On Sat, Aug 06, 2016 at 08:16:27PM -0700, Andi Kleen wrote:
> Alexey Dobriyan <adobri...@gmail.com> writes:
> > -
> > + seq_printf(m, "State:\t%s", get_task_state(p));
> > +
> > + seq_puts(m, "\nTgid:\t");
>
> The only different should be the format string.
>
> Scanning the format string really shouldn't be that expensive?!?
Surprise, it is (see my reply to Al).
What seq_put_decimal_ull() did is the equivalent of
seq << "foo";
seq << bar;
seq << '\n';
No precisions, not widths, no padding, no upper and lowercasing.
> It would be better if you could find out why that is slow and optimize
> it. Then you would benefit every seq_printf user, not just this
> special case.
>
> Perhaps it could benefit from some of the bit masking tricks to
> scan the string with wider tests than a word.
And then what? Parsing format string is still be there.
This is first line of profile of the first function (format_decode)
│ static noinline_for_stack
│ int format_decode(const char *fmt, struct printf_spec *spec)
│ {
10.38 │ push %rbp <===
1.07 │ mov %rsp,%rbp
1.09 │ push %r12
4.51 │ mov %rsi,%r12
1.40 │ push %rbx
1.86 │ mov %rdi,%rbx
│ sub $0x8,%rsp
It is so bloated that gcc needs to be asked to not screw up with stack
size.
