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

[Steven Rostedt]

On Mon, 17 Feb 2020 14:03:56 -0800
"Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:

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

A day you have off, but still find yourself working ;-)

> 
> 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.

Hey! Does that mean we can add a static branch for that check?


struct static_key rcu_booting = STATIC_KEY_INIT_TRUE;

[...]

	if (READ_ONCE(rcu_state.gp_flags) ||
	    (current == t && !in_irq() && !in_serving_softirq())
		return;

	if (static_branch_unlikely(&rcu_booting) && !t)
		return;

At end of boot:

	static_key_disable(&rcu_booting);

That way we can really micro-optimize the slow path, and it basically
becomes a nop!

-- Steve

> 
> 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