On Fri, 21 Sep 2012, Paul E. McKenney wrote:
> On Fri, Sep 21, 2012 at 06:08:59PM +0000, Paul Walmsley wrote:
>
> > As far as I know, our only idle entry point is in
> > arch/arm/common/process.c:cpu_idle().
>
> In mainline, this is arch/arm/kernel/process.c, correct?
Indeed; sorry about that, mistyped.
> > Looking at the x86 idle entry, they call rcu_idle_{enter,exit}() inside
> > {stop,start}_critical_timings(). Making that change here didn't help.
>
> The reason x86 does this is that they have idle notifiers deeper in the
> idle loop that use RCU read-side critical sections. So this was an
> expected result.
OK
> > Also tried commenting out the code from the stop_critical_timings() call
> > to the WARN_ON(irqs_disabled()), and adding a local_irq_enable(). That
> > also didn't help, which suggests that the problem is not caused by the
> > OMAP-specific PM idle code.
>
> I must admit that you make a convincing case here. Though it does leave
> me wondering what is different about Panda (and MX28, IIRC).
Given the dependency on CONFIG_NO_HZ, the stalls are probably dependent on
the userspace in use. The userspaces here are quite minimal and so allow
the system to stay idle for relatively long periods of time.
> I may take your advice of remote access to a Panda board, though that
> is likely to take a bit of time due to timezones. Regardless of the
> underlying issue here, I clearly need to make the stall-warning messages
> do a better job of printing out needed information.
If you've got a patch in mind for that, I'll boot it here.
One other observation. omap2plus_defconfig sets CONFIG_NO_HZ=y but
doesn't set CONFIG_RCU_FAST_NO_HZ. The stall warning messages still
appear when CONFIG_RCU_FAST_NO_HZ=y. One of them is attached below (with
CONFIG_RCU_CPU_STALL_INFO set as well, obviously).
As an aside, in the CONFIG_RCU_FAST_NO_HZ=y build, I dropped a printk()
into rcu_idle_gp_timer_func() and it doesn't look like it ever executed.
- Paul
[ 305.832000] INFO: rcu_sched self-detected stall on CPU
[ 305.834838] 1: (2 GPs behind) idle=5b1/1/0 drain=0 . timer=4294967295
[ 305.838378] (t=17463 jiffies)
[ 305.840118] [<c001be10>] (unwind_backtrace+0x0/0xf0) from [<c00ad65c>] (rcu_pending+0xd0/0x540)
[ 305.844848] [<c00ad65c>] (rcu_pending+0xd0/0x540) from [<c00ae5cc>] (rcu_check_callbacks+0x110/0x198)
[ 305.849884] [<c00ae5cc>] (rcu_check_callbacks+0x110/0x198) from [<c0053800>] (update_process_times+0x38/0x68)
[ 305.855285] [<c0053800>] (update_process_times+0x38/0x68) from [<c008cf40>] (tick_sched_timer+0x80/0xec)
[ 305.860473] [<c008cf40>] (tick_sched_timer+0x80/0xec) from [<c006942c>] (__run_hrtimer+0x7c/0x1e0)
[ 305.865356] [<c006942c>] (__run_hrtimer+0x7c/0x1e0) from [<c006a210>] (hrtimer_interrupt+0x11c/0x2d0)
[ 305.870361] [<c006a210>] (hrtimer_interrupt+0x11c/0x2d0) from [<c001a54c>] (twd_handler+0x30/0x44)
[ 305.875274] [<c001a54c>] (twd_handler+0x30/0x44) from [<c00a8128>] (handle_percpu_devid_irq+0x90/0x13c)
[ 305.880371] [<c00a8128>] (handle_percpu_devid_irq+0x90/0x13c) from [<c00a489c>] (generic_handle_irq+0x30/0x48)
[ 305.885833] [<c00a489c>] (generic_handle_irq+0x30/0x48) from [<c0014fb8>] (handle_IRQ+0x4c/0xac)
[ 305.890624] [<c0014fb8>] (handle_IRQ+0x4c/0xac) from [<c000864c>] (gic_handle_irq+0x28/0x5c)
[ 305.895233] [<c000864c>] (gic_handle_irq+0x28/0x5c) from [<c04fbc64>] (__irq_svc+0x44/0x5c)
[ 305.899780] Exception stack(0xde86ff88 to 0xde86ffd0)
[ 305.902526] ff80: 0004c062 00000001 00000000 de8660c0 de86e000 c07c42c8
[ 305.906982] ffa0: c05075a0 c074bdd0 00000000 411fc092 c074bff0 00000000 00000001 de86ffd0
[ 305.911437] ffc0: 0004c063 c00152b0 20000113 ffffffff
[ 305.914184] [<c04fbc64>] (__irq_svc+0x44/0x5c) from [<c00152b0>] (default_idle+0x20/0x44)
[ 305.918640] [<c00152b0>] (default_idle+0x20/0x44) from [<c00154dc>] (cpu_idle+0x9c/0x114)
[ 305.923126] [<c00154dc>] (cpu_idle+0x9c/0x114) from [<804f4a34>] (0x804f4a34)
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