linux-kernel - Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize

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Message-ID: <20181112005329.GG3056@worktop>
Date:   Mon, 12 Nov 2018 01:53:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched()
 with synchronize_rcu()

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.