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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Huacai Chen <chenhuacai@kernel.org>
Cc: Huacai Chen <chenhuacai@loongson.cn>, Xuerui Wang <kernel@xen0n.name>,
	loongarch@lists.linux.dev, Clark Williams <clrkwllms@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-rt-devel@lists.linux.dev, Guo Ren <guoren@kernel.org>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] LoongArch: Allow to enable PREEMPT_RT
Message-ID: <20241114113018.Ilo9ZsQo@linutronix.de>
References: <20241108091545.4182229-1-chenhuacai@loongson.cn>
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 <20241114103111.5W5ZY0D4@linutronix.de>
 <CAAhV-H72YSNBGutYPOVi=S7nwLb6YOiQOFqnimAp=9D0wAJc3g@mail.gmail.com>
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On 2024-11-14 19:19:26 [+0800], Huacai Chen wrote:
> > > > Why is ntpd/chronyd service affecting this? Is it running at prio 99?
> > > > Otherwise it should not be noticed.
> > > No, ntpd/chronyd doesn't affect latency. But they may trigger RTC
> > > synchronization every 11 minutes, and RTC synchronization affects
> > > latency. We can keep ntpd/chronyd running but disable RTC
> > > synchronization by configuration, this is the least aggressive method.
> >
> > What is "RTC synchronization" in this context?
> Means the sync_hw_clock() function in kernel/time/ntp.c, it can be
> enabled/disabled by chronyd configuration:

But what exactly is sync_hw_clock() doing that is causing a problem
here? The clock on HW is updated. The access to the RTC clock is
preemptible.

> Huacai

Sebastian