Messages in this thread | | | Date | Mon, 25 Jul 2016 10:49:21 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter() |
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On Mon, Jul 25, 2016 at 07:01:17PM +0200, Oleg Nesterov wrote: > Paul, sorry for delay. > > On 07/21, Paul E. McKenney wrote: > > > > On Thu, Jul 21, 2016 at 07:34:36PM +0200, Oleg Nesterov wrote: > > > On 07/20, Paul E. McKenney wrote: > > > > > > > > On Wed, Jul 20, 2016 at 07:16:03PM +0200, Oleg Nesterov wrote: > > > > > > > > > Now, suppose we add the additional enter/exit's: > > > > > > > > > > freeze_super(sb) > > > > > { > > > > > // this doesn't block > > > > > __rcu_sync_enter(SEM3); > > > > > __rcu_sync_enter(SEM2); > > > > > __rcu_sync_enter(SEM1); > > > > > > > > > > down_write(&sb->s_umount); > > > > > if (NEED_TO_FREEZE) { > > > > > percpu_down_write(SEM1); > > > > > > > > The above waits for the grace period initiated by __rcu_sync_enter(), > > > > correct? Presumably "yes", because it will invoke rcu_sync_enter(), which > > > > will see the state as GP_ENTER, and will thus wait. > > > > > > But if down_write() blocks and/or NEED_TO_FREEZE takes some time it > > > could already see the GP_PASSED state, or at least it can sleep less. > > > > > > > But your point is that if !NEED_TO_FREEZE, we will get here without > > > > waiting for a grace period. > > > > > > > > But why aren't the __rcu_sync_enter() and rcu_sync_exit() calls inside > > > > the "if" statement? > > > > > > Yes, if we do __rcu_sync_enter() inside "if", then rcu_sync_exit() can't > > > hit GP_ENTER. > > > > > > But why we should disallow this use-case? It does not complicate the code > > > at all. > > > > I do agree that it doesn't complicate the current implementation. > > But it relies on a global lock, so I am not at all confident that this > > implementation is the final word. > > Hmm. which global lock? Or did you mean freeze_super(), not rcu_sync?
OK, you are right, they are per-superblock locks rather than being global locks. Still, given that workloads that hammer a single filesystem hard are quite common, it might still eventually be of some concern.
> > And speaking of global locks, failing to discourage the pattern above > > means that the code is unnecessarily acquiring three global locks, > > which doesn't seem like a good thing to me. > > Well, I do not agree, but this wasn't written by me. Just in case, all these > locks above are not really global, they are per-sb, but this is minor.
Agreed.
> And the patches which changed sb->s_writers to use percpu_rw_semaphore/rcu_sync > didn't change this logic. > > Except the old implementation was buggy, and the readers were slower than now. > > > I agree that there are use cases for beginning-of-time __rcu_sync_enter() > > or whatever we end up naming it. > > OK, at least iiuc you agree that cgroup_init() can use __rcu_sync_enter(). > As for other potential use-cases, we will disccuss this later. I will have > to CC you anyway ;) > > So I'll send v2 with renames after I test it. Thanks again.
Sounds good!
Thanx, Paul
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