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

Suren Baghdasaryan
2022-09-05T18:03:57Z

On Mon, Sep 5, 2022 at 1:12 AM Michal Hocko <mhocko@suse.com> wrote:
>
> 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?

Correct, it's a comparison with the allocation fast path, so in a
sense represents the worst case scenario. However at the same time the
measurements are fair because they measure the overheads against the
same meaningful baseline, therefore can be used for comparison.

>
> 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.

I used whatever is default and didn't try other mechanisms. Don't
think the difference would be orders of magnitude better though.

>
> 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.

That's one of those questions which are very difficult to answer (if
even possible) because that would depend on the use scenario. If the
workload allocates frequently then adding the overhead will likely
affect it, otherwise might not be even noticeable. In general, in
pre-production testing we try to minimize the difference in
performance and memory profiles between the software we are testing
and the production one. From that point of view, the smaller the
overhead, the better. I know it's kinda obvious but unfortunately I
have no better answer to that question.

For the memory overhead, in my early internal proposal with assumption
of 10000 instrumented allocation call sites, I've made some
calculations for an 8GB 8-core system (quite typical for Android) and
ended up with the following:

per-cpu counters atomic counters
page_ext references 16MB 16MB
slab object references 10.5MB 10.5MB
alloc_tags 900KB 312KB
Total memory overhead 27.4MB 26.8MB

so, about 0.34% of the total memory. Our implementation has changed
since then and the number might not be completely correct but it
should be in the ballpark.
I just checked the number of instrumented calls that we currently have
in the 6.0-rc3 built with defconfig and it's 165 page allocation and
2684 slab allocation sites. I readily accept that we are probably
missing some allocations and additional modules can also contribute to
these numbers but my guess it's still less than 10000 that I used in
my calculations.
I don't claim that 0.34% overhead is low enough to be always
acceptable, just posting the numbers to provide some reference points.

> --
> Michal Hocko
> SUSE Labs