linux-kernel - Re: [PATCH] sched: Proxy yields to donor tasks

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Message-ID: <20251107081254.110947-1-sieberf@amazon.com>
Date: Fri, 7 Nov 2025 10:12:53 +0200
From: Fernand Sieber <sieberf@...zon.com>
To: kernel test robot <lkp@...el.com>, <mingo@...hat.com>,
	<peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<vschneid@...hat.com>
CC: <oe-kbuild-all@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<dwmw@...zon.co.uk>, <jschoenh@...zon.de>, <liuyuxua@...zon.com>
Subject: Re: [PATCH] sched: Proxy yields to donor tasks

On Thu, Nov 07, 2025 at 02:31:16PM +0800, kernel test robot wrote:
> sparse warnings: (new ones prefixed by >>)
>
>>> kernel/sched/rt.c:529:55: sparse: warning: incorrect type in initializer (different address spaces)
>>> kernel/sched/rt.c:976:39: sparse: warning: incorrect type in initializer (different address spaces)

Thank you for the report.

The warnings occur because my patch changes scheduler yield functions from
accessing rq->curr to rq->donor, both of which have identical __rcu annotations.
These are pre-existing annotation issues in the scheduler code, which has been
directly accessing RCU-annotated pointers without proper dereferencing, relying
on runqueue lock protection instead.

If we do want to address these sparse warnings, I can make modifications like:

-               curr = rq->donor;
+               curr = rcu_dereference_protected(rq->donor, lockdep_is_held(&rq->__lock));

This would have zero runtime overhead while satisfying sparse's RCU checking.

Peter, let me know if these warnings should be addressed.

Thanks,
Fernand

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