From: Joel Fernandes <joel@joelfernandes.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Nikolay Borisov <nborisov@suse.com>,
linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
linux-doc@vger.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption
Date: Thu, 18 Oct 2018 20:58:44 -0700 [thread overview]
Message-ID: <20181019035844.GA141835@joelaf.mtv.corp.google.com> (raw)
In-Reply-To: <20181018225223.42641c73@vmware.local.home>
On Thu, Oct 18, 2018 at 10:52:23PM -0400, Steven Rostedt wrote:
> On Thu, 18 Oct 2018 19:25:29 -0700
> Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > On Thu, Oct 18, 2018 at 09:50:35PM -0400, Steven Rostedt wrote:
> > > On Thu, 18 Oct 2018 18:26:45 -0700
> > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > > Yes, local_irq_restore is light weight, and does not check for reschedules.
> > > >
> > > > I was thinking of case where ksoftirqd is woken up, but does not run unless
> > > > we set the NEED_RESCHED flag. But that should get set anyway since probably
> > > > ksoftirqd is of high enough priority than the currently running task..
> > > >
> > > > Roughly speaking the scenario could be something like:
> > > >
> > > > rcu_read_lock();
> > > > <-- IPI comes in for the expedited GP, sets exp_hint
> > > > local_irq_disable();
> > > > // do a bunch of stuff
> > > > rcu_read_unlock(); <-- This calls the rcu_read_unlock_special which raises
> > > > the soft irq, and wakesup softirqd.
> > >
> > > If softirqd is of higher priority than the current running task, then
> > > the try_to_wake_up() will set NEED_RESCHED of the current task here.
> > >
> >
> > Yes, only *if*. On my system, ksoftirqd is CFS nice 0. I thought expedited
> > grace periods are quite important and they should complete quickly which is
> > the whole reason for interrupting rcu read sections with an IPI and stuff.
> > IMO there should be no harm in setting NEED_RESCHED unconditionally anyway
> > for possible benefit of systems where the ksoftirqd is not of higher priority
> > than the currently running task, and we need to run it soon on the CPU. But
> > I'm Ok with whatever Paul and you want to do here.
>
>
> Setting NEED_RESCHED unconditionally wont help. Because even if we call
> schedule() ksoftirqd will not be scheduled! If it's CFS nice 0, and the
> current task still has quota to run, if you call schedule, you'll just
> waste time calculating that the current task should still be running.
> It's equivalent to calling yield() (which is why we removed all yield()
> users in the kernel, because *all* of them were buggy!). This is *why*
> it only calls schedule *if* softirqd is of higher priority.
Yes, ok. you are right the TTWU path should handle setting the NEED_RESCHED
flag or not and unconditionally setting it does not get us anything. I had to
go through the code a bit since it has been a while since I explored it.
So Paul, I'm Ok with your latest patch for the issue we discussed and don't
think much more can be done barring raising of ksofitrqd priorities :-) So I
guess the synchronize_rcu_expedited will just cope with the deal between
local_irq_enable and the next scheduling point.. :-)
thanks,
- Joel
next prev parent reply other threads:[~2018-10-19 3:58 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-14 21:29 [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption Joel Fernandes (Google)
2018-10-14 23:17 ` Paul E. McKenney
2018-10-15 2:08 ` Joel Fernandes
2018-10-15 2:13 ` Joel Fernandes
2018-10-15 2:33 ` Paul E. McKenney
2018-10-15 2:47 ` Joel Fernandes
2018-10-15 2:50 ` Joel Fernandes
2018-10-15 6:05 ` Nikolay Borisov
2018-10-15 11:21 ` Paul E. McKenney
2018-10-15 19:39 ` Joel Fernandes
2018-10-15 19:54 ` Paul E. McKenney
2018-10-15 20:15 ` Joel Fernandes
2018-10-15 21:08 ` Paul E. McKenney
2018-10-16 11:26 ` Paul E. McKenney
2018-10-16 20:41 ` Joel Fernandes
2018-10-17 16:11 ` Paul E. McKenney
2018-10-17 18:15 ` Joel Fernandes
2018-10-17 20:33 ` Paul E. McKenney
2018-10-18 2:07 ` Joel Fernandes
2018-10-18 14:46 ` Paul E. McKenney
2018-10-19 0:03 ` Joel Fernandes
2018-10-19 0:19 ` Paul E. McKenney
2018-10-19 1:12 ` Steven Rostedt
2018-10-19 1:27 ` Joel Fernandes
2018-10-19 1:26 ` Joel Fernandes
2018-10-19 1:50 ` Steven Rostedt
2018-10-19 2:25 ` Joel Fernandes
2018-10-19 2:52 ` Steven Rostedt
2018-10-19 3:58 ` Joel Fernandes [this message]
2018-10-19 12:07 ` Paul E. McKenney
2018-10-19 17:24 ` Joel Fernandes
2018-10-19 18:11 ` Paul E. McKenney
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