Re: [PATCH 0/8] perf: add ability to sample physical data addresses

Peter Zijlstra
2013-07-30T09:02:14Z

On Tue, Jul 30, 2013 at 10:51:46AM +0200, Stephane Eranian wrote:
> On Tue, Jul 30, 2013 at 10:37 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Tue, Jul 30, 2013 at 10:02:01AM +0200, Stephane Eranian wrote:
> >> > Ahh. We don't put the useful bits in the mmap event; we'll need to fix
> >> > that too then ;-)
> >> >
> >> > Doing so is going to be a bit of a bother since we use the tail of
> >> > PERF_RECORD_MMAP for filenames and thus aren't particularly extensible.
> >> >
> >> > This would mean doing something like PERF_RECORD_MMAP2 and some means
> >> > for userspace to requrest the new events instead of the old one.
> >> >
> >> Tracking mmaps even for shmat() won't cover the paging cases. When you page a
> >> page back in, it most likely gets a different physical page. How would
> >> we track that
> >> case too using the same approach?
> >
> > It doesn't matter. Even if a page ends up being a different physical
> > page, it will always be the same sb:inode:pgoffset. You should be able
> > to always uniquely identify a (shared) page by that triplet.
> >
> Ok, so you're saying that triplet uniquely identifies a virtual page
> regardless of
> the physical page it is mapped onto. If the physical page changes because
> of paging, we keep the same triplet and therefore we can still detect the false
> sharing.

Exactly.

> > So if we create a net MMAP record that includes the device (substitute
> > for the superblock) and inode information we should be good.
>
> I will try that. I am not familiar with mm, so where do we find the
> device? Inside
> the vma?

Take a peek at fs/proc/task_mmu.c:show_map_vma(), its the code used to
print /proc/$PID/maps and displays all stuff we want.