From: David Woodhouse <dwmw2@infradead.org>
To: Peter Zijlstra <peterz@infradead.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: mhillenb@amazon.de, linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested
Date: Mon, 09 Jul 2018 10:18:55 +0100 [thread overview]
Message-ID: <1531127935.18697.57.camel@infradead.org> (raw)
In-Reply-To: <20180709085351.GC2476@hirez.programming.kicks-ass.net>
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On Mon, 2018-07-09 at 10:53 +0200, Peter Zijlstra wrote:
> On Fri, Jul 06, 2018 at 10:11:50AM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 06, 2018 at 06:29:05PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jul 06, 2018 at 03:53:30PM +0100, David Woodhouse wrote:
> > > >
> > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > index e4d4e60..89f5814 100644
> > > > --- a/include/linux/sched.h
> > > > +++ b/include/linux/sched.h
> > > > @@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
> > > >
> > > > static __always_inline bool need_resched(void)
> > > > {
> > > > - return unlikely(tif_need_resched());
> > > > + return unlikely(tif_need_resched()) ||
> > > > + rcu_urgent_qs_requested();
> > > > }
> > > Instead of making need_resched() touch two cachelines, I think I would
> > > prefer adding resched_cpu() to rcu_request_urgent_qs_task().
>
> > I used to do something like this, but decided that whacking each holdout
> > CPU over the head ten times a second was a bit much.
>
> This is only called from the !list_empty(rcu_tasks_holdout) loop in
> rcu_tasks_kthread afaict, and that has a
> schedule_timeout_interruptible(HZ) in it, which I read as once a second.
>
> Which seems like an entirely reasonable amount of time to kick a task.
> Not scheduling for a second is like an eternity.
If that is our only "fix" for KVM, then wouldn't that mean that things
like expand_fdtable() would be *expected* to take "an eternity" when
another CPU happens to be in the guest? Because vcpu_run() would still
loop until the task gets kicked after a second?
Of course, we can explicitly put a check into the KVM loop, but that
brings me back to my original concern — why is it OK to do it there as
a special case and not for the general case construct of
if (need_resched) { drop_local_locks(); cond_resched(); get_local_locks(); }
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next prev parent reply other threads:[~2018-07-09 9:19 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11 ` Paul E. McKenney
2018-07-06 17:14 ` David Woodhouse
2018-07-06 21:12 ` Paul E. McKenney
2018-07-09 8:58 ` Peter Zijlstra
2018-07-09 8:53 ` Peter Zijlstra
2018-07-09 9:18 ` David Woodhouse [this message]
2018-07-09 10:44 ` Peter Zijlstra
2018-07-09 10:56 ` David Woodhouse
2018-07-09 11:06 ` Peter Zijlstra
2018-07-09 11:12 ` David Woodhouse
2018-07-09 11:31 ` Peter Zijlstra
2018-07-09 12:34 ` Paul E. McKenney
2018-07-09 12:47 ` David Woodhouse
2018-07-09 14:30 ` Paul E. McKenney
2018-07-09 12:55 ` Peter Zijlstra
2018-07-09 12:57 ` David Woodhouse
2018-07-09 13:02 ` Peter Zijlstra
2018-07-09 14:29 ` Paul E. McKenney
2018-07-09 14:43 ` Peter Zijlstra
2018-07-09 14:54 ` Paul E. McKenney
2018-07-09 15:26 ` Peter Zijlstra
2018-07-09 16:34 ` Paul E. McKenney
2018-07-09 16:44 ` Paul E. McKenney
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney
2018-07-09 20:35 ` David Woodhouse
2018-07-09 20:42 ` Paul E. McKenney
2018-07-09 20:45 ` David Woodhouse
2018-07-09 21:05 ` Paul E. McKenney
2018-07-09 22:08 ` Paul E. McKenney
2018-07-11 10:57 ` David Woodhouse
2018-07-11 12:51 ` Paul E. McKenney
2018-07-11 12:58 ` David Woodhouse
2018-07-11 14:25 ` Paul E. McKenney
2018-07-11 14:23 ` David Woodhouse
2018-07-11 14:43 ` Paul E. McKenney
2018-07-11 16:49 ` Paul E. McKenney
2018-07-11 17:03 ` David Woodhouse
2018-07-11 17:48 ` Paul E. McKenney
2018-07-11 18:01 ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20 ` Paul E. McKenney
2018-07-11 18:36 ` Paul E. McKenney
2018-07-11 18:39 ` Christian Borntraeger
2018-07-11 20:27 ` Paul E. McKenney
2018-07-11 20:54 ` David Woodhouse
2018-07-11 21:09 ` Paul E. McKenney
2018-07-11 21:11 ` Christian Borntraeger
2018-07-11 21:32 ` Paul E. McKenney
2018-07-11 21:39 ` Christian Borntraeger
2018-07-11 23:47 ` Paul E. McKenney
2018-07-12 8:31 ` David Woodhouse
2018-07-12 11:00 ` Christian Borntraeger
2018-07-12 11:10 ` David Woodhouse
2018-07-12 11:58 ` Christian Borntraeger
2018-07-12 12:04 ` Christian Borntraeger
2018-07-11 23:37 ` Paul E. McKenney
2018-07-12 2:15 ` Paul E. McKenney
2018-07-12 6:21 ` Christian Borntraeger
2018-07-12 9:52 ` David Woodhouse
2018-07-11 18:31 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17 ` Paul E. McKenney
2018-07-11 20:19 ` David Woodhouse
2018-07-11 21:08 ` Paul E. McKenney
2018-07-12 12:00 ` David Woodhouse
2018-07-12 12:53 ` Paul E. McKenney
2018-07-12 16:17 ` Paul E. McKenney
2018-07-16 15:40 ` Paul E. McKenney
2018-07-17 8:19 ` David Woodhouse
2018-07-17 12:56 ` Paul E. McKenney
2018-07-18 15:36 ` Paul E. McKenney
2018-07-18 16:01 ` David Woodhouse
2018-07-18 16:37 ` Paul E. McKenney
2018-07-18 19:41 ` David Woodhouse
2018-07-18 20:17 ` Paul E. McKenney
2018-07-19 0:26 ` Frederic Weisbecker
2018-07-19 6:45 ` Christian Borntraeger
2018-07-19 7:20 ` David Woodhouse
2018-07-19 10:23 ` Christian Borntraeger
2018-07-19 12:55 ` Paul E. McKenney
2018-07-19 13:14 ` Frederic Weisbecker
2018-07-19 13:36 ` David Woodhouse
2018-07-19 17:09 ` Paul E. McKenney
2018-07-23 8:08 ` David Woodhouse
2018-07-23 12:22 ` Paul E. McKenney
2018-07-19 0:32 ` Frederic Weisbecker
2018-07-19 3:11 ` Paul E. McKenney
2018-07-19 6:16 ` David Woodhouse
2018-07-19 13:17 ` Frederic Weisbecker
2018-07-19 13:15 ` Frederic Weisbecker
2018-07-10 9:24 ` Peter Zijlstra
2018-07-10 16:26 ` Paul E. McKenney
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