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Date:   Mon, 12 Nov 2018 01:12:33 +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: <20181112001233.GC3056@worktop>
References: <20181111194104.GA4787@linux.ibm.com>
 <20181111194410.6368-23-paulmck@linux.ibm.com>
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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().

So why are we doing this?