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Date:   Mon, 12 Nov 2018 14:21:12 -0800
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
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()
Reply-To: paulmck@linux.ibm.com
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On Mon, Nov 12, 2018 at 07:17:41PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 12, 2018 at 05:28:52AM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 12, 2018 at 10:00:47AM +0100, Peter Zijlstra wrote:
> 
> > > Still, better safe than sorry. It was a rather big change in behaviour,
> > > so it wouldn't have been strange to call that out.
> > 
> > This guy:
> > 
> > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds")
> > 
> > Has a commit log that says:
> > 
> > 	Now that RCU-preempt knows about preemption disabling, its
> > 	implementation of synchronize_rcu() works for synchronize_sched(),
> > 	and likewise for the other RCU-sched update-side API members.
> > 	This commit therefore confines the RCU-sched update-side code
> > 	to CONFIG_PREEMPT=n builds, and defines RCU-sched's update-side
> > 	API members in terms of those of RCU-preempt.
> > 
> > That last phrase seems pretty explicit.  What am I missing here?
> 
> That does not explicitly state that because RCU-preempt
> synchornize_rcu() can take _much_ longer, the new synchronize_sched()
> can now take _much_ longer too.
> 
> So when someone bisects a problem to this commit; and he reads the
> Changelog, he might get the impression that was unexpected.

Of course, a preempt_disable() section of code can still be preempted
by the underlying hypervisor, so in a surprisingly large fraction of
the installed base, there really isn't that much difference.

> > Not that it matters, given that I know of no way to change a mainlined
> > commit log.  I suppose I could ask Jon if he would be willing to take
> > a 2018 RCU API LWN article, if that would help.
> 
> Yes, it is water under the bridge; but Changelogs should be explicit
> about behavioural changes.
> 
> And while the merged RCU has the semantic behaviour required, the timing
> behaviour did change significantly.

When running on bare metal, potentially.  From what I see, preemption
of RCU read-side critical sections is the exception rather than the rule.
And again, when running on hypervisors, even irq-disable regions of code
can be preempted.  (And yes, there is work in flight to allow RCU to deal
with this.)

> > > > > 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.
> > 
> > ???  Did you mean to say "necessary doesn't imply sufficient"?  If so,
> > what else do you feel is missing?
> 
> No, I meant to say that your original Changelog only states that
> sync_rcu now covers rcu-sched behaviour.  Which means that the change is
> sufficient.
> 
> It completely and utterly fails to explain _why_ you're doing the
> change. Ie. you do not address why it is necessary.
> 
> A Changelog should always explain why the change is needed.
> 
> In this case because you want to get rid of the sync_sched() api.

Right, which is stated in your suggested wording above.  So I am still
not seeing what you want added to this:

	"Since synchronize_sched() is now equivalent to synchronize_rcu(),
	replace the synchronize_sched() usage such that we can eventually
	remove the interface."

								Thanx, Paul