RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers

[Finn Thain]

On Wed, 24 Feb 2021, Song Bao Hua (Barry Song) wrote:

> 
> Realtime requirement is definitely a true requirement on ARM Linux.
> 
> I once talked/worked  with some guys who were using ARM for realtime
> system.
> The feasible approaches include:
> 1. Dual OS(RTOS + Linux): e.g.  QNX+Linux XENOMAI+Linux L4+Linux
> 2. preempt-rt
> Which is continuously maintained like:
> https://lore.kernel.org/lkml/20210218201041.65fknr7bdplwqbez@xxxxxxxxxxxxx/
> 3. bootargs isolcpus=
> to isolate a cpu for a specific realtime task or interrupt
> https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux_for_real_time/7/html/tuning_guide/isolating_cpus_using_tuned-profiles-realtime
> 4. ARM FIQ which has separate fiq API, an example in fsl sound:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/soc/fsl/imx-pcm-fiq.c
> 5. Let one core invisible to Linux
> Running non-os system and rtos on the core
> 

Regarding Linux systems, it appears that approach 3 could theoretically 
achieve minimal interrupt latency for a given device without requiring any 
interrupt nesting. But the price is one CPU core which is not going to 
work on a uniprocessor system.

> Honestly, I've never seen anyone who depends on irq priority to support 
> realtime in ARM Linux though ARM's RTOS-es use it quite commonly.
> 

Perhaps you don't work with uniprocessor ARM Linux systems?

> Once preempt_rt is enabled, those who want a fast irq environment need a 
> no_thread flag, or need to set its irq thread to higher sched_fifo/rr 
> priority.
> 

Thanks for the tip.

> [...]
> 
> Anyway, the debate is long enough, let's move to some more important 
> things. I appreciate that you shared a lot of knowledge of m68k.
> 

No problem.

> Thanks
> Barry
>