On Wed, Jan 03, 2024 at 09:56:42AM -0800, Paul E. McKenney wrote: > On Wed, Jan 03, 2024 at 06:35:20PM +0100, Uladzislau Rezki wrote: > > On Wed, Jan 03, 2024 at 06:47:30AM -0800, Paul E. McKenney wrote: > > > On Wed, Jan 03, 2024 at 02:16:00PM +0100, Uladzislau Rezki wrote: > > > > On Tue, Jan 02, 2024 at 11:25:13AM -0800, Paul E. McKenney wrote: > > > > > On Tue, Jan 02, 2024 at 01:52:26PM +0100, Uladzislau Rezki wrote: > > > > > > Hello, Paul! > > > > > > > > > > > > Sorry for late answer, it is because of holidays :) > > > > > > > > > > > > > > > > The problem is that, we are limited in number of "wait-heads" which we > > > > > > > > > > add as a marker node for this/current grace period. If there are more clients > > > > > > > > > > and there is no a wait-head available it means that a system, the deferred > > > > > > > > > > kworker, is slow in processing callbacks, thus all wait-nodes are in use. > > > > > > > > > > > > > > > > > > > > That is why we need an extra grace period. Basically to repeat our try one > > > > > > > > > > more time, i.e. it might be that a current grace period is not able to handle > > > > > > > > > > users due to the fact that a system is doing really slow, but this is rather > > > > > > > > > > a corner case and is not a problem. > > > > > > > > > > > > > > > > > > But in that case, the real issue is not the need for an extra grace > > > > > > > > > period, but rather the need for the wakeup processing to happen, correct? > > > > > > > > > Or am I missing something subtle here? > > > > > > > > > > > > > > > > > Basically, yes. If we had a spare dummy-node we could process the users > > > > > > > > by the current GP(no need in extra). Why we may not have it - it is because > > > > > > > > like you pointed: > > > > > > > > > > > > > > > > - wake-up issue, i.e. wake-up time + when we are on_cpu; > > > > > > > > - slow list process. For example priority. The kworker is not > > > > > > > > given enough CPU time to do the progress, thus "dummy-nodes" > > > > > > > > are not released in time for reuse. > > > > > > > > > > > > > > > > Therefore, en extra GP is requested if there is a high flow of > > > > > > > > synchronize_rcu() users and kworker is not able to do a progress > > > > > > > > in time. > > > > > > > > > > > > > > > > For example 60K+ parallel synchronize_rcu() users will trigger it. > > > > > > > > > > > > > > OK, but what bad thing would happen if that was moved to precede the > > > > > > > rcu_seq_start(&rcu_state.gp_seq)? That way, the requested grace period > > > > > > > would be the same as the one that is just now starting. > > > > > > > > > > > > > > Something like this? > > > > > > > > > > > > > > start_new_poll = rcu_sr_normal_gp_init(); > > > > > > > > > > > > > > /* Record GP times before starting GP, hence rcu_seq_start(). */ > > > > > > > rcu_seq_start(&rcu_state.gp_seq); > > > > > > > ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq); > > > > > > > > > > > > > I had a concern about the case when rcu_sr_normal_gp_init() handles what > > > > > > we currently have, in terms of requests. Right after that there is/are > > > > > > extra sync requests which invoke the start_poll_synchronize_rcu() but > > > > > > since a GP has been requested before it will not request an extra one. So > > > > > > "last" incoming users might not be processed. > > > > > > > > > > > > That is why i have placed the rcu_sr_normal_gp_init() after a gp_seq is > > > > > > updated. > > > > > > > > > > > > I can miss something, so please comment. Apart of that we can move it > > > > > > as you proposed. > > > > > > > > > > Couldn't that possibility be handled by a check in rcu_gp_cleanup()? > > > > > > > > > It is controlled by the caller anyway, i.e. if a new GP is needed. > > > > > > > > I am not 100% sure it is as straightforward as it could look like to > > > > handle it in the rcu_sr_normal_gp_cleaup() function. At least i see > > > > that we need to access to the first element of llist and find out if > > > > it is a wait-dummy-head or not. If not we know there are extra incoming > > > > calls. > > > > > > > > So that way requires extra calling of start_poll_synchronize_rcu(). > > > > > > If this is invoked early enough in rcu_gp_cleanup(), all that needs to > > > happen is to set the need_gp flag. Plus you can count the number of > > > requests, and snapshot that number at rcu_gp_init() time and check to > > > see if it changed at rcu_gp_cleanup() time. Later on, this could be > > > used to reduce the number of wakeups, correct? > > > > > You mean instead of waking-up a gp-kthread just continue processing of > > new users if they are exist? If so, i think, we can implement it as separate > > patches. > > Agreed, this is an optimization, and thus should be a separate patch. > > > > > I can add a comment about your concern and we can find the best approach > > > > later, if it is OK with you! > > > > > > I agree that this should be added via a later patch, though I have not > > > yet given up on the possibility that this patch might be simple enough > > > to be later in this same series. > > > > > Maybe there is a small misunderstanding. Please note, the rcu_sr_normal_gp_init() > > function does not request any new gp, i.e. our approach does not do any extra GP > > requests. It happens only if there are no any dummy-wait-head available as we > > discussed it earlier. > > The start_poll_synchronize_rcu() added by your patch 4/7 will request > an additional grace period because it is invoked after rcu_seq_start() > is called, correct? Or am I missing something subtle here? > <snip> + // New poll request after rnp unlock + if (start_new_poll) + (void) start_poll_synchronize_rcu(); + <snip> The "start_new_poll" is set to "true" only when _this_ GP is not able to handle anything and there are outstanding users. It happens when the rcu_sr_normal_gp_init() function was not able to insert a dummy separator to the llist, because there were no left dummy-nodes(fixed number of them) due to the fact that all of them are "in-use". The reason why there are no dummy-nodes is because of slow progress because it is done by dedicated kworker. I can trigger it, i mean when we need an addition GP, start_new_pool is 1, only when i run 20 000 processes concurrently in a tight loop: <snip> while (1) synchronize_rcu(); <snip> in that scenario we start to ask for an addition GP because we are not up to speed, i.e. a system is slow in processing callbacks and we need some time until wait-node/nodes is/are released for reuse. We need a next GP to move it forward, i.e. to repeat a try of attaching a dummy-node. -- Uladzislau Rezki
