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SubjectRe: + printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch added to -mm tree
Hello,

On (12/02/15 15:57), akpm@linux-foundation.org wrote:
[..]
> @console_may_schedule tracks whether console_sem was acquired through lock
> or trylock. If the former, we're inside a sleepable context and
> console_conditional_schedule() performs cond_resched(). This allows
> console drivers which use console_lock for synchronization to yield while
> performing time-consuming operations such as scrolling.
>
> However, the actual console outputting is performed while holding irq-safe
> logbuf_lock, so console_unlock() clears @console_may_schedule before
> starting outputting lines. Also, only a few drivers call
> console_conditional_schedule() to begin with. This means that when a lot
> of lines need to be output by console_unlock(), for example on a console
> registration, the task doing console_unlock() may not yield for a long
> time on a non-preemptible kernel.
>
> If this happens with a slow console devices, for example a serial console,
> the outputting task may occupy the cpu for a very long time. Long enough
> to trigger softlockup and/or RCU stall warnings, which in turn pile more
> messages, sometimes enough to trigger the next cycle of warnings
> incapacitating the system.
>
> Fix it by making console_unlock() insert cond_resched() between lines if
> @console_may_schedule.

CPU2 still can cause lots of troubles. consider

CPU0 CPU1 CPU2
printk
... printk_deferred
printk wake_up_klogd
wake_up_klogd_work_func
console_trylock
console_unlock

printk_deferred() may be issued by scheduler, for example.

-ss

> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Calvin Owens <calvinowens@fb.com>
> Acked-by: Jan Kara <jack@suse.com>
> Cc: Dave Jones <davej@codemonkey.org.uk>
> Cc: Kyle McMartin <kyle@kernel.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
> kernel/printk/printk.c | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff -puN kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles kernel/printk/printk.c
> --- a/kernel/printk/printk.c~printk-do-cond_resched-between-lines-while-outputting-to-consoles
> +++ a/kernel/printk/printk.c
> @@ -2234,13 +2234,24 @@ void console_unlock(void)
> static u64 seen_seq;
> unsigned long flags;
> bool wake_klogd = false;
> - bool retry;
> + bool do_cond_resched, retry;
>
> if (console_suspended) {
> up_console_sem();
> return;
> }
>
> + /*
> + * Console drivers are called under logbuf_lock, so
> + * @console_may_schedule should be cleared before; however, 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 */
> @@ -2312,6 +2323,9 @@ skip:
> call_console_drivers(level, ext_text, ext_len, text, len);
> start_critical_timings();
> local_irq_restore(flags);
> +
> + if (do_cond_resched)
> + cond_resched();
> }
> console_locked = 0;
>
> _
>
> Patches currently in -mm which might be from tj@kernel.org are
>
> printk-do-cond_resched-between-lines-while-outputting-to-consoles.patch
>
> --
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