linux-kernel - Re: [PATCH 3/3] LoongArch: Allow to enable PREEMPT

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Message-ID: <20241114132956.wafcHvaB@linutronix.de>
Date: Thu, 14 Nov 2024 14:29:56 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, Xuerui Wang <kernel@...0n.name>,
	loongarch@...ts.linux.dev, Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-rt-devel@...ts.linux.dev, Guo Ren <guoren@...nel.org>,
	Jiaxun Yang <jiaxun.yang@...goat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] LoongArch: Allow to enable PREEMPT_RT

On 2024-11-14 19:36:38 [+0800], Huacai Chen wrote:
> On Thu, Nov 14, 2024 at 7:30 PM Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> >
> > 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.
> This is a platform-specific problem, our RTC driver is
> drivers/rtc/rtc-loongson.c, the write operation to RTC register is
> slow.

Ach okay. So the pure read on the slow bus causes a delay because the
CPU is stalled. That is not limited to chrony but should also have an
affect if the user uses hwclock to read/ write the time.

> Huacai
 
Sebastian