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

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Message-ID: <20241114103111.5W5ZY0D4@linutronix.de>
Date: Thu, 14 Nov 2024 11:31:11 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Huacai Chen <chenhuacai@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>, 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 2024-11-08 17:15:45 [+0800], Huacai Chen wrote:
> It is really time.
> 
> LoongArch has all the required architecture related changes, that have
> been identified over time, in order to enable PREEMPT_RT. With the recent
> printk changes, the last known road block has been addressed.
> 
> Allow to enable PREEMPT_RT on LoongArch.
> 
> Below are the latency data from cyclictest on a 4-core Loongson-3A5000
> machine, with a "make -j8" kernel building workload in the background.
> 
> 1. PREEMPT kernel with default configuration:
> 
> ./cyclictest -a -t -m -i200 -d0 -p99
> policy: fifo: loadavg: 8.78 8.96 8.64 10/296 64800
> T: 0 ( 4592) P:99 I:200 C:14838617 Min:   3 Act:    6 Avg:    8 Max:    844
> T: 1 ( 4593) P:99 I:200 C:14838765 Min:   3 Act:    9 Avg:    8 Max:    909
> T: 2 ( 4594) P:99 I:200 C:14838510 Min:   3 Act:    7 Avg:    8 Max:    832
> T: 3 ( 4595) P:99 I:200 C:14838631 Min:   3 Act:    8 Avg:    8 Max:    931
> 
> 2. PREEMPT_RT kernel with default configuration:
> 
> ./cyclictest -a -t -m -i200 -d0 -p99
> policy: fifo: loadavg: 10.38 10.47 10.35 9/336 77788
> T: 0 ( 3941) P:99 I:200 C:19439626 Min:   3 Act:   12 Avg:    8 Max:    227
> T: 1 ( 3942) P:99 I:200 C:19439624 Min:   2 Act:   11 Avg:    8 Max:    184
> T: 2 ( 3943) P:99 I:200 C:19439623 Min:   3 Act:    4 Avg:    7 Max:    223
> T: 3 ( 3944) P:99 I:200 C:19439623 Min:   2 Act:   10 Avg:    7 Max:    226
> 
> 3. PREEMPT_RT kernel with tuned configuration:
> 
> ./cyclictest -a -t -m -i200 -d0 -p99
> policy: fifo: loadavg: 10.52 10.66 10.62 12/334 109397
> T: 0 ( 4765) P:99 I:200 C:29335186 Min:   3 Act:    6 Avg:    8 Max:     62
> T: 1 ( 4766) P:99 I:200 C:29335185 Min:   3 Act:   10 Avg:    8 Max:     52
> T: 2 ( 4767) P:99 I:200 C:29335184 Min:   3 Act:    8 Avg:    8 Max:     64
> T: 3 ( 4768) P:99 I:200 C:29335183 Min:   3 Act:   12 Avg:    8 Max:     53
> 
> Main instruments of tuned configuration include: Disable the boot rom
> space in BIOS for kernel, in order to avoid speculative access to low-
> speed memory; Disable CPUFreq scaling; Disable RTC synchronization in
> the ntpd/chronyd service.

If "rom space in BIOS for kernel" is a thing you should document it
somewhere or issue a warning at boot. I don't know what the latency
impact is here and if this is needed at all during runtime.

Why is ntpd/chronyd service affecting this? Is it running at prio 99?
Otherwise it should not be noticed.

Is lockdep complaining in any workloads?
Is CONFIG_DEBUG_ATOMIC_SLEEP leading to any complains?


> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>

Sebastian