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SubjectRe: [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)
+ rcu_all_qs();
ret = 1;
 \ /
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