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

andrey.konovalov
2023-09-13T17:14:52Z

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.

---

Changes v1->v2:
- Rework API to stack_depot_save_flags(STACK_DEPOT_FLAG_GET) +
stack_depot_put.
- Add CONFIG_STACKDEPOT_MAX_FRAMES Kconfig option.
- Switch stack depot to using list_head's.
- Assorted minor changes, see the commit message for each path.

Andrey Konovalov (19):
lib/stackdepot: check disabled flag when fetching
lib/stackdepot: simplify __stack_depot_save
lib/stackdepot: drop valid bit from handles
lib/stackdepot: add depot_fetch_stack helper
lib/stackdepot: use fixed-sized slots for stack records
lib/stackdepot: fix and clean-up atomic annotations
lib/stackdepot: rework helpers for depot_alloc_stack
lib/stackdepot: rename next_pool_required to new_pool_required
lib/stackdepot: store next pool pointer in new_pool
lib/stackdepot: store free stack records in a freelist
lib/stackdepot: use read/write lock
lib/stackdepot: use list_head for stack record links
kmsan: use stack_depot_save instead of __stack_depot_save
lib/stackdepot, kasan: add flags to __stack_depot_save and rename
lib/stackdepot: add refcount for records
lib/stackdepot: allow users to evict stack traces
kasan: remove atomic accesses to stack ring entries
kasan: check object_size in kasan_complete_mode_report_info
kasan: use stack_depot_put for tag-based modes

include/linux/stackdepot.h | 59 ++++--
lib/Kconfig | 10 +-
lib/stackdepot.c | 410 ++++++++++++++++++++++++-------------
mm/kasan/common.c | 7 +-
mm/kasan/generic.c | 9 +-
mm/kasan/kasan.h | 2 +-
mm/kasan/report_tags.c | 27 +--
mm/kasan/tags.c | 24 ++-
mm/kmsan/core.c | 7 +-
9 files changed, 356 insertions(+), 199 deletions(-)

--
2.25.1