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Subjectrcu self-detected stall messages on OMAP3, 4 boards

Hi Paul

Recently several of us have been seeing "INFO: rcu_sched self-detected
stall on CPU { 0} (t=20611 jiffies)" stack tracebacks on various OMAP3
and 4 board.

I only noticed it during v3.6-rc3, but I suspect it's been happening
for users at least since May:

The only quasi-reproducible test case that I've found so far
is to boot the board with serial console enabled to a login prompt, then
wait for a few minutes, then send a keypress to the board via serial.
The tracebacks I get look like this:

[ 467.480712] INFO: rcu_sched self-detected stall on CPU { 0} (t=20611 jiffies)
[ 467.484741] [<c001b7cc>] (unwind_backtrace+0x0/0xf0) from [<c00acc94>] (rcu_check_callbacks+0x180/0x630)
[ 467.489929] [<c00acc94>] (rcu_check_callbacks+0x180/0x630) from [<c0052b18>] (update_process_times+0x38/0x68)
[ 467.495361] [<c0052b18>] (update_process_times+0x38/0x68) from [<c008c04c>] (tick_sched_timer+0x80/0xec)
[ 467.500518] [<c008c04c>] (tick_sched_timer+0x80/0xec) from [<c0068544>] (__run_hrtimer+0x7c/0x1e0)
[ 467.505401] [<c0068544>] (__run_hrtimer+0x7c/0x1e0) from [<c0069328>] (hrtimer_interrupt+0x11c/0x2d0)
[ 467.510437] [<c0069328>] (hrtimer_interrupt+0x11c/0x2d0) from [<c001a04c>] (twd_handler+0x30/0x44)
[ 467.515350] [<c001a04c>] (twd_handler+0x30/0x44) from [<c00a71a0>] (handle_percpu_devid_irq+0x90/0x13c)
[ 467.520477] [<c00a71a0>] (handle_percpu_devid_irq+0x90/0x13c) from [<c00a3914>] (generic_handle_irq+0x30/0x48)
[ 467.525939] [<c00a3914>] (generic_handle_irq+0x30/0x48) from [<c0014c58>] (handle_IRQ+0x4c/0xac)
[ 467.530731] [<c0014c58>] (handle_IRQ+0x4c/0xac) from [<c0008478>] (gic_handle_irq+0x28/0x5c)
[ 467.535339] [<c0008478>] (gic_handle_irq+0x28/0x5c) from [<c04f8ce4>] (__irq_svc+0x44/0x5c)
[ 467.539886] Exception stack(0xc0729f58 to 0xc0729fa0)
[ 467.542663] 9f40: 00047f2a 00000001
[ 467.547119] 9f60: 00000000 c074a940 c0728000 c07c4b08 c05045a0 c074be20 00000000 411fc092
[ 467.551574] 9f80: c074c040 00000000 00000001 c0729fa0 00047f2b c0014f50 20000113 ffffffff
[ 467.556030] [<c04f8ce4>] (__irq_svc+0x44/0x5c) from [<c0014f50>] (default_idle+0x20/0x44)
[ 467.560485] [<c0014f50>] (default_idle+0x20/0x44) from [<c001517c>] (cpu_idle+0x9c/0x114)
[ 467.564971] [<c001517c>] (cpu_idle+0x9c/0x114) from [<c06d77b0>] (start_kernel+0x2b4/0x304)

Looks like this message was added as of commit
a858af2875fb291d0f4b0a4419fefbf03c2379c0 ("rcu: Print scheduling-clock
information on RCU CPU stall-warning messages").

Do you have any suggestions for how we can determine what is causing

Here's an example of a kernel config that we use:

A few observations that may or may not be relevant: we use NO_HZ, and
we also have a clockevents timer that is relatively slow to program.


- Paul

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