linux-kernel - Re: [PATCH v2 00/19] stackdepot: allow evicting stack traces

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Message-ID: <CA+fCnZckOM0ycja3-=08=B3jwoWrYgn1w91eT=b6no9EN0UWLw@mail.gmail.com>
Date:   Thu, 5 Oct 2023 22:35:45 +0200
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Marco Elver <elver@...gle.com>,
        Alexander Potapenko <glider@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        Evgenii Stepanov <eugenis@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>,
        andrey.konovalov@...ux.dev
Subject: Re: [PATCH v2 00/19] stackdepot: allow evicting stack traces

On Wed, Sep 13, 2023 at 7:14 PM <andrey.konovalov@...ux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@...gle.com>
>
> Currently, the stack depot grows indefinitely until it reaches its
> capacity. Once that happens, the stack depot stops saving new stack
> traces.
>
> This creates a problem for using the stack depot for in-field testing
> and in production.
>
> For such uses, an ideal stack trace storage should:
>
> 1. Allow saving fresh stack traces on systems with a large uptime while
>    limiting the amount of memory used to store the traces;
> 2. Have a low performance impact.
>
> Implementing #1 in the stack depot is impossible with the current
> keep-forever approach. This series targets to address that. Issue #2 is
> left to be addressed in a future series.
>
> This series changes the stack depot implementation to allow evicting
> unneeded stack traces from the stack depot. The users of the stack depot
> can do that via new stack_depot_save_flags(STACK_DEPOT_FLAG_GET) and
> stack_depot_put APIs.
>
> Internal changes to the stack depot code include:
>
> 1. Storing stack traces in fixed-frame-sized slots; the slot size is
>    controlled via CONFIG_STACKDEPOT_MAX_FRAMES (vs precisely-sized
>    slots in the current implementation);
> 2. Keeping available slots in a freelist (vs keeping an offset to the next
>    free slot);
> 3. Using a read/write lock for synchronization (vs a lock-free approach
>    combined with a spinlock).
>
> This series also integrates the eviction functionality in the tag-based
> KASAN modes.
>
> Despite wasting some space on rounding up the size of each stack record,
> with CONFIG_STACKDEPOT_MAX_FRAMES=32, the tag-based KASAN modes end up
> consuming ~5% less memory in stack depot during boot (with the default
> stack ring size of 32k entries). The reason for this is the eviction of
> irrelevant stack traces from the stack depot, which frees up space for
> other stack traces.
>
> For other tools that heavily rely on the stack depot, like Generic KASAN
> and KMSAN, this change leads to the stack depot capacity being reached
> sooner than before. However, as these tools are mainly used in fuzzing
> scenarios where the kernel is frequently rebooted, this outcome should
> be acceptable.
>
> There is no measurable boot time performance impact of these changes for
> KASAN on x86-64. I haven't done any tests for arm64 modes (the stack
> depot without performance optimizations is not suitable for intended use
> of those anyway), but I expect a similar result. Obtaining and copying
> stack trace frames when saving them into stack depot is what takes the
> most time.
>
> This series does not yet provide a way to configure the maximum size of
> the stack depot externally (e.g. via a command-line parameter). This will
> be added in a separate series, possibly together with the performance
> improvement changes.

Hi Marco and Alex,

Could you PTAL at the not-yet-reviewed patches in this series when you
get a chance?

Thanks!