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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <laijs@cn.fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 04/16] rcu: Remove redundant check for an online CPU
Date: Wed, 23 Jul 2014 07:14:24 -0700	[thread overview]
Message-ID: <20140723141424.GT11241@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJhHMCAgrnxPTqTJjQ+Yw8LFgQBM3VHm6yWNf_Ct0nvAw-CmoQ@mail.gmail.com>

On Wed, Jul 23, 2014 at 10:01:14AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 9:41 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Jul 23, 2014 at 09:23:47AM -0400, Pranith Kumar wrote:
> >> On Wed, Jul 23, 2014 at 8:09 AM, Paul E. McKenney
> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> > On Wed, Jul 23, 2014 at 01:09:41AM -0400, Pranith Kumar wrote:
> >> >> rcu_prcess_callbacks() is the softirq handler for RCU which is raised from
> >> >> invoke_rcu_core() which is called from __call_rcu_core().
> >> >>
> >> >> Each of these three functions checks if the cpu is online. We can remove the
> >> >> redundant ones. This commit removes one of these redundant check.
> >> >>
> >> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> >> >
> >> > Sorry, but no.  There can be a long delay between raise_softirq() and
> >> > this function starting, particularly if ksoftirqd gets involved.  The
> >> > CPU could easily go offline in the meantime.
> >> >
> >>
> >> That makes sense. I guess one of the other two checks in
> >> __call_rcu_core() or invoke_rcu_core() can go then?
> >
> > What would have to be the case for this suggested change to be safe?
> > Does that condition in fact hold?
> >
> 
> The only scenario which is unsafe is when this thread is preempted and
> scheduled on a dying CPU. In that case we should turn off interrupts.
> 
> Actually, checking the code, I see that interrupts are turned off
> already before we call _call_rcu_core(). So I am not sure such a case
> will happen. On the other hand invoke_rcu_core() will be a one line
> function which might as well be in-lined to avoid the double cpu
> online check.
> 
> What am I missing?

I am not sure.  Perhaps the fact that __call_rcu_core() doesn't call
invoke_rcu_core() unless the condition holds (which means that you
cannnot remove the check from __call_rcu_core()) or maybe the fact that
invoke_rcu_core() is called from many other places, which means that
you might not be able to remove the check from invoke_rcu_core().

							Thanx, Paul


  reply	other threads:[~2014-07-23 14:14 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-23  5:09 [PATCH 00/16] rcu: Some minor fixes and cleanups Pranith Kumar
2014-07-23  5:09 ` [PATCH 01/16] rcu: Use rcu_num_nodes instead of NUM_RCU_NODES Pranith Kumar
2014-07-23  5:09 ` [PATCH 02/16] rcu: Check return value for cpumask allocation Pranith Kumar
2014-07-23 12:06   ` Paul E. McKenney
2014-07-23 12:49     ` Pranith Kumar
2014-07-23 17:14     ` Pranith Kumar
2014-07-23 18:01       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 03/16] rcu: Fix comment for gp_state field values Pranith Kumar
2014-07-23  5:09 ` [PATCH 04/16] rcu: Remove redundant check for an online CPU Pranith Kumar
2014-07-23 12:09   ` Paul E. McKenney
2014-07-23 13:23     ` Pranith Kumar
2014-07-23 13:41       ` Paul E. McKenney
2014-07-23 14:01         ` Pranith Kumar
2014-07-23 14:14           ` Paul E. McKenney [this message]
2014-07-23 15:07             ` Pranith Kumar
2014-07-23 15:21               ` Pranith Kumar
2014-07-23  5:09 ` [PATCH 05/16] rcu: Add noreturn attribute to boost kthread Pranith Kumar
2014-07-23  5:09 ` [PATCH 06/16] rcu: Clear gp_flags only when actually starting new gp Pranith Kumar
2014-07-23 12:13   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 07/16] rcu: Save and restore irq flags in rcu_gp_cleanup() Pranith Kumar
2014-07-23 12:16   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 08/16] rcu: Clean up rcu_spawn_one_boost_kthread() Pranith Kumar
2014-07-23  5:09 ` [PATCH 09/16] rcu: Remove redundant check for online cpu Pranith Kumar
2014-07-23 12:21   ` Paul E. McKenney
2014-07-23 12:59     ` Pranith Kumar
2014-07-23 13:50       ` Paul E. McKenney
2014-07-23 14:12         ` Pranith Kumar
2014-07-23 14:23           ` Paul E. McKenney
2014-07-23 15:11             ` Pranith Kumar
2014-07-23 15:30               ` Paul E. McKenney
2014-07-23 15:44                 ` Pranith Kumar
2014-07-23 19:15                   ` Paul E. McKenney
2014-07-23 20:01                     ` Pranith Kumar
2014-07-23 20:16                     ` Pranith Kumar
2014-07-23 20:23                       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 10/16] rcu: Check for RCU_FLAG_GP_INIT bit in gp_flags for spurious wakeup Pranith Kumar
2014-07-23 12:23   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 11/16] rcu: Check for spurious wakeup using return value Pranith Kumar
2014-07-23 12:26   ` Paul E. McKenney
2014-07-24  2:36     ` Pranith Kumar
2014-07-24  3:43       ` Paul E. McKenney
2014-07-24  4:03         ` Pranith Kumar
2014-07-24 18:12           ` Paul E. McKenney
2014-07-24 19:59             ` Pranith Kumar
2014-07-24 20:27               ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 12/16] rcu: Rename rcu_spawn_gp_kthread() to rcu_spawn_kthreads() Pranith Kumar
2014-07-23  5:09 ` [PATCH 13/16] rcu: Spawn nocb kthreads from rcu_prepare_kthreads() Pranith Kumar
2014-07-23  5:09 ` [PATCH 14/16] rcu: Remove redundant checks for rcu_scheduler_fully_active Pranith Kumar
2014-07-23 12:27   ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 15/16] rcu: Check for a nocb cpu before trying to spawn nocb threads Pranith Kumar
2014-07-23 12:28   ` Paul E. McKenney
2014-07-23 13:14     ` Pranith Kumar
2014-07-23 13:42       ` Paul E. McKenney
2014-07-23  5:09 ` [PATCH 16/16] rcu: kvm.sh: Fix error when you pass --cpus argument Pranith Kumar
2014-07-23 12:31   ` Paul E. McKenney
2014-07-23 14:45 ` [PATCH 00/16] rcu: Some minor fixes and cleanups Paul E. McKenney
2014-08-27  1:10   ` Pranith Kumar
2014-08-27  3:20     ` Paul E. McKenney

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