linux-kernel - Re: [RFC PATCH 00/10] x86: undwarf unwinder

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Date:   Thu, 1 Jun 2017 14:33:20 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 00/10] x86: undwarf unwinder

On 06/01/2017, 02:17 PM, Peter Zijlstra wrote:
> On Thu, Jun 01, 2017 at 06:58:20AM -0500, Josh Poimboeuf wrote:
>>> Being able to generate more optimal code in the hottest code paths of the kernel 
>>> is the _real_, primary upstream kernel benefit of a different debuginfo method - 
>>> which has to be weighed against the pain of introducing a new unwinder. But this 
>>> submission does not talk about that aspect at all, which should be fixed I think.
>>
>> Actually I devoted an entire one-sentence paragraph to performance in
>> the documentation:
>>
>>   The simpler debuginfo format also enables the unwinder to be relatively
>>   fast, which is important for perf and lockdep.
>>
>> But I'll try to highlight that a little more.
> 
> That's relative to a DWARF unwinder. It doesn't appear to be possible to
> get anywhere near a frame-pointer unwinder due to having to do this
> log(n) lookup for every single frame.

This is ~ 20 times faster than my DWARF unwinder by a quick measurement
(20000 calls to save_stack_trace via single vfs_write).

perf profile, if you care:

__save_stack_trace
|
|--65.89%--unwind_next_frame
|          |
|          |--53.64%--__undwarf_lookup
|          |
|           --5.30%--deref_stack_reg
|                     |
|                      --2.32%--stack_access_ok
|
|--24.17%--__unwind_start
|          |
|          |--21.52%--unwind_next_frame
|          |          |
|          |          |--14.24%--__undwarf_lookup
|          |          |
|          |           --2.98%--deref_stack_reg
|          |                     |
|          |                      --1.32%--stack_access_ok
|          |
|           --1.32%--get_stack_info
|                     |
|                      --0.66%--in_task_stack
|
|--3.31%--unwind_get_return_address
|          __kernel_text_address
|          |
|          |--0.99%--is_ftrace_trampoline
|          |
|          |--0.99%--__is_insn_slot_addr
|          |          |
|          |           --0.66%--__rcu_read_unlock
|          |
|           --0.66%--is_bpf_text_address
|
 --1.66%--save_stack_address


-- 
js
suse labs