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authorFrederic Weisbecker <fweisbec@gmail.com>2009-12-06 00:52:56 +0100
committerFrederic Weisbecker <fweisbec@gmail.com>2009-12-06 08:27:22 +0100
commitb625b3b3b740e177a1148594cd3ad5ff52f35315 (patch)
treeae47ed49c18b46a0a12521307b411ca8f53028f4
parent7f33f9c5cc3c99aeaf4d266a7ed502b828115a53 (diff)
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 <fweisbec@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: "K. Prasad" <prasad@linux.vnet.ibm.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "H. Peter Anvin" <hpa@zytor.com>
-rw-r--r--arch/x86/kernel/entry_64.S6
1 files changed, 3 insertions, 3 deletions
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 722df1b1152..0f08a0cea3e 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)