Hi Ian,
On Sat, Oct 17, 2026 at 10:37:04PM -0700, Ian Rogers wrote:
> This series adds a new BPF sample filter predicate, `lock_held(<lock>)`
> (along with `lock_held_read(<lock>)` / `lock_held_shared(<lock>)`,
> `lock_held_write(<lock>)` / `lock_held_exclusive(<lock>)`, and `:R` / `:W`
> mode suffixes), to `perf record --filter`, allowing perf event samples to
> be recorded only when a specified kernel lock is held (or not held with
> `!`) by the running kernel thread or CPU. It also extends both
> `lock_held()` and `perf lock contention --use-bpf` to support RCU and
> SRCU, and fixes a pre-existing `lock_stat` filtering bug in `perf lock
> contention`.
Awesome! Looks really useful!
>
> For example:
>
> # perf record -a -g -e cycles --filter 'lock_held(&cgroup_mutex)' -- ...
> # perf record -a -g -e cycles --filter 'lock_held_read(&mmap_lock)' -- ...
> # perf record -a -g -e cycles --filter 'lock_held(&mmap_lock:W)' -- ...
> # perf record -a -g -e cycles --filter 'lock_held_read(rcu)' -- ...
> # perf lock contention -a -b -Y rcu,srcu -- sleep 1
>
> An important use-case is detecting memory allocations or direct memory
> reclaim occurring while kernel locks are held, which can stall lock
> waiters for extended periods under memory pressure:
>
> # Direct memory reclaim triggered while holding any mutex or rwsem:
> # perf record -a -g -e vmscan:mm_vmscan_direct_reclaim_begin \
> --filter 'lock_held(mutex) || lock_held(rwsem)' -- ...
>
> # Page allocations permitted to enter direct reclaim (__GFP_DIRECT_RECLAIM)
> # while holding a mutex or rwsem:
> # perf record -a -g -e kmem:mm_page_alloc \
> --filter 'gfp_flags & 0x400' \
> --filter 'lock_held(mutex) || lock_held(rwsem)' -- ...
It seems the first filter is processed by the kernel tracepoint
filtering logic and the second is passed to the BPF filter, right?
Does the ordering matter?
>
> # Major page faults occurring while holding a mutex or rwsem:
> # perf record -a -g -e major-faults \
> --filter 'lock_held(mutex) || lock_held(rwsem)' -- ...
>
> Supported `<lock>` specifications match `perf lock contention`:
> - Global/static kernel lock symbols (with or without leading `&`, e.g.
> `lock_held(&cgroup_mutex)` or `lock_held(cgroup_mutex)`)
It used to have no '&' for global locks. While having it is more
natural, I think it's better to leave it for slab locks so that we can
distinguish them easily.
> - Hexadecimal kernel lock addresses
> - Slab cache names prefixed with `&` (e.g. `lock_held(&dentry)`,
> `lock_held_read(&dentry)`)
> - Well-known per-process, semi-global, and RCU locks: `mmap_lock`,
> `siglock`, `rq_lock`, `zone_lock`, `rcu`, `rcu-trace`
> - Lock types and flags: `mutex`, `spinlock`, `rwsem`, `rwsem:R`,
> `rwsem:W`, `rwlock`, `rwlock:R`, `rwlock:W`, `rt-mutex`,
> `percpu-rwsem`, `pcpu-sem:W`, `srcu`, `srcu:R`, `srcu:W`, `rcu:R`,
> `rcu:W`, `semaphore`
>
> Limitations and weaknesses in determining whether a lock is held:
> - Inlined lock/unlock operations:
> - When the kernel is configured to inline spinlock or rwlock operations
> (`CONFIG_INLINE_SPIN_LOCK`, `CONFIG_INLINE_READ_LOCK`,
> `CONFIG_INLINE_WRITE_LOCK`, `!CONFIG_UNINLINE_SPIN_UNLOCK`,
> `CONFIG_INLINE_*_UNLOCK*`, etc.), `_raw_spin_lock*` /
> `_raw_spin_unlock*` and `_raw_read_lock*` / `_raw_write_lock*` are
> inlined into callers and have no BTF function entry points for
> `fentry`/`fexit` to attach to. `configure_lock_progs()` verifies that
> both acquire and release entry points exist in BTF before enabling a
> lock family.
> - Fast-path `percpu_down_read()` / `percpu_up_read()` and
> `srcu_read_lock_fast()` / `srcu_read_unlock_fast()` are inlined
> per-CPU counter increments without function calls, so only
> `percpu_down_write()` / `percpu_up_write()` and out-of-line
> `__srcu_read_lock*()` / `__srcu_read_unlock*()` can be hooked.
> - On `!CONFIG_PREEMPT_RCU` kernels, `rcu_read_lock()` compiles to
> `preempt_disable()` and does not maintain
> `current->rcu_read_lock_nesting` in `task_struct`, so `lock_held(rcu)`
> read-side inspection requires `CONFIG_PREEMPT_RCU`.
Looks ok. What happens if user specified unsupported locks - e.g. rcu:R
on !CONFIG_PREEMPT_RCU.
> - Lack of an inspectable owner field on non-exclusive / spinning locks:
> - While `struct mutex`, `struct rt_mutex`, and write-locked
> `struct rw_semaphore` store the owning `task_struct *` in `lock->owner`
> (allowing `perf_sample_filter` to inspect kernel memory directly on
> sample overflow when filtering by lock address), spinlocks, rwlocks,
> semaphores, SRCU, and reader-locked `rw_semaphore`s (where
> `__up_read()` does not clear `sem->owner` when `!CONFIG_DEBUG_RWSEMS`)
> do not maintain an authoritative per-task owner pointer.
So do you use the owner field for the supported locks? Then maybe it
can skip tracing mutex_lock() and mutex_unlock(). Does it already do?
> - Tracking these locks (as well as slab-cache and lock-type filters)
> relies on `fexit` (acquire) and `fentry` (release) BPF hooks
> maintaining the `held_locks` BPF hash map:
> - Any such lock already held before `perf record` attaches its BPF
> programs will not be present in `held_locks` until it is released
> and re-acquired.
Right, but this can be mitigated when it runs for long.
> - The attached `fentry`/`fexit` hooks execute on every kernel lock
> acquire and release for the hooked lock family across the system,
> adding tracing overhead outside of perf sample handling.
This is a real problem. As it touches the fastpath it can cause a lot
of overhead if locking is frequent. But having this capability would be
very useful anyway!
Thanks,
Namhyung
>
> Patch overview:
> 1. "perf lock contention: Share common BPF and lock type definitions in
> lock_data.h": Moves `lock_type_table` to `util/lock-contention.c` and
> `LCB_F_*` flags, CO-RE structs, and common BPF lock helpers
> (`get_lock_owner`, `get_current_mmap_lock`, `get_current_siglock`,
> `get_lock_addr`, `check_lock_type`, `do_collect_lock_syms`,
> `do_slab_cache_iter`) into `tools/perf/util/bpf_skel/lock_data.h`.
> 2. "perf bpf-filter: Add lock_held() sample filter": Implements
> `lock_held()`, `lock_held_read()`/`lock_held_shared()`,
> `lock_held_write()`/`lock_held_exclusive()`, and `:R`/`:W` mode
> suffixes using direct lock owner inspection where possible (`mutex`,
> `rt_mutex`, write-locked `rw_semaphore`) and a `held_locks` BPF map
> updated by selectively attached `fentry`/`fexit` hooks, supporting
> both PMU/software events (`SEC("perf_event")`) and tracepoint events
> (`SEC("tracepoint")`).
> 3. "perf test: Add tests for lock_held() BPF sample filter": Adds unit
> tests in `tests/parse-events.c` and shell tests in
> `tests/shell/record_bpf_filter.sh`.
> 4. "perf bpf-filter: Extend lock_held() to support RCU, Tasks Trace RCU,
> and SRCU": Supports `rcu` (`rcu:R` via direct
> `current->rcu_read_lock_nesting` inspection with zero tracing hooks;
> `rcu:W` via `synchronize_rcu*()` / `rcu_barrier()` hooks),
> `rcu-trace` (via `current->trc_reader_nesting`), and `srcu`
> (`srcu:R` via `__srcu_read_lock*` / `__srcu_read_unlock*`; `srcu:W`
> via `synchronize_srcu*()` / `srcu_barrier()`).
> 5. "perf lock contention: Extend BPF lock contention to RCU and SRCU":
> Extends `perf lock contention --use-bpf` to track RCU and SRCU
> grace-period and barrier waits (`-Y rcu`, `-Y srcu`, `-L rcu`).
> 6. "perf lock contention: Filter events before creating lock_stat":
> Fixes `report_lock_contention_begin_event()` in `builtin-lock.c` to
> apply `-Y`, `-L`, and `-S` filters before calling
> `lock_stat_findnew()`.
>
> Changes in v3:
> - Move the writer-only `rw_semaphore` check out of `get_lock_owner()` in
> `lock_data.h` into `is_rwsem_writer_owner()` in `sample_filter.bpf.c`
> so `perf lock contention -b -o` still reports reader owners when a
> writer contends on a reader-owned `rw_semaphore`, and scope
> `is_direct_lock_owner()` by lock type (`PBF_LOCK_DIRECT_MUTEX`,
> `PBF_LOCK_DIRECT_RWSEM`, `PBF_LOCK_DIRECT_RTMUTEX`).
> - Add `SEC("tracepoint") int perf_tp_sample_filter(void *ctx)` so BPF
> sample filters (`lock_held()`, `uid`, `gid`, etc.) can attach to
> `PERF_TYPE_TRACEPOINT` events (such as `kmem:mm_page_alloc` and
> `vmscan:mm_vmscan_direct_reclaim_begin`), check `libbpf_get_error()`
> when attaching, and document memory allocation / direct reclaim
> filtering with `lock_held()`.
> - Return early in `init_numa_data()` in `bpf-filter.c` when `node_data`
> cannot be resolved and check `node_data_addr != 0` in
> `do_collect_zone_lock()`.
> - Avoid double-counting `_raw_read_lock_irq` / `_raw_read_lock_bh` in
> `sample_filter.bpf.c` on `CONFIG_GENERIC_LOCKBREAK=y &&
> !CONFIG_DEBUG_LOCK_ALLOC` kernels where they call
> `_raw_read_lock_irqsave()`.
> - Offset `rcu_state_addr` by 1 byte so it is distinct from
> `&rcu_state.node[0].lock` at offset 0 of `rcu_state`, and pass `bool
> is_rcu` to `do_contention_end()` in `lock_contention.bpf.c` so
> `tp_btf/contention_end` and RCU/SRCU `fexit` hooks never cross-match
> timestamp maps.
>
> Changes in v2:
> - Guard `BPF_CORE_READ(mutex, owner.counter)` with `bpf_core_field_exists`
> and fall back to `struct rt_mutex_base` in `get_lock_owner()` so CO-RE
> relocation succeeds on `CONFIG_PREEMPT_RT=y` kernels.
> - Add `CONFIG_DEBUG_LOCK_ALLOC=y` lock entry points
> (`_mutex_lock_nest_lock`, `_mutex_lock_killable`,
> `_mutex_trylock_nest_lock`, `_down_write_nest_lock`,
> `_raw_spin_lock_nest_lock`, `_rt_mutex_lock_nest_lock`) to
> `sample_filter.bpf.c` and `lock_progs[]`.
> - Verify in `supported_lock_prog_groups()` that both acquire and all
> unlock BTF entry points exist for a lock family before enabling its
> tracing hooks, preventing acquire-only attachment when spinlock/rwlock
> unlock functions are inlined.
> - Enable `LOCK_PROG_PERCPU_RWSEM` alongside `LOCK_PROG_RWSEM_WRITE` when
> a symbol name ends in `rwsem` (such as `cgroup_threadgroup_rwsem`).
> - Gate `record_lock_acquired(ctx, LCB_F_READ)` in `fexit_downgrade_write`
> on `track_rwsem_read` and verify non-zero read/write counts in
> `record_lock_released_addr()` before decrementing aggregate slab/type
> counts.
> - Adjust stack skip depth by 2 for `fentry` RCU/SRCU programs in
> `lock_contention.bpf.c` and recognize `synchronize_rcu*`,
> `synchronize_srcu*`, `rcu_barrier`, and `srcu_barrier` in
> `machine__is_lock_function()`.
> - Add patch 6 fixing `report_lock_contention_begin_event()` to check
> type, address, and callstack filters before creating `lock_stat`
> entries.
>
> Assisted-by: Antigravity:gemini-3.1-pro
>
> Ian Rogers (6):
> perf lock contention: Share common BPF and lock type definitions in
> lock_data.h
> perf bpf-filter: Add lock_held() sample filter
> perf test: Add tests for lock_held() BPF sample filter
> perf bpf-filter: Extend lock_held() to support RCU, Tasks Trace RCU,
> and SRCU
> perf lock contention: Extend BPF lock contention to RCU and SRCU
> perf lock contention: Filter events before creating lock_stat
>
> tools/perf/Documentation/perf-lock.txt | 7 +-
> tools/perf/Documentation/perf-record.txt | 64 +-
> tools/perf/builtin-lock.c | 88 +-
> tools/perf/tests/parse-events.c | 121 ++
> tools/perf/tests/shell/lock_contention.sh | 14 +
> tools/perf/tests/shell/record_bpf_filter.sh | 134 ++
> tools/perf/util/bpf-filter.c | 1271 ++++++++++++++++-
> tools/perf/util/bpf-filter.h | 13 +
> tools/perf/util/bpf-filter.l | 31 +-
> tools/perf/util/bpf-filter.y | 46 +-
> tools/perf/util/bpf_lock_contention.c | 171 ++-
> .../perf/util/bpf_skel/lock_contention.bpf.c | 448 +++---
> tools/perf/util/bpf_skel/lock_data.h | 360 ++++-
> tools/perf/util/bpf_skel/sample-filter.h | 77 +
> tools/perf/util/bpf_skel/sample_filter.bpf.c | 1225 ++++++++++++++++
> tools/perf/util/bpf_skel/vmlinux/vmlinux.h | 7 +
> tools/perf/util/lock-contention.c | 22 +
> tools/perf/util/lock-contention.h | 15 +
> tools/perf/util/machine.c | 11 +
> tools/perf/util/parse-events.c | 3 +-
> 20 files changed, 3741 insertions(+), 387 deletions(-)
>
> --
> 2.56.0.212.g129ede20d9-goog
>
