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Date:   Mon, 12 Nov 2018 10:00:47 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, mingo@kernel.org,
        jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched()
 with synchronize_rcu()
Message-ID: <20181112090047.GN3056@worktop>
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On Sun, Nov 11, 2018 at 06:24:55PM -0800, Paul E. McKenney wrote:

> > > 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.

Still, better safe than sorry. It was a rather big change in behaviour,
so it wouldn't have been strange to call that out.

> > 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?

I'm not, I'm just taking issue with the Changelog.

> > 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.

Please, sufficient doesn't imply necessary etc.. A changelog should
always clarify why we do the patch.