> -----Original Message-----
> From: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Sent: Wednesday, February 17, 2021 2:50 AM
>
> On Mon, Feb 15, 2021 at 05:53:25PM +0900, Sangmoon Kim wrote:
> > The event allows us to trace the RCU stall when
> > sysctl_panic_on_rcu_stall is disabled.
> >
> > The first parameter is the name of RCU flavour like other trace
> > events. The second one shows us which function detected stalls.
> >
> > The RCU stall is mainly caused by external factors such as interrupt
> > handling or task scheduling or something else. Therefore, this event
> > uses TRACE_EVENT macro, not dedicated one, so that someone interested
> > in the RCU stall can use it without CONFIG_RCU_TRACE.
> >
> > Signed-off-by: Sangmoon Kim <sangmoon.kim@xxxxxxxxxxx>
>
> The patch looks plausible, but I have to ask... Why not instead just
> get the existing information out of the console log?
>
> Thanx, Paul
This can provide a trigger point for the RCU stall warning.
If a module in the kernel wants to trace the stall for debugging purposes,
there is a cost of continuing to parse the console log.
This tracepoint is useful because it is hard to pay these costs
especially on mobile devices.
Thanks,
Sangmoon
>
> > ---
> > include/trace/events/rcu.h | 28 ++++++++++++++++++++++++++++
> > kernel/rcu/tree_exp.h | 2 ++
> > kernel/rcu/tree_stall.h | 2 ++
> > 3 files changed, 32 insertions(+)
> >
> > diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> > index 155b5cb43cfd..8476f3161bd0 100644
> > --- a/include/trace/events/rcu.h
> > +++ b/include/trace/events/rcu.h
> > @@ -432,6 +432,34 @@ TRACE_EVENT_RCU(rcu_fqs,
> > __entry->cpu, __entry->qsevent)
> > );
> >
> > +/*
> > + * Tracepoint for RCU stall events. Takes a string identifying the RCU flavor
> > + * and a string identifying which function detected the RCU stall as follows:
> > + *
> > + * "StallDetected": Scheduler-tick detects other CPU's stalls.
> > + * "SelfDetected": Scheduler-tick detects a current CPU's stall.
> > + * "ExpeditedStall": Expedited grace period detects stalls.
> > + */
> > +TRACE_EVENT(rcu_stall_warning,
> > +
> > + TP_PROTO(const char *rcuname, const char *msg),
> > +
> > + TP_ARGS(rcuname, msg),
> > +
> > + TP_STRUCT__entry(
> > + __field(const char *, rcuname)
> > + __field(const char *, msg)
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->rcuname = rcuname;
> > + __entry->msg = msg;
> > + ),
> > +
> > + TP_printk("%s %s",
> > + __entry->rcuname, __entry->msg)
> > +);
> > +
> > #endif /* #if defined(CONFIG_TREE_RCU) */
> >
> > /*
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 8760b6ead770..c16618284cb2 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -566,6 +566,8 @@ static void synchronize_rcu_expedited_wait(void)
> > dump_cpu_task(cpu);
> > }
> > }
> > + trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
> > +
> > jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
> > }
> > }
> > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > index 70d48c52fabc..e93df4fac5b1 100644
> > --- a/kernel/rcu/tree_stall.h
> > +++ b/kernel/rcu/tree_stall.h
> > @@ -531,6 +531,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
> >
> > rcu_check_gp_kthread_starvation();
> >
> > + trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
> > panic_on_rcu_stall();
> >
> > rcu_force_quiescent_state(); /* Kick them all. */
> > @@ -575,6 +576,7 @@ static void print_cpu_stall(unsigned long gps)
> > jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> >
> > + trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
> > panic_on_rcu_stall();
> >
> > /*
> > --
> > 2.17.1
> >
