Re: [RFC PATCH 00/30] Code tagging framework and applications

Michal Hocko
2022-09-05T08:12:37Z

On Sun 04-09-22 18:32:58, Suren Baghdasaryan wrote:
> On Thu, Sep 1, 2022 at 12:15 PM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > Yes, tracking back the call trace would be really needed. The question
> > is whether this is really prohibitively expensive. How much overhead are
> > we talking about? There is no free lunch here, really. You either have
> > the overhead during runtime when the feature is used or on the source
> > code level for all the future development (with a maze of macros and
> > wrappers).
>
> As promised, I profiled a simple code that repeatedly makes 10
> allocations/frees in a loop and measured overheads of code tagging,
> call stack capturing and tracing+BPF for page and slab allocations.
> Summary:
>
> Page allocations (overheads are compared to get_free_pages() duration):
> 6.8% Codetag counter manipulations (__lazy_percpu_counter_add + __alloc_tag_add)
> 8.8% lookup_page_ext
> 1237% call stack capture
> 139% tracepoint with attached empty BPF program

Yes, I am not surprised that the call stack capturing is really
expensive comparing to the allocator fast path (which is really highly
optimized and I suspect that with 10 allocation/free loop you mostly get
your memory from the pcp lists). Is this overhead still _that_ visible
for somehow less microoptimized workloads which have to take slow paths
as well?

Also what kind of stack unwinder is configured (I guess ORC)? This is
not my area but from what I remember the unwinder overhead varies
between ORC and FP.

And just to make it clear. I do realize that an overhead from the stack
unwinding is unavoidable. And code tagging would logically have lower
overhead as it performs much less work. But the main point is whether
our existing stack unwiding approach is really prohibitively expensive
to be used for debugging purposes on production systems. I might
misremember but I recall people having bigger concerns with page_owner
memory footprint than the actual stack unwinder overhead.
--
Michal Hocko
SUSE Labs