linux-kernel - Re: [PATCH] sched_ext: Clear direct dispatch state on dequeue when dsq is NULL

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Message-ID: <aXnqsCeN--uzQQ8V@gpd4>
Date: Wed, 28 Jan 2026 11:53:36 +0100
From: Andrea Righi <arighi@...dia.com>
To: Daniel Hodges <hodgesd@...a.com>
Cc: tj@...nel.org, void@...ifault.com, changwoo@...lia.com,
	sched-ext@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched_ext: Clear direct dispatch state on dequeue when
 dsq is NULL

On Wed, Jan 21, 2026 at 04:31:02PM -0800, Daniel Hodges wrote:
...
> > Long story short, the only thing that was working reliably for me was to
> > clear ddsp_dsq_id and ddsp_enq_flags in select_task_rq_scx(), but I thought
> > it was a bit too overkill and then I've never finished to investigate the
> > real issue...
> > 
> > In conclusion, I think this is fixing some of these warnings that we see
> > and it's probably good to apply it, but it's not fixing all of them.
> > 
> > Anyway, I'll do some tests with this patch and report back!
> > 
> > Thanks,
> > -Andrea
> 
> Sounds good, I hit this running cosmos on a moderately loaded machine.
> I'll see if I can get a reproducer made and do some more testing.

This is with 6.19.0-rc7 + this fix on top:

 WARNING: kernel/sched/ext.c:1282 at scx_dsq_insert_commit+0xf2/0x120, CPU#13: alacritty/6070

Which is WARN_ON_ONCE(p->scx.ddsp_dsq_id != SCX_DSQ_INVALID) in
mark_direct_dispatch().

Triggered almost immediately after loading scx_cosmos on my laptop. I'll
also try to find a better reproducer (ideally inside a VM).

-Andrea