Messages in this thread | | | From | Wander Costa <> | Date | Fri, 7 Jan 2022 17:16:23 -0300 | Subject | Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices |
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On Fri, Jan 7, 2022 at 4:03 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > On Fri, Nov 12, 2021 at 06:57:55AM -0800, Paul E. McKenney wrote: > > On Fri, Nov 12, 2021 at 11:42:39AM -0300, Wander Costa wrote: > > > On Thu, Nov 11, 2021 at 10:42 PM Sergey Senozhatsky > > > <senozhatsky@chromium.org> wrote: > > > > > > > > On (21/11/11 16:59), Wander Lairson Costa wrote: > > > > > > > > > > If we have a reasonable large dataset to flush in the printk ring > > > > > buffer in the presence of a slow console device (like a serial port > > > > > with a low baud rate configured), the RCU stall detector may report > > > > > warnings. > > > > > > > > > > This patch suppresses RCU stall warnings while flushing the ring buffer > > > > > to the console. > > > > > > > > > [..] > > > > > +extern int rcu_cpu_stall_suppress; > > > > > + > > > > > +static void rcu_console_stall_suppress(void) > > > > > +{ > > > > > + if (!rcu_cpu_stall_suppress) > > > > > + rcu_cpu_stall_suppress = 4; > > > > > +} > > > > > + > > > > > +static void rcu_console_stall_unsuppress(void) > > > > > +{ > > > > > + if (rcu_cpu_stall_suppress == 4) > > > > > + rcu_cpu_stall_suppress = 0; > > > > > +} > > > > > + > > > > > /** > > > > > * console_unlock - unlock the console system > > > > > * > > > > > @@ -2634,6 +2648,9 @@ void console_unlock(void) > > > > > * and cleared after the "again" goto label. > > > > > */ > > > > > do_cond_resched = console_may_schedule; > > > > > + > > > > > + rcu_console_stall_suppress(); > > > > > + > > > > > again: > > > > > console_may_schedule = 0; > > > > > > > > > > @@ -2645,6 +2662,7 @@ void console_unlock(void) > > > > > if (!can_use_console()) { > > > > > console_locked = 0; > > > > > up_console_sem(); > > > > > + rcu_console_stall_unsuppress(); > > > > > return; > > > > > } > > > > > > > > > > @@ -2716,8 +2734,10 @@ void console_unlock(void) > > > > > > > > > > handover = console_lock_spinning_disable_and_check(); > > > > > printk_safe_exit_irqrestore(flags); > > > > > - if (handover) > > > > > + if (handover) { > > > > > + rcu_console_stall_unsuppress(); > > > > > return; > > > > > + } > > > > > > > > > > if (do_cond_resched) > > > > > cond_resched(); > > > > > @@ -2738,6 +2758,8 @@ void console_unlock(void) > > > > > retry = prb_read_valid(prb, next_seq, NULL); > > > > > if (retry && console_trylock()) > > > > > goto again; > > > > > + > > > > > + rcu_console_stall_unsuppress(); > > > > > } > > > > > > > > May be we can just start touching watchdogs from printing routine? > > > > > > > Hrm, console_unlock is called from vprintk_emit [0] with preemption > > > disabled. and it already has the logic implemented to call > > > cond_resched when possible [1]. > > > > > > [0] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2244 > > > [1] https://elixir.bootlin.com/linux/latest/source/kernel/printk/printk.c#L2719 > > > > So when we are having problems is when console_may_schedule == 0? > > Just following up... Any progress on this? The ability to suppress RCU > CPU stall warnings due to console slowness would likely be valuable to > quite a few people. >
My understanding is that the consensus is that the proper fix is the printk threads currently under work and it wouldn't take long before it is ready to review.
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