linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-Id: <20180709142932.GO3593@linux.vnet.ibm.com>
Date:   Mon, 9 Jul 2018 07:29:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Mon, Jul 09, 2018 at 03:02:27PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 02:55:16PM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 09, 2018 at 05:34:57AM -0700, Paul E. McKenney wrote:
> > > But KVM defeats this by checking need_resched() before invoking
> > > cond_resched().
> > 
> > That's not wrong or even uncommon I think.
> 
> In fact, I think we recently put that pattern in crypto code in order to
> break up very long kernel_fpu sections.

OK, so here are our options:

1.	Add the RCU conditional to need_resched(), as David suggests.
	Peter has concerns about overhead.

2.	Create a new need_resched_rcu_qs() that is to be used when
	deciding whether or not to do cond_resched().  This would
	exact the overhead only where it is needed, but is one more
	thing for people to get wrong.

3.	Revert my changes to de-emphasize cond_resched_rcu_qs(),
	and go back to sprinkling cond_resched_rcu_qs() throughout
	the code.  This also is one more thing for people to get wrong,
	and might well eventually convert all cond_resched() calls to
	cond_resched_rcu_qs(), which sure seems like a failure mode to me.

4.	Others?

> Note that you also 'broke' cond_resched_lock() as that no longer matches
> cond_resched().

Given that cond_resched_lock() was there first, I believe that you can
just say "broke" without the quote marks.  :-/

Given that this code is releasing and acquiring a lock, I believe that
the patch below should cure this, aside from also needing to check
whether RCU needs a quiescent state.  Any other similar gotchas out there?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 537bced8f4bc..b559b556f464 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5017,6 +5017,7 @@ int __cond_resched_lock(spinlock_t *lock)
 			preempt_schedule_common();
 		else
 			cpu_relax();
+		rcu_all_qs();
 		ret = 1;
 		spin_lock(lock);
 	}