Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Joel Fernandes]

On Fri, Jul 12, 2019 at 03:32:40PM +0900, Byungchul Park wrote:
> On Thu, Jul 11, 2019 at 03:58:39PM -0400, Joel Fernandes wrote:
> > Hmm, speaking of grace period durations, it seems to me the maximum grace
> > period ever is recorded in rcu_state.gp_max. However it is not read from
> > anywhere.
> > 
> > Any idea why it was added but not used?
> > 
> > I am interested in dumping this value just for fun, and seeing what I get.
> > 
> > I wonder also it is useful to dump it in rcutorture/rcuperf to find any
> > issues, or even expose it in sys/proc fs to see what worst case grace periods
> > look like.
> 
> Hi,
> 
> 	commit ae91aa0adb14dc33114d566feca2f7cb7a96b8b7
> 	rcu: Remove debugfs tracing
> 
> removed all debugfs tracing, gp_max also included.
> 
> And you sounds great. And even looks not that hard to add it like,
> 
> :)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index ad9dc86..86095ff 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1658,8 +1658,10 @@ static void rcu_gp_cleanup(void)
>  	raw_spin_lock_irq_rcu_node(rnp);
>  	rcu_state.gp_end = jiffies;
>  	gp_duration = rcu_state.gp_end - rcu_state.gp_start;
> -	if (gp_duration > rcu_state.gp_max)
> +	if (gp_duration > rcu_state.gp_max) {
>  		rcu_state.gp_max = gp_duration;
> +		trace_rcu_grace_period(something something);
> +	}

Yes, that makes sense. But I think it is much better off as a readable value
from a virtual fs. The drawback of tracing for this sort of thing are:
 - Tracing will only catch it if tracing is on
 - Tracing data can be lost if too many events, then no one has a clue what
   the max gp time is.
 - The data is already available in rcu_state::gp_max so copying it into the
   trace buffer seems a bit pointless IMHO
 - It is a lot easier on ones eyes to process a single counter than process
   heaps of traces.

I think a minimal set of RCU counters exposed to /proc or /sys should not
hurt and could do more good than not. The scheduler already does this for
scheduler statistics. I have seen Peter complain a lot about new tracepoints
but not much (or never) about new statistics.

Tracing has its strengths but may not apply well here IMO. I think a counter
like this could be useful for tuning of things like the jiffies_*_sched_qs,
the stall timeouts and also any other RCU knobs. What do you think?

- Joel