On 07/20/2011 03:57 PM, 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.

Can anyone confirm whether page faults while walking the stack are
normal for PPC? We really want to use the context switch event with
callchains and need to understand whether this behavior is normal. Of
course if it is normal, a way to address the problem without a panic
will be needed.

Thanks,
David

 [<b0180e00>]rb_erase+0x1b4/0x3e8
 [<b00430f4>]__dequeue_entity+0x50/0xe8
 [<b0043304>]set_next_entity+0x178/0x1bc
 [<b0043440>]pick_next_task_fair+0xb0/0x118
 [<b02ada80>]schedule+0x500/0x614
 [<b02afaa8>]rwsem_down_failed_common+0xf0/0x264
 [<b02afca0>]rwsem_down_read_failed+0x34/0x54
 [<b02aed4c>]down_read+0x3c/0x54
 [<b0023b58>]do_page_fault+0x114/0x5e8
 [<b001e350>]handle_page_fault+0xc/0x80
 [<b0022dec>]perf_callchain+0x224/0x31c
 [<b009ba70>]perf_prepare_sample+0x240/0x2fc
 [<b009d760>]__perf_event_overflow+0x280/0x398
 [<b009d914>]perf_swevent_overflow+0x9c/0x10c
 [<b009db54>]perf_swevent_ctx_event+0x1d0/0x230
 [<b009dc38>]do_perf_sw_event+0x84/0xe4
 [<b009dde8>]perf_sw_event_context_switch+0x150/0x1b4
 [<b009de90>]perf_event_task_sched_out+0x44/0x2d4
 [<b02ad840>]schedule+0x2c0/0x614
 [<b0047dc0>]__cond_resched+0x34/0x90
 [<b02adcc8>]_cond_resched+0x4c/0x68
 [<b00bccf8>]move_page_tables+0xb0/0x418
 [<b00d7ee0>]setup_arg_pages+0x184/0x2a0
 [<b0110914>]load_elf_binary+0x394/0x1208
 [<b00d6e28>]search_binary_handler+0xe0/0x2c4
 [<b00d834c>]do_execve+0x1bc/0x268
 [<b0015394>]sys_execve+0x84/0xc8
 [<b001df10>]ret_from_syscall+0x0/0x3c

Thanks,
David