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

Andrey Konovalov
2023-10-05T20:36:01Z

On Wed, Sep 13, 2023 at 7:14 PM <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.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!