Re: [PATCH 4/4] sched/fair: Revert 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 02, 2026 at 10:12:34AM +0100, Peter Zijlstra wrote:
> On Sat, Jan 31, 2026 at 04:21:39PM +0100, Peter Zijlstra wrote:
> > On Sat, Jan 31, 2026 at 09:47:07AM +0800, Zhang Qiao wrote:
> > 
> > > >  	if (se->on_rq) {
> > > >  		/* commit outstanding execution time */
> > > >  		update_curr(cfs_rq);
> > > > -		update_entity_lag(cfs_rq, se);
> > > > -		se->deadline -= se->vruntime;
> > > > +		avruntime = avg_vruntime(cfs_rq);
> > > > +		se->vlag = entity_lag(avruntime, se);
> > > 
> > > 
> > > vlag is updated here. Considering vlag and vprot share the same union, updating
> > > vlag will overwrite vprot. Is it right to call protect_slice() (which use vprot)
> > > after this update?
> > 
> > Oh you are quite right; I'm sure Ingo had a patch removing that union,
> > but clearly that's not been merged yet.
> > 
> > Sorry about that mistake; I'll make a new version on Monday.
> 
> After looking at things, I think the best option is to simply remove
> that union.
> 
> The thing is, I can fix reweight to respect this union, but there are
> more problems, notably the sched_change pattern will not only call
> put_prev_task/set_next_task, it will actually dequeue/enqueue the thing
> and therefore 'temporarily' use the vlag field, destroying the vprot
> value.
> 
> And yes, we can fix all that, but I'm thinking that at that point the
> union is more trouble than its worth.
> 

And its already gone -- I managed to consistently look at the wrong
tree. So all should be good.

---
commit 80390ead2080071cbd6f427ff8deb94d10a4a50f
Author: Ingo Molnar <mingo@kernel.org>
Date:   Wed Nov 26 05:31:28 2025 +0100

    sched/fair: Separate se->vlag from se->vprot
    
    There's no real space concerns here and keeping these fields
    in a union makes reading (and tracing) the scheduler code harder.
    
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Link: https://patch.msgid.link/20251201064647.1851919-4-mingo@kernel.org

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d395f2810fac..bf96a7d595e2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -586,15 +586,10 @@ struct sched_entity {
 	u64				sum_exec_runtime;
 	u64				prev_sum_exec_runtime;
 	u64				vruntime;
-	union {
-		/*
-		 * When !@on_rq this field is vlag.
-		 * When cfs_rq->curr == se (which implies @on_rq)
-		 * this field is vprot. See protect_slice().
-		 */
-		s64                     vlag;
-		u64                     vprot;
-	};
+	/* Approximated virtual lag: */
+	s64				vlag;
+	/* 'Protected' deadline, to give out minimum quantums: */
+	u64				vprot;
 	u64				slice;
 
 	u64				nr_migrations;