Messages in this thread | | | Subject | Re: [RFC PATCH] perf report: add sort by file lines | From | Lin Ming <> | Date | Wed, 30 Mar 2011 00:45:55 +0800 |
| |
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.
> > 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? How to know the access type(read or write)?
> > 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?
Thanks, Lin Ming
| |