linux-kernel - Re: [tip: sched/urgent] sched/deadline: Fix dl

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Message-ID: <20251009092648.GW3419281@noisy.programming.kicks-ass.net>
Date: Thu, 9 Oct 2025 11:26:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, John Stultz <jstultz@...gle.com>,
	x86@...nel.org,
	'Linux Samsung SOC' <linux-samsung-soc@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [tip: sched/urgent] sched/deadline: Fix dl_server getting stuck

On Mon, Sep 29, 2025 at 05:19:27PM +0200, Marek Szyprowski wrote:

> Thanks for some hints, but unfortunately ARM64 doesn't support 
> earlyprintk, so I was not able to use this method.

I've been told it is called earlycon= on arm; still sets up
early_console and make early_printk() work.

My patches just re-route everything printk() into early_printk().

> However I've played a bit with this code and found that this strange 
> wake-up of the CPU7 seems to be caused by the timer. If I restore
> 
>    if (!dl_se->server_has_tasks(dl_se))
>            return HRTIMER_NORESTART;
> 
> part in the dl_server_timer, the everything works again as before this 
> patch.

Right, so something on your platform is not working right. Stray timers
should not wake up CPUs.

> This issue is however not Exynos5433 ARM 64bit specific. Similar lockup 
> happens on Exynos5422 ARM 32bit boards, although there is no message in 
> that case. Does it mean that handling of the hrtimers on Exynos boards 
> is a bit broken in the context of CPU hotplug? 

There is this thread:

  https://lkml.kernel.org/r/20251009080007.GH3245006@noisy.programming.kicks-ass.net

I suspect this might be the same issue and we've made a little progress
there.