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@kernel.org 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@kernel.org wrote:
[ . . . ]
Here is the list, though it is early in the morning here:
RCU_NONIDLE().Peter's patch, if it turns out to hoist your code out of what RCU considers to be the idle loop.If the problem is trace events, use the _rcuidle() variant of the trace event. Instead of trace_blah(), use trace_blah_rcuidle().Switch from RCU (as in rcu_read_lock()) to SRCU (as in srcu_read_lock()).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.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