linux-kernel - Re: [PATCH] rcu: Make jiffies_till_sched

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Date:   Fri, 12 Jul 2019 06:02:42 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Byungchul Park <byungchul.park@....com>, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

On Fri, Jul 12, 2019 at 08:51:16AM -0400, Joel Fernandes wrote:
> 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?

Is this one of those cases where eBPF is the answer, regardless of
the question?  ;-)

							Thanx, Paul