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

[Paul E. McKenney]

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?

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")

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()?

> > > 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();
	}

							Thanx, Paul