On Fri, 29 Aug 2025 13:54:08 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 29 Aug 2025 at 11:11, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > The idea is this (pseudo code):
> >
> > user_stack_trace() {
> > foreach vma in each stack frame:
> > key = hash(vma->vm_file);
> > if (!lookup(key)) {
> > trace_file_map(key, generate_path(vma), generate_buildid(vma));
> > add_into_hash(key);
> > }
> > }
>
> I see *zero* advantage to this. It's only doing stupid things that
> cost extra, and only because you don't want to do the smart thing that
> I've explained extensively that has *NONE* of these overheads.
>
> Just do the parsing at parse time. End of story.
What does "parsing at parse time" mean?
>
> Or don't do this at all. Justy forget the whole thing entirely. Throw
> the patch that started this all away, and just DON'T DO THIS.
Maybe we are talking past each other.
When I get a user space stack trace, I get the virtual addresses of each of
the user space functions. This is saved into an user stack trace event in
the ring buffer that usually gets mapped right to a file for post
processing.
I still do the:
user_stack_trace() {
foreach addr each stack frame
vma = vma_lookup(mm, addr);
callchain[i++] = (addr - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);
Are you saying that this shouldn't be done either? And to just record the
the virtual address in the chain and the vma->vm_start and
vma->vm_pgoff in another event? Where the post processing could do the
math? This other event could also record the path and build id.
The question is, when do I record this vma event? How do I know it's new?
I can't rely too much on other events (like mmap) and such as those events
may have occurred before the tracing started. I have to have some way to
know if the vma has been saved previously, which was why I had the hash
lookup, and only add vma's on new instances.
My main question is, when do I record the vma data event?
-- Steve
