Re: [PATCH v3 00/19] stackdepot: allow evicting stack traces

[Andrey Konovalov]

On Fri, Nov 3, 2023 at 10:37 PM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
>
> On Tue, Oct 24, 2023 at 3:14 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > 1. I know fixed-sized slots are need for eviction to work, but have
> > you evaluated if this causes some excessive memory waste now? Or is it
> > negligible?
>
> With the current default stack depot slot size of 64 frames, a single
> stack trace takes up ~3-4x on average compared to precisely sized
> slots (KMSAN is closer to ~4x due to its 3-frame-sized linking
> records).
>
> However, as the tag-based KASAN modes evict old stack traces, the
> average total amount of memory used for stack traces is ~0.5 MB (with
> the default stack ring size of 32k entries).
>
> I also have just mailed an eviction implementation for Generic KASAN.
> With it, the stack traces take up ~1 MB per 1 GB of RAM while running
> syzkaller (stack traces are evicted when they are flushed from
> quarantine, and quarantine's size depends on the amount of RAM.)
>
> The only problem is KMSAN. Based on a discussion with Alexander, it
> might not be possible to implement the eviction for it. So I suspect,
> with this change, syzbot might run into the capacity WARNING from time
> to time.
>
> The simplest solution would be to bump the maximum size of stack depot
> storage to x4 if KMSAN is enabled (to 512 MB from the current 128 MB).
> KMSAN requires a significant amount of RAM for shadow anyway.
>
> Would that be acceptable?
>
> > If it turns out to be a problem, one way out would be to partition the
> > freelist into stack size classes; e.g. one for each of stack traces of
> > size 8, 16, 32, 64.
>
> This shouldn't be hard to implement.
>
> However, as one of the perf improvements, I'm thinking of saving a
> stack trace directly into a stack depot slot (to avoid copying it).
> With this, we won't know the stack trace size before it is saved. So
> this won't work together with the size classes.

On a second thought, saving stack traces directly into a stack depot
slot will require taking the write lock, which will badly affect
performance, or using some other elaborate locking scheme, which might
be an overkill.

> > 2. I still think switching to the percpu_rwsem right away is the right
> > thing, and not actually a downside. I mentioned this before, but you
> > promised a follow-up patch, so I trust that this will happen. ;-)
>
> First thing on my TODO list wrt perf improvements :)
>
> > Acked-by: Marco Elver <elver@xxxxxxxxxx>
> >
> > The series looks good in its current state. However, see my 2
> > higher-level comments above.
>
> Thank you!