On (12/10/15 23:56), Sergey Senozhatsky wrote:
>
> A silly minor nitpick
>
ah.. nope, I lied.
this part
>+static int printk_start_offload_kthreads(void)
> {
>- struct console *con;
> int i;
> struct task_struct *task;
>
>+ /* Does handover of printing make any sense? */
>+ if (printk_offload_chars == 0 || num_possible_cpus() <= 1)
>+ return 0;
>+ for (i = 0; i < PRINTING_TASKS; i++) {
>+ if (printing_kthread[i])
>+ continue;
>+ task = kthread_run(printing_task, NULL, "print/%d", i);
>+ if (IS_ERR(task))
>+ goto out_err;
>+ printing_kthread[i] = task;
>+ }
>+ return 0;
>+out_err:
>+ pr_err("printk: Cannot create printing thread: %ld\n", PTR_ERR(task));
>+ /* Disable offloading if creating kthreads failed */
>+ printk_offload_chars = 0;
>+ return PTR_ERR(task);
>+}
we can keep printk_offload_chars if we have created at least one thread,
in theory. if we are in heavy or nearly hevy (which is 'a process will spin in
unlock_console() with preemption_disabled for far too long') printk traffic case
(and that's what I want to fix from my side) then we have at least 1 process
doing
printk->console_unlock() very often, so breaking that 'while (1)'
console_unlock()
loop is a good thing here. which may be a corner case.
if we want to zero `printk_offload_chars' then how about bringing back Tejun
Heo's
"printk: do cond_resched() between lines while outputting to consoles"?
Besides, we need this for !CONFIG_PRINTK_OFFLOAD kernel.
===8<====
if printk_start_offload_kthreads() has failed or if for some
other reason `printk_offload_chars' ended up to be 0, we still
need to try to break up long console_unlock() loops and try to
reschedule.
Signed-off-by: Sergey Senozhatsky <[email protected]>
---
kernel/printk/printk.c | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 21b0fb9..fc8c493 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2285,6 +2285,16 @@ static bool cpu_stop_printing(int printed_chars)
return false;
}
+static bool cpu_should_cond_resched(bool do_cond_resched)
+{
+ /* Oops? Print everything now to maximize chances user will see it */
+ if (oops_in_progress)
+ return false;
+ if (!printk_offload_chars && do_cond_resched)
+ return true;
+ return false;
+}
+
/**
* console_unlock - unlock the console system
*
@@ -2309,7 +2319,7 @@ void console_unlock(void)
unsigned long flags;
bool wake_klogd = false;
bool retry;
- bool hand_over = false;
+ bool hand_over = false, do_cond_resched;
int printed_chars = 0;
if (console_suspended) {
@@ -2317,6 +2327,15 @@ void console_unlock(void)
return;
}
+ /*
+ * We may end up dumping a lot of lines, for example, if called
+ * from console registration path, and should invoke cond_resched()
+ * between lines if allowable. Not doing so can cause a very long
+ * scheduling stall on a slow console leading to RCU stall and
+ * softlockup warnings which exacerbate the issue with more
+ * messages practically incapacitating the system.
+ */
+ do_cond_resched = console_may_schedule;
console_may_schedule = 0;
/* flush buffered message fragment immediately to console */
@@ -2397,6 +2416,12 @@ skip:
call_console_drivers(level, ext_text, ext_len, text, len);
start_critical_timings();
printed_chars += len;
+
+ if (unlikely(cpu_should_cond_resched(do_cond_resched))) {
+ raw_spin_unlock_irqrestore(&print_lock, flags);
+ cond_resched();
+ raw_spin_lock_irqsave(&print_lock, flags);
+ }
}
/* Release the exclusive_console once it is used */
--
2.6.3
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