Re: perf PPC: kernel panic with callchains and context switch events

[Paul Mackerras]

On Wed, Jul 20, 2011 at 03:57:51PM -0600, David Ahern wrote:
> I am hoping someone familiar with PPC can help understand a panic that
> is generated when capturing callchains with context switch events.
> 
> Call trace is below. The short of it is that walking the callchain
> generates a page fault. To handle the page fault the mmap_sem is needed,
> but it is currently held by setup_arg_pages. setup_arg_pages calls
> shift_arg_pages with the mmap_sem held. shift_arg_pages then calls
> move_page_tables which has a cond_resched at the top of its for loop. If
> the cond_resched() is removed from move_page_tables everything works
> beautifully - no panics.
> 
> So, the question: is it normal for walking the stack to trigger a page
> fault on PPC? The panic is not seen on x86 based systems.

Walking the user stack can certainly generate a page fault; walking
the kernel stack should never generate a page fault.  If any page
fault is generated reading the user stack frame, we're supposed to
detect that and fall back to walking the page tables manually (see
read_user_stack_64() in arch/powerpc/kernel/perf_callchain.c).  I
think I need to check our __get_user_inatomic() implementation.

I don't think removing the cond_resched() from move_page_tables is the
right answer.

Paul.
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