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

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Message-ID: <0e1e61f6-6561-412b-9d45-1a000b3adc21@samsung.com>
Date: Thu, 9 Oct 2025 13:42:58 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Peter Zijlstra <peterz@...radead.org>
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>, Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [tip: sched/urgent] sched/deadline: Fix dl_server getting stuck

On 09.10.2025 11:26, Peter Zijlstra wrote:
> 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.

ARM and ARM64 are almost completely separate architectures. 
CONFIG_EARLY_PRINTK is not available on ARM64.


> 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.

Indeed it looks like a similar issue and the proposed patch fixes the 
problem on my Exynos based boards! Thanks! I will post my comment and 
Tested-by tag there.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland