On Wed, Oct 07, 2020 at 03:34:38PM -0700, Paul E. McKenney wrote: > On Sun, Oct 04, 2020 at 10:11:32PM -0400, Joel Fernandes (Google) wrote: > > During testing, I see it is possible that rcu_pending() returns 1 when > > offloaded callbacks are ready to execute thus raising the RCU softirq. > > > > However, softirq does not execute offloaded callbacks. They are executed in a > > kthread which is awakened independent of the softirq. > > > > This commit therefore avoids raising the softirq in the first place. That's > > probably a good thing considering that the purpose of callback offloading is to > > reduce softirq activity. > > > > Passed 30 minute tests of TREE01 through TREE09 each. > > > > On TREE08, I notice that there is atmost 150us from when the softirq was > > NOT raised when ready cbs were present, to when the ready callbacks were > > invoked by the rcuop thread. This also further confirms that there is no > > need to raise the softirq for ready cbs in the first place. > > > > Cc: neeraju@xxxxxxxxxxxxxx > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > Looks good, applied, thank you! I reworked things a bit based on > previous patches and to more precisely capture why this patch does > not cause additional problems. Please let me know if I messed > anything up. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 33847a34a2d261354a79b4a24d9d37222e8ec888 > Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Date: Wed Oct 7 13:50:36 2020 -0700 > > rcu/tree: nocb: Avoid raising softirq for offloaded ready-to-execute CBs > > Testing showed that rcu_pending() can return 1 when offloaded callbacks > are ready to execute. This invokes RCU core processing, for example, > by raising RCU_SOFTIRQ, eventually resulting in a call to rcu_core(). > However, rcu_core() explicitly avoids in any way manipulating offloaded > callbacks, which are instead handled by the rcuog and rcuoc kthreads, > which work independently of rcu_core(). > > One exception to this independence is that rcu_core() invokes > do_nocb_deferred_wakeup(), however, rcu_pending() also checks > rcu_nocb_need_deferred_wakeup() in order to correctly handle this case, > invoking rcu_core() when needed. > > This commit therefore avoids needlessly invoking RCU core processing > by checking rcu_segcblist_ready_cbs() only on non-offloaded CPUs. > This reduces overhead, for example, by reducing softirq activity. > > This change passed 30 minute tests of TREE01 through TREE09 each. > > On TREE08, there is at most 150us from the time that rcu_pending() chose > not to invoke RCU core processing to the time when the ready callbacks > were invoked by the rcuoc kthread. This provides further evidence that > there is no need to invoke rcu_core() for offloaded callbacks that are > ready to invoke. > > Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Thanks!
