[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <20190705150932.GO26519@linux.ibm.com>
Date: Fri, 5 Jul 2019 08:09:32 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Byungchul Park <byungchul.park@....com>,
linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
kernel-team@...roid.com
Subject: Re: [PATCH] rcuperf: Make rcuperf kernel test more robust for
!expedited mode
On Fri, Jul 05, 2019 at 08:24:50AM -0400, Joel Fernandes wrote:
> On Fri, Jul 05, 2019 at 12:52:31PM +0900, Byungchul Park wrote:
> > On Thu, Jul 04, 2019 at 10:40:44AM -0700, Paul E. McKenney wrote:
> > > On Thu, Jul 04, 2019 at 12:34:30AM -0400, Joel Fernandes (Google) wrote:
> > > > It is possible that the rcuperf kernel test runs concurrently with init
> > > > starting up. During this time, the system is running all grace periods
> > > > as expedited. However, rcuperf can also be run for normal GP tests.
> > > > Right now, it depends on a holdoff time before starting the test to
> > > > ensure grace periods start later. This works fine with the default
> > > > holdoff time however it is not robust in situations where init takes
> > > > greater than the holdoff time to finish running. Or, as in my case:
> > > >
> > > > I modified the rcuperf test locally to also run a thread that did
> > > > preempt disable/enable in a loop. This had the effect of slowing down
> > > > init. The end result was that the "batches:" counter in rcuperf was 0
> > > > causing a division by 0 error in the results. This counter was 0 because
> > > > only expedited GPs seem to happen, not normal ones which led to the
> > > > rcu_state.gp_seq counter remaining constant across grace periods which
> > > > unexpectedly happen to be expedited. The system was running expedited
> > > > RCU all the time because rcu_unexpedited_gp() would not have run yet
> > > > from init. In other words, the test would concurrently with init
> > > > booting in expedited GP mode.
> > > >
> > > > To fix this properly, let us check if system_state if SYSTEM_RUNNING
> > > > is set before starting the test. The system_state approximately aligns
> >
> > Just minor typo..
> >
> > To fix this properly, let us check if system_state if SYSTEM_RUNNING
> > is set before starting the test. ...
> >
> > Should be
> >
> > To fix this properly, let us check if system_state is set to
> > SYSTEM_RUNNING before starting the test. ...
>
> That's a fair point. I wonder if Paul already fixed it up in his tree,
> however I am happy to resend if he hasn't. Paul, how would you like to handle
> this commit log nit?
>
> it is just 'if ..' to 'is SYSTEM_RUNNING'
It now reads as follows:
To fix this properly, this commit waits until system_state is
set to SYSTEM_RUNNING before starting the test. This change is
made just before kernel_init() invokes rcu_end_inkernel_boot(),
and this latter is what turns off boot-time expediting of RCU
grace periods.
I dropped the last paragraph about late_initcall(). And I suspect that
the last clause from rcu_gp_is_expedited() can be dropped:
bool rcu_gp_is_expedited(void)
{
return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
rcu_scheduler_active == RCU_SCHEDULER_INIT;
}
This is because rcu_expedited_nesting is initialized to 1, and is
decremented in rcu_end_inkernel_boot(), which is called long after
rcu_scheduler_active has been set to RCU_SCHEDULER_RUNNING, which
happens at core_initcall() time. So if the last clause says "true",
so does the second-to-last clause.
The similar check in rcu_gp_is_normal() is still need, however, to allow
the power-management subsystem to invoke synchronize_rcu() just after
the scheduler has been initialized, but before RCU is aware of this.
So, how about the commit shown below?
Thanx, Paul
------------------------------------------------------------------------
commit 1f7e72efe3c761c2b34da7b59e01ad69c657db10
Author: Paul E. McKenney <paulmck@...ux.ibm.com>
Date: Fri Jul 5 08:05:10 2019 -0700
rcu: Remove redundant "if" condition from rcu_gp_is_expedited()
Because rcu_expedited_nesting is initialized to 1 and not decremented
until just before init is spawned, rcu_expedited_nesting is guaranteed
to be non-zero whenever rcu_scheduler_active == RCU_SCHEDULER_INIT.
This commit therefore removes this redundant "if" equality test.
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 249517058b13..64e9cc8609e7 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -136,8 +136,7 @@ static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
*/
bool rcu_gp_is_expedited(void)
{
- return rcu_expedited || atomic_read(&rcu_expedited_nesting) ||
- rcu_scheduler_active == RCU_SCHEDULER_INIT;
+ return rcu_expedited || atomic_read(&rcu_expedited_nesting);
}
EXPORT_SYMBOL_GPL(rcu_gp_is_expedited);
Powered by blists - more mailing lists