Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:5 as permitted sender) client-ip=2620:137:e000::3:5;
MIME-Version: 1.0
References: <cover.1698077459.git.andreyknvl@google.com> <CANpmjNNoJQoWzODAbc4naq--b+LOfK76TCbx9MpL8+4x9=LTiw@mail.gmail.com>
 <CA+fCnZeQ6nkCbkOR4GqGQ9OzprGNNrXvrOqqsJP0Vr3uJKLdrQ@mail.gmail.com>
In-Reply-To: <CA+fCnZeQ6nkCbkOR4GqGQ9OzprGNNrXvrOqqsJP0Vr3uJKLdrQ@mail.gmail.com>
From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Mon, 6 Nov 2023 18:41:28 +0100
Message-ID: <CA+fCnZeEkj2TdZ37jFOcYcA-+2BwHta7thMZxPf7n2N4Rt--tw@mail.gmail.com>
Subject: Re: [PATCH v3 00/19] stackdepot: allow evicting stack traces
To:     Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>
Cc:     andrey.konovalov@linux.dev, Dmitry Vyukov <dvyukov@google.com>,
        Vlastimil Babka <vbabka@suse.cz>, kasan-dev@googlegroups.com,
        Evgenii Stepanov <eugenis@google.com>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Fri, Nov 3, 2023 at 10:37=E2=80=AFPM Andrey Konovalov <andreyknvl@gmail.=
com> wrote:
>
> On Tue, Oct 24, 2023 at 3:14=E2=80=AFPM Marco Elver <elver@google.com> wr=
ote:
> >
> > 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@google.com>
> >
> > The series looks good in its current state. However, see my 2
> > higher-level comments above.
>
> Thank you!