Re: [PATCH 3/3] LoongArch: Allow to enable PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-11-14 19:36:38 [+0800], Huacai Chen wrote:
> On Thu, Nov 14, 2024 at 7:30 PM Sebastian Andrzej Siewior
> <bigeasy@linutronix.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