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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()
Date: Mon, 12 Nov 2018 01:53:29 +0100	[thread overview]
Message-ID: <20181112005329.GG3056@worktop> (raw)
In-Reply-To: <20181112004528.GA4170@linux.ibm.com>

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.

  reply	other threads:[~2018-11-12  0:53 UTC|newest]

Thread overview: 79+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-11 19:41 [PATCH tip/core/rcu 0/41] More RCU flavor consolidation cleanup for v4.21/v5.0 Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 01/41] rcu: Remove unused rcu_state externs Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 02/41] rcu: Fix rcu_{node,data} comments about gp_seq_needed Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 03/41] rcu: Eliminate synchronize_rcu_mult() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 04/41] rcu: Consolidate the RCU update functions invoked by sync.c Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 05/41] sched/membarrier: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 06/41] sparc/oprofile: Convert timer_stop() to use synchronize_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 07/41] s390/mm: Convert tlb_table_flush() to use call_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 08/41] powerpc: Convert hugepd_free() " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 09/41] crypto/pcrypt: Replace synchronize_rcu_bh() with synchronize_rcu() Paul E. McKenney
2018-11-16  5:56   ` Herbert Xu
2018-11-27 17:19     ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 10/41] drivers/ipmi: Replace synchronize_sched() " Paul E. McKenney
2018-11-12 15:38   ` Corey Minyard
2018-11-12 16:04     ` Paul E. McKenney
2018-11-12 16:15       ` Corey Minyard
2018-11-12 16:28         ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 11/41] ethernet/sis: " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 12/41] ethernet/realtek: " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 13/41] ethernet/intel/ixgbe: " Paul E. McKenney
2018-11-13 16:54   ` [Intel-wired-lan] " Bowers, AndrewX
2018-11-11 19:43 ` [PATCH tip/core/rcu 14/41] drivers/vhost: Replace synchronize_rcu_bh() " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 15/41] cpufreq/intel_pstate: Replace synchronize_sched() " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 16/41] cpufreq/cpufreq_governor: " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 17/41] fs/file: " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 18/41] tracing: Replace synchronize_sched() and call_rcu_sched() Paul E. McKenney
2018-11-12  2:09   ` Steven Rostedt
2018-11-11 19:43 ` [PATCH tip/core/rcu 19/41] main: Replace rcu_barrier_sched() with rcu_barrier() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 20/41] kprobes: eplace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-12  3:00   ` Masami Hiramatsu
2018-11-12  3:19     ` Paul E. McKenney
2018-11-13 18:08       ` Masami Hiramatsu
2018-11-13 19:22         ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 21/41] lockdep: Replace " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 22/41] sched/membarrier: " Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 23/41] sched: Replace " Paul E. McKenney
2018-11-12  0:12   ` Peter Zijlstra
2018-11-12  0:45     ` Paul E. McKenney
2018-11-12  0:53       ` Peter Zijlstra [this message]
2018-11-12  1:47         ` Paul E. McKenney
2018-11-12  2:07           ` Peter Zijlstra
2018-11-12  2:24             ` Paul E. McKenney
2018-11-12  9:00               ` Peter Zijlstra
2018-11-12 13:28                 ` Paul E. McKenney
2018-11-12 18:17                   ` Peter Zijlstra
2018-11-12 20:20                     ` Steven Rostedt
2018-11-12 22:21                     ` Paul E. McKenney
2018-11-26 19:59                       ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 24/41] modules: Replace synchronize_sched() and call_rcu_sched() Paul E. McKenney
2018-11-12 12:48   ` Jessica Yu
2018-11-12 16:31     ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 25/41] workqueue: Replace call_rcu_sched() with call_rcu() Paul E. McKenney
2018-11-13 15:48   ` Tejun Heo
2018-11-13 19:21     ` Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 26/41] events: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 27/41] percpu-refcount: Replace call_rcu_sched() with call_rcu() Paul E. McKenney
2018-11-13 15:48   ` Tejun Heo
2018-11-11 19:43 ` [PATCH tip/core/rcu 28/41] slab: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 29/41] mm: Replace call_rcu_sched() with call_rcu() Paul E. McKenney
2018-11-11 19:43 ` [PATCH tip/core/rcu 30/41] netfilter: Replace call_rcu_bh(), rcu_barrier_bh(), and synchronize_rcu_bh() Paul E. McKenney
2018-12-01 11:35   ` Pablo Neira Ayuso
2018-11-11 19:44 ` [PATCH tip/core/rcu 31/41] net/sched: Replace call_rcu_bh() and rcu_barrier_bh() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 32/41] net/core: Replace call_rcu_bh() and synchronize_rcu_bh() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 33/41] net/bridge: Replace call_rcu_bh() and rcu_barrier_bh() Paul E. McKenney
2018-11-13  0:29   ` Nikolay Aleksandrov
2018-11-11 19:44 ` [PATCH tip/core/rcu 34/41] percpu-rwsem: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-13 15:49   ` Tejun Heo
2018-11-11 19:44 ` [PATCH tip/core/rcu 35/41] types: Remove call_rcu_bh() and call_rcu_sched() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 36/41] cgroups: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-13 15:50   ` Tejun Heo
2018-11-11 19:44 ` [PATCH tip/core/rcu 37/41] livepatch: " Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 38/41] net/core/skmsg: Replace call_rcu_sched() with call_rcu() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 39/41] net/decnet: Replace rcu_barrier_bh() with rcu_barrier() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 40/41] tools/kernel.h: Replace synchronize_sched() with synchronize_rcu() Paul E. McKenney
2018-11-11 19:44 ` [PATCH tip/core/rcu 41/41] rcutorture/formal: " Paul E. McKenney
2018-11-12 14:07 ` [PATCH tip/core/rcu 0/41] More RCU flavor consolidation cleanup for v4.21/v5.0 Mathieu Desnoyers
2018-11-12 16:01   ` Paul E. McKenney
2018-11-12 21:40     ` Sasha Levin
2018-11-12 22:16       ` Paul E. McKenney

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