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Date:   Mon, 12 Nov 2018 01:53:29 +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: <20181112005329.GG3056@worktop>
References: <20181111194104.GA4787@linux.ibm.com>
 <20181111194410.6368-23-paulmck@linux.ibm.com>
 <20181112001233.GC3056@worktop>
 <20181112004528.GA4170@linux.ibm.com>
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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?

> > 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.