On Mon, Feb 17, 2020 at 05:21:31PM -0500, Steven Rostedt wrote: > 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! ;-) ;-) ;-) There might be some place where static branches would be of use in RCU, but unfortunately none that I can think of on the read side. Though I am once again making progress on Lai Jiangshan's patchset, which might one day result in inlining of __rcu_read_lock() and __rcu_read_unlock() in PREEMPT=y kernels. Thanx, Paul > -- 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 >