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@xxxxxxxxxx> 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
