From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
mingo@kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org,
john.stultz@linaro.org, dimitrysh@google.com, romlem@google.com,
ccross@google.com, tkjos@google.com
Subject: Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter()
Date: Mon, 25 Jul 2016 19:01:17 +0200 [thread overview]
Message-ID: <20160725170116.GA24149@redhat.com> (raw)
In-Reply-To: <20160722032641.GE7094@linux.vnet.ibm.com>
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?
> 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.
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.
Oleg.
next prev parent reply other threads:[~2016-07-25 17:01 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-14 18:25 [PATCH 0/2] locking/percpu-rwsem: Optimizations/tweaks Peter Zijlstra
2016-07-14 18:25 ` [PATCH 1/2] locking/percpu-rwsem: Optimize readers and reduce global impact Peter Zijlstra
2016-07-15 16:30 ` Oleg Nesterov
2016-07-15 19:47 ` Peter Zijlstra
2016-07-18 18:23 ` kbuild test robot
2016-07-18 22:51 ` kbuild test robot
2016-07-14 18:25 ` [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob Peter Zijlstra
2016-07-14 18:37 ` Peter Zijlstra
2016-07-14 18:43 ` Oleg Nesterov
2016-07-14 18:56 ` Peter Zijlstra
2016-07-14 19:20 ` Peter Zijlstra
2016-07-14 19:29 ` Paul E. McKenney
2016-07-14 19:38 ` Peter Zijlstra
2016-07-14 19:54 ` Paul E. McKenney
2016-07-15 13:27 ` Oleg Nesterov
2016-07-15 13:39 ` Paul E. McKenney
2016-07-15 13:45 ` Oleg Nesterov
2016-07-15 15:38 ` Paul E. McKenney
2016-07-15 16:49 ` Oleg Nesterov
2016-07-15 18:01 ` Paul E. McKenney
2016-07-16 17:10 ` [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter() Oleg Nesterov
2016-07-16 18:40 ` Oleg Nesterov
2016-07-18 11:54 ` Peter Zijlstra
2016-07-18 13:44 ` Oleg Nesterov
2016-07-19 20:50 ` Paul E. McKenney
2016-07-20 15:13 ` Oleg Nesterov
2016-07-20 20:58 ` Paul E. McKenney
2016-07-21 17:34 ` Oleg Nesterov
2016-07-20 17:16 ` Oleg Nesterov
2016-07-20 21:31 ` Paul E. McKenney
2016-07-21 17:34 ` Oleg Nesterov
2016-07-22 3:26 ` Paul E. McKenney
2016-07-25 17:01 ` Oleg Nesterov [this message]
2016-07-25 17:05 ` John Stultz
2016-07-25 17:26 ` Oleg Nesterov
2016-08-09 8:48 ` Peter Zijlstra
2016-07-25 17:49 ` Paul E. McKenney
2016-07-15 13:42 ` [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob Oleg Nesterov
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