Re: [PATCH v3 4/7] rcu: Improve handling of synchronize_rcu() users

[Uladzislau Rezki <urezki@gmail.com>]

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().

I can add a comment about your concern and we can find the best approach
later, if it is OK with you!

--
Uladzislau Rezki