Hi community,

Any suggestions on how should I proceed with my issue?

Description of my issue:
-  My board kernel prints: "sched: RT throttling activated", then reboot 
(there is no back-trace in UART console).
-  I have sc16is752 (I2C based UART) on my board.
-  As Atheros AR9331 does not have dedicated I2C controller, I am using 
bitbang I2C.
-  This is likely related to below line of sc16is752 driver 
(drivers//tty/serial/sc16is7xx.c)
         ret = devm_request_threaded_irq(dev, irq, NULL, sc16is7xx_ist,
-  './ps -eo pid,nice,policy,cmd' confirms that above is a realtime 
kernel thread (with SCHED_FIFO scheduling policy)
-  Occurrence rate is about once or several times a day, after I stress 
I2C/UART traffic.
-  Occurrence rate is much lower if I do *NOT* stress I2C/UART traffic.

My setup:
-  Atheros AR9331 chip, which is MIPS32 24kc
-  OpenWRT tag 15.05
-  Linux kernel v3.18.29
-  Not preempt kernel (CONFIG_PREEMPT_NONE=y)
-  As AR9331 does not have dedicated I2C controller, I am using bitbang 
I2C as below:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/drivers/i2c/algos/i2c-algo-bit.c?h=linux-3.18.y
-  source code of sc16is752 driver sc16is7xx.c is here:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/drivers/tty/serial/sc16is7xx.c?h=linux-3.18.y

Question 1): There seems a fundamental question about OpenWRT mips linux 
kernel?
-  OpenWRT tag 15.05
-  In OpenWRT file target/linux/ar71xx/Makefile, there is CPU_TYPE=34kc
-  Atheros AR9331 datasheet says it is 24kc (rather than 34kc)
-  With CPU_TYPE=34kc, it uses gcc options "-mips32r2 -mtune=34kc"

Question 2): I have no idea what causes the reboot? It is a chip 
hardware reboot, or function similar to emergency_restart() is called 
somewhere in the kernel? Kernel console only shows "sched: RT throttling 
activated" and reboot, there is no back-trace.

If I enable preempt kernel (CONFIG_PREEMPT=y), "RT throttling activated" 
still happens, and there is no reboot.
-  After "RT throttling activated" happens, I use "top -d 1" to check 
CPU usage, I observed at least once sc16is752 threaded-irq handler 
kernel thread occupies 95% of CPU for some seconds (not sure how many 
seconds), then sc16is752 threaded-irq handler CPU usage drops to normal, 
and the whole OpenWRT/Linux system is normal.
-  I did observe once that sc16is752 threaded-irq handler CPU usage 
stays at 95% forever, it seems like a dead-lock inside sc16is752 
threaded-irq handler.
-  It seems a dead-lock similar (in some way?) to below patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-4.4.y&id=d78006d2345f87889918a8a7aa3764628ff84263

Question 3) Is there any explanation why board does not reboot when I 
enable preempt kernel (CONFIG_PREEMPT=y)?

Question 4) I am not sure if this is related to MIPS32r2 34kc (with 
9-stage pipeline multi-threaded processor core), while Atheros AR9331 is 
24kc (with 8-stage pipeline processor core)?
https://www.imgtec.com/mips/classic/


I have a temporary work-around:
-  In bitbang I2C driver, use hrtimer-backed usleep_range() (rather than 
busywait udelay()), then this issue is gone (verified by stress test).
-  As I do not know how long it takes to arm hrtimer on AR9331 platform, 
I use usleep_range(10us, 20us), this greatly reduces bitbang I2C 
throughput, and reduces sc16is752 (I2C based UART) baudrate.

Thanks.
Xuebing Wang