On Wed, Sep 02, 2020 at 06:07:05PM +0200, Ulf Hansson wrote: > On Wed, 2 Sep 2020 at 15:52, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Wed, Sep 02, 2020 at 08:49:11AM +0200, Ulf Hansson wrote: > > > On Tue, 1 Sep 2020 at 17:00, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > [ . . . ] > > > > > > Here is the list, though it is early in the morning here: > > > > > > > > 1. RCU_NONIDLE(). > > > > > > > > 2. Peter's patch, if it turns out to hoist your code out of what > > > > RCU considers to be the idle loop. > > > > > > > > 3. If the problem is trace events, use the _rcuidle() variant of the > > > > trace event. Instead of trace_blah(), use trace_blah_rcuidle(). > > > > > > > > 4. Switch from RCU (as in rcu_read_lock()) to SRCU (as in > > > > srcu_read_lock()). > > > > > > > > 5. Take Peter's patch a step further, moving the rcu_idle_enter() > > > > and rcu_idle_exit() calls as needed. But please keep in mind > > > > that these two functions require that irqs be disabled by their > > > > callers. > > > > > > > > 6. If RCU_NONIDLE() in inconvenient due to early exits and such, > > > > you could use the rcu_irq_enter_irqson() and rcu_irq_exit_irqson() > > > > functions that it calls. > > > > > > > > Do any of those help? > > > > > > Yes, they will, in one way or the other. Thanks for providing me with > > > all the available options. > > > > > > BTW, I still don't get what good rcu_idle_enter|exit() does, but I am > > > assuming those need to be called at some point before the CPU goes to > > > sleep. > > > > These functions allow RCU to leave idle CPUs undisturbed. If they > > were not invoked, RCU would periodically IPI idle CPUs to verify that > > there were no RCU readers running on them. This would be quite bad for > > battery lifetime, among other things. So the call to rcu_idle_enter() > > tells RCU that it may safely completely ignore this CPU until its next > > call to rcu_idle_exit(). > > Alright, thanks for explaining this, much appreciated. > > So in one way, we would also like to call rcu_idle_enter(), as soon as > we know there is no need for the RCU to be active. To prevent > unnecessary IPIs I mean. :-) Well, the IPIs don't happen until the better part of a second into the grace period. So delaying an rcu_idle_enter() a few microseconds, as Peter Zijlstra is proposing, is absolutely no problem whatsoever. And once the rcu_idle_enter() happens, the RCU grace-period kthread's next scan of the CPUs will see that this CPU needs to be ignored, so no more IPIs for it until it does the next rcu_idle_exit(), rcu_irq_enter(), or any of a number of other things that cause RCU to once again pay attention to that CPU. Thanx, Paul
