From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B48CD47F2D1; Mon, 19 Oct 2026 07:00:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792393259; cv=none; b=VK5FJJY4cEZICOLkHO5N5/ciKwtbG1Z9IQAYvTcBIru0RcfOuTnltMGr3NmiX/6Sw/5QbbyFe6GoxaIaBcBvlieEjtfg8i2M5cgdEakIsdVNK/aLViURpt6XUAg8MaEcT4Tc+ahTXzTjMFOFFLdRdfi5Ok8nkAxyqLxQXp8P5ag= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792393259; c=relaxed/simple; bh=lmf1l6RMAuNPSACVzdfAOmPqOBgNn10PJwdnLs7abzc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NFqOh+mWtovSW5V9IBUcF2XgDfkQs5gTPyaj5h9LopwkEGWy/rpPENkJ1n/JwnTL1OAK476EecvYsZ6YeRhWfXvE6co62UpuHn3yMWXxoqiSPpyJJKxS+CYvnLPCBZ1SRN/Y6CafTyKIlBueWH4uSW2BEDLiVEjZOCe1TyfjaNM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Vfrn5aK8; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Vfrn5aK8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D67BA1F000C4; Mon, 19 Oct 2026 07:00:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792393257; bh=ioyI8Oq0pOeBpMOUrxgYFO/bGmu6pBTLTYcXOY5fj8k=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Vfrn5aK89mCRaN7vRbtv96nso7z5YUmaKII6+jiZHmGc3fAsctjnC+tz1o6gfc4pW mc31CN3EMHrojkLkpaMlzy+6IdEiljJL+CLpgnWAjtm1RoxhIy+A1XmYJLv81RAlGv 4GGqbBE7o2nYBpzeMQ0N0DMGkOXopExklsmigeVXKODBiLPs9n78HUNZvoSdwLy1GO qmK+hJR8GQI/2pIjKFW5nSSytziKblj9iyVBl1V/jkzFVBhGyBp27blj+t9i4XhJ0S 4uOaLl+DGHHKK+EoRsLt4OSKOfFa5RIJ7hwnirk0Eh3T/aRb3RS+g03jzArCojy6MP EGsVhZplRttdw== Date: Mon, 19 Oct 2026 00:00:55 -0700 From: Namhyung Kim To: Ian Rogers Cc: acme@kernel.org, adrian.hunter@intel.com, james.clark@linaro.org, jolsa@kernel.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, rcu@vger.kernel.org Subject: Re: [PATCH v3 0/6] perf bpf-filter / lock contention: Add lock_held() sample filter and RCU/SRCU support Message-ID: References: <20261017171437.2066343-1-irogers@google.com> <20261018053710.2572636-1-irogers@google.com> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20261018053710.2572636-1-irogers@google.com> 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()` > (along with `lock_held_read()` / `lock_held_shared()`, > `lock_held_write()` / `lock_held_exclusive()`, 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 `` 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 >