Petr Mladek wrote: > On Wed 2016-03-02 21:01:03, Tetsuo Handa wrote: > > I have a question about "printk: set may_schedule for some of > > console_trylock() callers" in linux-next.git. > > > > I'm trying to dump information of all threads which might be relevant > > to stalling inside memory allocator. But it seems to me that since this > > patch changed to allow calling cond_resched() from printk() if it is > > safe to do so, it is now possible that the thread which invoked the OOM > > killer can sleep for minutes with the oom_lock mutex held when my dump is > > in progress. I want to release oom_lock mutex as soon as possible so > > that other threads can call out_of_memory() to get TIF_MEMDIE and exit > > their allocations. > > > > So, how can I prevent printk() triggered by out_of_memory() from sleeping > > for minutes with oom_lock mutex held? Guard it with preempt_disable() / > > preempt_enable() ? Guard it with rcu_read_lock() / rcu_read_unlock() ? > > > > preempt_disable() / preempt_enable() would do the job. I see. Thank you. > The question is where to put it. If you are concerned about > the delay, you might want to disable preemption around > the whole locked area, so that it works reasonable also > in the preemptive kernel. > We had a similar problem in the past. I'll again propose http://lkml.kernel.org/r/201509191605.CAF13520.QVSFHLtFJOMOOF@xxxxxxxxxxxxxxxxxxx . > I am looking forward to have the console printing offloaded > into the workqueues. Then printk() will become consistently > "fast" operation and will cause less surprises like this. > That's a good news. I was wishing that there were a dedicated kernel thread which does printk() operation. While at it, I ask for an API which waits for printk buffer to be flushed (something like below) so that a watchdog thread which might dump thousands of threads from sleepable context (like my dump) can avoid "** XXX printk messages dropped **" messages. ---------- diff --git a/include/linux/console.h b/include/linux/console.h index ea731af..11e936c 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -147,6 +147,7 @@ extern int unregister_console(struct console *); extern struct console *console_drivers; extern void console_lock(void); extern int console_trylock(void); +extern void wait_console_flushed(unsigned long timeout); extern void console_unlock(void); extern void console_conditional_schedule(void); extern void console_unblank(void); diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 9917f69..2eb60df 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -121,6 +121,15 @@ static int __down_trylock_console_sem(unsigned long ip) up(&console_sem);\ } while (0) +static int __down_timeout_console_sem(unsigned long timeout, unsigned long ip) +{ + if (down_timeout(&console_sem, timeout)) + return 1; + mutex_acquire(&console_lock_dep_map, 0, 1, ip); + return 0; +} +#define down_timeout_console_sem(timeout) __down_timeout_console_sem((timeout), _RET_IP_) + /* * This is used for debugging the mess that is the VT code by * keeping track if we have the console semaphore held. It's @@ -2125,6 +2134,21 @@ int console_trylock(void) } EXPORT_SYMBOL(console_trylock); +void wait_console_flushed(unsigned long timeout) +{ + might_sleep(); + + if (down_timeout_console_sem(timeout)) + return; + if (console_suspended) { + up_console_sem(); + return; + } + console_locked = 1; + console_may_schedule = 1; + console_unlock(); +} + int is_console_locked(void) { return console_locked; ---------- > Best Regards, > Petr > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>