Re: [RFC] Proposal for ptrace improvements

[Denys Vlasenko]

On Fri, Mar 4, 2011 at 3:40 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Fri, Mar 04, 2011 at 03:31:15PM +0100, Denys Vlasenko wrote:
>> On Fri, Mar 4, 2011 at 3:07 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
>> > Hello,
>> >
>> > On Fri, Mar 04, 2011 at 02:59:32PM +0100, Denys Vlasenko wrote:
>> >> I would rather speed strace up than slow it down further, even if
>> >> slightly.
>> >
>> > The question to ask is at what cost?  If mostly unnoticeable slow down
>> > makes the API cleaner, I'll go that way.  Everything is a tradeoff.
>>
>> # time sh -c 'ls -lR /usr/share >/dev/null'
>> real  0m2.633s
>>
>> strace without PTRACE_GETSIGINFO:
>> real  0m47.023s
>> real  0m48.799s
>> real  0m47.695s
>>
>> strace with PTRACE_GETSIGINFO:
>> real  0m51.958s
>> real  0m53.773s
>> real  0m51.625s
>
> Great, numbers, so it's ~10 slow down.  Gees, with or without that
> change, strace(2) is heavy, >18 times slower than without.

Here's a typical sequence of operations strace is doing:

wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 10445
ptrace(PTRACE_GETSIGINFO, 10445, 0, {si_signo=SIGTRAP, si_code=0x5,
si_pid=10445, si_uid=0, si_value={int=0, ptr=0}}) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*ORIG_RAX, [0x36]) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*CS, [0x23]) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*RAX, [0xffffffffffffffda]) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*RBX, [0x1]) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*RCX, [0x5401]) = 0
ptrace(PTRACE_PEEKUSER, 10445, 8*RDX, [0xffb83f10]) = 0
ptrace(PTRACE_SYSCALL, 10445, 0x1, SIG_0) = 0

IOW: strace wastes a lot of time just transiting to the kernel and back
with simple syscalls to get the information.

What would help here is a "vectorized" waitpid operation
which retrieves much more information in one go:

waitpid_vec(int max_results, int status_vector[], siginfo_t
si_vector[], struct pt_regs reg_vector[])

-- 
vda
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