Messages in this thread | | | From | Wander Costa <> | Date | Fri, 12 Nov 2021 11:42:39 -0300 | Subject | Re: [PATCH v2 1/1] printk: suppress rcu stall warnings caused by slow console devices |
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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
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