linux-kernel - Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running

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Date:   Mon, 17 Feb 2020 14:03:56 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
 before GP kthread is running

On Mon, Feb 17, 2020 at 03:25:17PM -0500, Steven Rostedt wrote:
> On Sat, 15 Feb 2020 05:42:08 -0800
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > 
> > And does the following V2 look better?
> > 
> 
> For the issue I brought up, yes. But now I have to ask...
> 
> > @@ -1252,10 +1253,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
> >   */
> >  static void rcu_gp_kthread_wake(void)
> >  {
> > -	if ((current == rcu_state.gp_kthread &&
> > -	     !in_irq() && !in_serving_softirq()) ||
> > -	    !READ_ONCE(rcu_state.gp_flags) ||
> > -	    !rcu_state.gp_kthread)
> > +	struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
> > +
> > +	if ((current == t && !in_irq() && !in_serving_softirq()) ||
> > +	    !READ_ONCE(rcu_state.gp_flags) || !t)
> 
> Why not test !t first? As that is the fastest operation in the if
> statement, and will shortcut all the other operations if it is true.
> 
> As I like to micro-optimize ;-), for or (||) statements, I like to add
> the fastest operations first. To me, that would be:
> 
> 	if (!t || READ_ONCE(rcu_state.gp_flags) ||
> 	    (current == t && !in_irq() && !in_serving_softirq()))
> 		return;
> 
> Note, in_irq() reads preempt_count which is not always a fast operation.

And what is a day without micro-optimization of a slowpath?  :-)

OK, let's see...

Grace-period kthread wakeups are normally mediated by rcu_start_this_gp(),
which uses a funnel lock to consolidate concurrent requests to start
a grace period.  If a grace period is already in progress, it refrains
from doing a wakeup because that means that the grace-period kthread
will check for another grace period being needed at the end of the
current grace period.

Exceptions include:

o	The wakeup reporting the last quiescent state of the current
	grace period.

o	Emergency situations such as callback overloads and RCU CPU stalls.

So on a busy system that is not overloaded, the common case is that
rcu_gp_kthread_wake() is invoked only once per grace period because there
is no emergency and there is a grace period in progress.  If this system
has short idle periods and a fair number of quiescent states, a reasonable
amount of idle time, then the last quiescent state will not normally be
detected by the grace-period kthread.  But workloads can of course vary.

The "!t" holds only during early boot.  So we could put a likely() around
the "t".  But more to the point, at runtime, "!t" would always be false,
so it really should be last in the list of "||" clauses.  This isn't
enough of a fastpath for a static branch to make sense.

The "!READ_ONCE(rcu_state.gp_flags)" will normally hold, though it is
false often enough to pay for itself.  Or has been in the past, anyway.
I suspect that access to the global variable rcu_state.gp_flags is not
always fast either.

So I am having difficulty talking myself into modifying this one given
the frequency of operations.

							Thanx, Paul