linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Date:	Fri, 11 Dec 2015 15:29:46 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Jan Kara <jack@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Srinivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long

On (12/11/15 13:27), Sergey Senozhatsky wrote:
[..]
> > static bool cpu_stop_printing(int printed_chars)
> > {
> >         /* Oops? Print everything now to maximize chances user will see it */
> >         if (oops_in_progress)
> >                 return false;
> >         if (!printk_offload_chars || printed_chars < printk_offload_chars)
> >                 return false;
> >         /*
> >          * Make sure we load fresh value of printing_tasks_spinning. Matches
> >          * the barrier in printing_task()
> >          */
> >         smp_rmb();
> >         if (atomic_read(&printing_tasks_spinning))
> >                 return true;
> >         wake_up(&print_queue);
> >
> -	return false;
> +	return true;
> > }

*just as an idea*, I was thinking about having two different offload
limits -- for user space processes doing console_unlock() for whatever
reason (printk in syscall or because of console_lock, etc.) and for
KTHREADS. the kernel threads can have normal offload_limit, while user
space processes can return back from syscal sooner (doing only half of
printk worload, for example). but this is probably too `custom', though
sort of make some sense.

(compile tested only)

---
 kernel/printk/printk.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 79915da..cff1dd1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -330,6 +330,7 @@ static struct kernel_param_ops offload_chars_ops = {
  * 0.1s maximum latency due to printing.
  */
 static unsigned int __read_mostly printk_offload_chars = 1000;
+static unsigned int __read_mostly printk_offload_limits[2] = {500, 1000};
 
 module_param_cb(offload_chars, &offload_chars_ops, &printk_offload_chars,
 		   S_IRUGO | S_IWUSR);
@@ -343,10 +344,14 @@ MODULE_PARM_DESC(offload_chars, "offload printing to console to a different"
  */
 static bool cpu_stop_printing(int printed_chars)
 {
+	bool type = current->flags & PF_KTHREAD;
+
 	/* Oops? Print everything now to maximize chances user will see it */
 	if (oops_in_progress)
 		return false;
-	if (!printk_offload_chars || printed_chars < printk_offload_chars)
+	if (!printk_offload_chars)
+		return false;
+	if (printed_chars < printk_offload_limits[type])
 		return false;
 	/*
 	 * Make sure we load fresh value of printing_tasks_spinning. Matches
@@ -2995,6 +3000,12 @@ out_err:
 	return ret;
 }
 
+static void offload_limits_set(void)
+{
+	printk_offload_limits[0] = printk_offload_chars >> 1;
+	printk_offload_limits[1] = printk_offload_chars;
+}
+
 static int offload_chars_set(const char *val, const struct kernel_param *kp)
 {
 	int ret;
@@ -3006,6 +3017,8 @@ static int offload_chars_set(const char *val, const struct kernel_param *kp)
 		mutex_unlock(&printing_kthread_mutex);
 		return ret;
 	}
+
+	offload_limits_set();
 	ret = printk_start_offload_kthreads();
 	mutex_unlock(&printing_kthread_mutex);
 	return ret;
@@ -3014,11 +3027,13 @@ static int offload_chars_set(const char *val, const struct kernel_param *kp)
 static void printk_offload_init(void)
 {
 	mutex_lock(&printing_kthread_mutex);
+	offload_limits_set();
 	if (num_possible_cpus() <= 1) {
 		/* Offloading doesn't make sense. Disable print offloading. */
 		printk_offload_chars = 0;
-	} else
+	} else {
 		printk_start_offload_kthreads();
+	}
 	mutex_unlock(&printing_kthread_mutex);
 }
 
-- 
2.6.4

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