On Thu, Oct 21, 2021 at 03:24:35PM -0700, Paul E. McKenney wrote: > > > > > So you can let the heavy real-time workloads > > > > > have slowish Qdisc switching and IPI the IPI-insensitive workloads: > > > > > > > > > > oldstate = start_poll_synchronize_rcu(); > > > > > > > > > > // do something. > > > > > > > > > > if (IS_ENABLED(CONFIG_PREEMPT_RT)) > > > > > cond_synchronize_rcu(oldstate); > > > > > else if (!poll_state_synchronize_rcu(oldstate)) > > > > > synchronize_rcu_expedited(); > > > > > > > > I was concerned about the system-wide impact synchronize_rcu_expedited() > > > > has, but this at least avoids doing it if it's unnecessary. If it's bad > > > > form to leave mini_Qdisc.rcu_state initialized to 0, this option is > > > > preferable from my perspective. > > > > > > This might well be the best approach if multiple swaps are at all common. > > > > > > But if needed, we really could do both: Make initial swaps really fast, > > > and improve the performance of later swaps. > > > > Using this the time for mini_qdisc_pair_swap() to complete was similar > > to my earlier patch with mini_Qdisc.rcu_state initialized to 0. So this > > seems acceptible for my purposes. > > Not sure which approach "this" refers to. For the moment, I am assuming > the last code fragment shown above. Yes, that is correct. Apologies that I wasn't clear. > > > > > Yet another approach is this: > > > > > > > > > > oldstate = start_poll_synchronize_rcu(); > > > > > > > > > > // do something. > > > > > > > > > > while (!poll_state_synchronize_rcu(oldstate)) > > > > > schedule_timeout_idle(1); > > > > > > > > > > The advantage of this one is that you get credit for partial progress > > > > > through grace periods, but it won't be anywhere near as fast as > > > > > synchronize_rcu_expedited(). > > > > > > > > If we need to initialize mini_Qdisc.rcu_state up front this approach > > > > probably won't improve things much for my problem. > > > > > > It all depends on how long between swaps. ;-) > > > > This approach helps, but I still see about 50% of the > > mini_qdisc_pair_swap() calls taking tens of milliseconds. This is > > similar to what I saw using cond_synchronize_rcu() with rcu_state > > initialized from get_state_synchronize_rcu(). > > OK, which means that the swaps are too closely spaced to get much > benefit about 50% of the time. Yes, or in my case it's the time between mini_qdisc_pair_init() and the first swap, which is why initializing rcu_state to 0 helped before. > > Wouldn't this also behave very poorly in the wrapping case you mentioned > > above? > > I confess, I have no idea what case is the "wrapping case". This question was based on a misunderstanding on my part, you can ignore it. > But any implementation that needs to wait for a full normal > (non-expedited) grace period will suffer at least 50% of the time > when running your workload. And of the above code fragments, only the > synchronize_rcu_expedited() variants avoid waiting for a full normal > grace period. > > > In the end, I'm happy to move forward using synchronize_rcu_expedited(). > > I'm going to test a bit more, and I should have a patch out sometime > > tomorrow. > > Works for me! At least given the CONFIG_PREEMPT_RT check. Yes, I included that check. Thanks, Seth
