Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu()

[Paul E. McKenney]

On Mon, Nov 12, 2018 at 03:07:10AM +0100, Peter Zijlstra wrote:
> On Sun, Nov 11, 2018 at 05:47:36PM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 12, 2018 at 01:53:29AM +0100, Peter Zijlstra wrote:
> > > On Sun, Nov 11, 2018 at 04:45:28PM -0800, Paul E. McKenney wrote:
> > > > On Mon, Nov 12, 2018 at 01:12:33AM +0100, Peter Zijlstra wrote:
> > > > > On Sun, Nov 11, 2018 at 11:43:52AM -0800, Paul E. McKenney wrote:
> > > > > > Now that synchronize_rcu() waits for preempt-disable regions of code
> > > > > > as well as RCU read-side critical sections, synchronize_sched() can be
> > > > > > replaced by synchronize_rcu().  This commit therefore makes this change.
> > > > > 
> > > > > Yes, but it also waits for an actual RCU quiestent state, which makes
> > > > > synchoinize_rcu() potentially much more expensive than an actual
> > > > > synchronize_sched().
> > > > 
> > > > None of the readers have changed.
> > > > 
> > > > For the updaters, if CONFIG_PREEMPT=n, synchronize_rcu() and
> > > > synchronize_sched() always were one and the same.  When CONFIG_PREEMPT=y,
> > > > synchronize_rcu() and synchronize_sched() are now one and the same.
> > > 
> > > The Changelog does not state this; and does the commit that makes that
> > > happen state the regression potential?
> > 
> > The Changelog says this:
> > 
> > 	Now that synchronize_rcu() waits for preempt-disable
> > 	regions of code as well as RCU read-side critical sections,
> > 	synchronize_sched() can be replaced by synchronize_rcu().
> > 	This commit therefore makes this change.
> > 
> > The "synchronize_rcu() waits for preempt-disable regions of code as
> > well as RCU read-side critical sections" seems pretty unambiguous to me.
> > Exactly what more are you wanting said there?
> 
> The quoted bit only states that synchronize_rcu() is sufficient; it does
> not say it is equivalent and the patch is a nop. It also doesn't say
> that the purpose is to get rid of the synchronize_sched() function.
> 
> > There were quite a few commits involved in making this happen.  Perhaps
> > the most pertinent are these:
> > 
> > 3e3100989869 ("rcu: Defer reporting RCU-preempt quiescent states when disabled")
> > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds")
> 
> The latter; it does not mention that this will possible make
> synchronize_sched() quite a bit more expensive on PREEMPT=y builds :/

In theory, sure.  In practice, people have switched any number of
things from RCU-sched to RCU and back without problems.

> > Normal grace periods are almost always quite long compared to typical
> > read-side critical sections, preempt-disable regions of code, and so on.
> > So in the common case this should be OK.  Or are you instead worried
> > about synchronize_sched_expedited()?
> 
> No, I still feel expedited should not exist at all ;-)

I figured as much.  ;-)

> But for PREEMPT=y synchronize_sched() can be quite a bit shorter than
> synchronize_rcu(), since we don't have to wait for preempted read side
> stuff.

Again, there are quite a few places that have managed that transition
without issue.  Why do you expect this change to have problems that have
not been seen elsewhere?

> > > > > So why are we doing this?
> > > > 
> > > > Given that synchronize_rcu() and synchronize_sched() are now always one
> > > > and the same, this is a distinction without a difference.
> > > 
> > > The Changelog did not state a reason for the patch. Therefore it is a
> > > bad patch.
> > 
> > ???  Here is the current definition of synchronize_sched() in mainline:
> > 
> > 	static inline void synchronize_sched(void)
> > 	{
> > 		synchronize_rcu();
> > 	}
> 
> Again, the patch didn't say that.
> 
> If the Changelog would've read something like:
> 
> "Since synchronize_sched() is now equivalent to synchronize_rcu(),
> replace the synchronize_sched() usage such that we can eventually remove
> the interface."
> 
> It would've been clear that the patch is a nop and what the purpose
> was.

I can easily make that change.

							Thanx, Paul