls issued from within the logbuf_lock spinlock section,
with some limitations:

if (unlikely(logbuf_cpu == smp_processor_id())) {
recursion_bug = true;
return;
}

raw_spin_lock(&logbuf_lock);
logbuf_cpu = this_cpu;
...
logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock);

so should, for instance, raw_spin_unlock() generate spin_dump(), printk()
will blow up (both sync and async), because logbuf_cpu is already reset.
it may look that async printk added another source of recursion - wake_up().
but, apparently, this is not exactly correct. because there is already a
wake_up() call in console_unlock() - up().

printk()
if (logbuf_cpu == smp_processor_id())
return;

raw_spin_lock(&logbuf_lock);
logbuf_cpu = this_cpu;
...
logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock);

console_trylock()
raw_spin_lock_irqsave(&sem->lock) << ***
raw_spin_unlock_irqsave(&sem->lock) << ***

console_unlock()
up()
raw_spin_lock_irqsave(&sem->lock) << ***
__up()
wake_up_process()
try_to_wake_up() << *** in may places


*** a printk() call from here will kill the system. either it will
recurse printk(), or spin forever in 'nested' printk() on one of
the already taken spin locks.

I had an idea of waking up a printk_kthread under logbuf_lock,
so `logbuf_cpu == smp_processor_id()' test would help here. But
it turned out to introduce a regression in printk() behaviour.
apart from that, it didn't fix any of the existing recursion
printks.

there is printk_deferred() printk that is supposed to be used for
printing under scheduler locks, but it won't help in all of the cases.

printk() has many issues.

> I will explain how:
>
> The try_to_wake_up() function takes us through the scheduler code (RT
> sched), to the hrtimer code, where we eventually call ktime_get() (for
> the MONOTONIC clock used for hrtimer). And this function has this:
>
> WARN_ON(timekeeping_suspended);
>
> This starts another printk while we are in the middle of
> wake_up_process() and the CPU tries to take the above lock again and
> gets stuck there :)
>
> This doesn't happen everytime because we don't always call ktime_get()
> and it is called only if hrtimer_active() returns false.
>
> This happened because of a WARN_ON() but it can happen anyway. Think
> about this case:
>
> - offline all CPUs, except 0
> - call any routine that prints messages after disabling interrupts,
> etc.
> - If any of the function within wake_up_process() does a print, we are
> screwed.
>
> So the thing is that we can't really call wake_up_process() in cases
> where the last CPU disables interrupts. And that's why my fixup patch
> (which moved to synchronous prints after suspend) really works.
>
> @Jan and Sergey: I would expect a patch from you guys to fix this
> properly :)
>
> Maybe something more in can_print_async() routine, like:
>
> only-one-cpu-online + irqs_disabled()
>

right. adding only (num_online_cpus() > 1) check to can_printk_async()
*in theory* can break some cases. for example, SMP system, with only
one online CPU, active rt_sched throttling (not necessarily because of
printk kthread, any other task will do), and some of interrupts services
by CPU0 keep calling printk(), so deferred printk IRQ will stay busy:

echo 0 > /sys/..../cpu{1..NR_CPUS}/online # only CPU0 is active

CPU0
sched()
printk_deferred()
IRQ
wake_up_klogd_work_func()
console_trylock()
console_unlock()

IRQ
printk()

IRQ
printk()
....
IRQ
printk()
...

console_sem_up()
return

with async printk here we can offload printing from IRQ.

the warning that you see is WARN_ON(timekeeping_suspended), so we have
timekeeping_suspended, checking for irqs_disabled() is a _bit_ non-intuitive,
I think, but in the existing scheme of things can work (at least tick_suspend()
comment suggests so). correct me if I'm wrong.


so... I think we can switch to sync printk mode in suspend_console() and
enable async printk from resume_console(). IOW, suspend/kexec are now
executed under sync printk mode.

we already call console_unlock() during suspend, which is synchronous,
many times (e.g. console_cpu_notify()).


something like below, perhaps. will this work for you?

---
kernel/printk/printk.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index bbb4180..786690e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -288,6 +288,11 @@ static u32 log_buf_len = __LOG_BUF_LEN;

/* Control whether printing to console must be synchronous. */
static bool __read_mostly printk_sync = true;
+/*
+ * Force sync printk mode during suspend/kexec, regardless whether
+ * console_suspend_enabled permits console suspend.
+ */
+static bool __read_mostly force_printk_sync;
/* Printing kthread for async printk */
static struct task_struct *printk_kthread;
/* When `true' printing thread has messages to print */
@@ -295,7 +300,7 @@ static bool printk_kthread_need_flush_console;

static inline bool can_printk_async(void)
{
- return !printk_sync && printk_kthread;
+ return !printk_sync && printk_kthread && !force_printk_sync;
}

/* Return log buffer address */
@@ -2027,6 +2032,7 @@ static bool suppress_message_printing(int level) { return false; }

/* Still needs to be defined for users */
DEFINE_PER_CPU(printk_func_t, printk_func);
+static bool __read_mostly force_printk_sync;

#endif /* CONFIG_PRINTK */

@@ -2163,6 +2169,8 @@ MODULE_PARM_DESC(console_suspend, "suspend console during suspend"
*/
void suspend_console(void)
{
+ force_printk_sync = true;
+
if (!console_suspend_enabled)
return;
printk("Suspending console(s) (use no_console_suspend to debug)\n");
@@ -2173,6 +2181,8 @@ void suspend_console(void)

void resume_console(void)
{
+ force_printk_sync = false;
+
if (!console_suspend_enabled)
return;
down_console_sem();
--
2.9.0.rc1
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