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    Subjectperf PPC: kernel panic with callchains and context switch events
    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.

    [<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


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