From b625b3b3b740e177a1148594cd3ad5ff52f35315 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Sun, 6 Dec 2009 00:52:56 +0100 Subject: [PATCH] x86: Fixup wrong debug exception frame link in stacktraces While dumping a stacktrace, the end of the exception stack won't link the frame pointer to the previous stack. The interrupted stack will then be considered as unreliable and ignored by perf, as the frame pointer is unreliable itself. This happens because we overwrite the frame pointer that links to the interrupted frame with the address of the exception stack. This is done in order to reserve space inside. But rbp has been chosen here only because it is not a scratch register, so that the address of the exception stack remains in rbp after calling do_debug(), we can then release the exception stack space without the need to retrieve its address again. But we can pick another non-scratch register to do that, so that we preserve the link to the interrupted stack frame in the stacktraces. Just randomly choose r12. Every registers are saved just before and restored just after calling do_debug(). And r12 is not used in the middle, which makes it a perfect candidate. Example: perf record -g -a -c 1 -f -e mem:$(tasklist_lock_addr):rw Before: 44.18% [k] _raw_read_lock | | --- |--6.31%-- waitid | |--4.26%-- writev | |--3.63%-- __select | |--3.15%-- __waitpid | | | |--28.57%-- 0x8b52e00000139f | | | |--28.57%-- 0x8b52e0000013c6 | | | |--14.29%-- 0x7fde786dc000 | | | |--14.29%-- 0x62696c2f7273752f | | | --14.29%-- 0x1ea9df800000000 | |--3.00%-- __poll After: 43.94% [k] _raw_read_lock | --- _read_lock | |--60.53%-- send_sigio | __kill_fasync | kill_fasync | evdev_pass_event | evdev_event | input_pass_event | input_handle_event | input_event | synaptics_process_byte | psmouse_handle_byte | psmouse_interrupt | serio_interrupt | i8042_interrupt | handle_IRQ_event | handle_edge_irq | handle_irq | __irqentry_text_start | ret_from_intr | | | |--30.43%-- __select | | | |--17.39%-- 0x454f15 | | | |--13.04%-- __read | | | |--13.04%-- vread_hpet | | | |--13.04%-- _xcb_lock_io | | | --13.04%-- 0x7f630878ce87 Note: it does not only affect perf events but also other stacktraces in x86-64. They were considered as unreliable once we quit the debug stack frame. Signed-off-by: Frederic Weisbecker Cc: Peter Zijlstra Cc: Paul Mackerras Cc: Arnaldo Carvalho de Melo Cc: "K. Prasad" Cc: Thomas Gleixner Cc: "H. Peter Anvin" --- arch/x86/kernel/entry_64.S | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 722df1b1152d..0f08a0cea3e0 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1076,10 +1076,10 @@ ENTRY(\sym) TRACE_IRQS_OFF movq %rsp,%rdi /* pt_regs pointer */ xorl %esi,%esi /* no error code */ - PER_CPU(init_tss, %rbp) - subq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%rbp) + PER_CPU(init_tss, %r12) + subq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%r12) call \do_sym - addq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%rbp) + addq $EXCEPTION_STKSZ, TSS_ist + (\ist - 1) * 8(%r12) jmp paranoid_exit /* %ebx: no swapgs flag */ CFI_ENDPROC END(\sym) -- GitLab