Re: [RFC PATCH] perf report: add sort by file lines

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2011-03-30 at 00:45 +0800, Lin Ming wrote:
> On Tue, 2011-03-29 at 17:54 +0800, Peter Zijlstra wrote:
> > On Tue, 2011-03-29 at 17:32 +0800, Lin Ming wrote:
> > > 
> > > Peter suggested to reverse map the reported IP (PEBS + fixup)
> > > to a data access using dwarf info.
> > > So I wrote this patch to see if the direction is right. 
> > 
> > I'm not sure this is quite the same thing, I'm not arguing this might
> > not be useful, but this is not about data access.
> > 
> > Suppose you have a line like:
> > 
> >   foo->bar->fubar = tmp->blah;
> > 
> > There's 3 indirections there, a line number doesn't even get you close
> > to knowing what data access triggered the event.
> > 
> > struct bar {
> >         int poekoe[5];
> >         int fubar;
> > };
> > 
> > struct foo {
> >         long poekoe[3];
> >         struct bar *bar;
> > };
> > 
> > struct tmp {
> >         long poekoe[4];
> >         int blah;
> > };
> > 
> > void foo(struct foo *foo, struct tmp *tmp)
> > {
> >         foo->bar->fubar = tmp->blah;
> > }
> > 
> > Which gives (somewhat simplified):
> > 
> > foo:
> > 	.cfi_startproc
> > 	pushq	%rbp
> > 	.cfi_def_cfa_offset 16
> > 	movq	%rsp, %rbp
> > 	.cfi_offset 6, -16
> > 	.cfi_def_cfa_register 6
> > 	movq	%rdi, -8(%rbp)
> > 	movq	%rsi, -16(%rbp)
> > 	movq	-8(%rbp), %rax    /* load foo arg from stack */
> > 	movq	24(%rax), %rax    /* load foo->bar */
> > 	movq	-16(%rbp), %rdx   /* load tmp arg from stack */
> > 	movl	32(%rdx), %edx    /* load tmp->blah */
> > 	movl	%edx, 20(%rax)    /* store bar->fubar */
> > 	leave
> > 	ret
> > 	.cfi_endproc
> 
> I need to have a close look at how dwarf cfi thing works.

You'll need more than CFI.

> > 
> > where I annotated the various moves with C comments.
> > 
> > Now depending on what exact IP you get using PEBS+fixup you could using
> > DWARF bits generate similar deductions from the code as I did in those
> > comments and thus know exactly what data member was accessed and how
> > (read/write).
> 
> Is it an unwind of the call frame stack to find out what data member was
> accessed?

No need to unwind stacks, DWARF should have information on function
local stack. It should be able to tell you the type of things like -8(%
rbp).

> How to know the access type(read or write)?

instruction decode, see if the memory operand is a source or target.

> > 
> > With that data you could then borrow some pahole code and annotate the
> > various data structures to illustrate read/write distributions, which
> > can then be used as input for data-reorder.
> 
> Could you explain a bit more about this?

$ pahole tmp.o
struct tmp {
        long int                   poekoe[4];            /*     0    32 */
        int                        blah;                 /*    32     4 */

        /* size: 40, cachelines: 1 */
        /* padding: 4 */
        /* last cacheline: 40 bytes */
};      /* definitions: 1 */

struct foo {
        long int                   poekoe[3];            /*     0    24 */
        struct bar *               bar;                  /*    24     8 */

        /* size: 32, cachelines: 1 */
        /* last cacheline: 32 bytes */
};      /* definitions: 1 */

struct bar {
        int                        poekoe[5];            /*     0    20 */
        int                        fubar;                /*    20     4 */

        /* size: 24, cachelines: 1 */
        /* last cacheline: 24 bytes */
};      /* definitions: 1 */

You could provide similar structure printout from the report function,
and add read/write statistics.