On 04.08.26 09:15, Dmitry Ilvokhin wrote:
The contended_release tracepoint landed in v7.2-rc2 for sleeping locks (4f070ccb4dc4 "locking: Add contended_release tracepoint to sleepable locks"). Spinlock support was dropped from that series. This one adds it for queued spinlocks.The existing contention_begin/contention_end tracepoints fire on the waiter side. The holder's identity and stack can be captured at contention_begin time (e.g. perf lock contention --lock-owner), but only for locks with an owner field to read: mutex and rwsem. qspinlock has none, so a contended spinlock cannot be attributed to its holder at all. Even where the owner can be read, it reflects the holder's state when a waiter arrives, not when the lock is released. This series adds a contended_release tracepoint to qspinlock that fires on the holder side when a lock with waiters is released. This provides: - Hold time estimation: when the holder's own acquisition was contended, its contention_end (acquisition) and contended_release can be correlated to measure how long the lock was held under contention. - The holder's stack at release time, which for spinlocks is not available by any other means. The unlock path might be quite hot, so the tracepoint is made as cheap as possible, to keep it usable in production: - x86 with PARAVIRT_SPINLOCKS=y, which is what distributions ship, swaps the unlock implementation via static_call() when the tracepoint is enabled. The disabled path is byte-identical to today's: the same inline movb, no NOP and no call. - Everywhere else a static-branch check is compiled into queued_spin_unlock(). On x86_64 that is a single NOP on the executed path, with the call to the traced helper emitted out of line and unreachable while the tracepoint is off. On other architectures a few more instructions to manage a stack frame land on the executed path too, so the generic path sits behind CONFIG_QUEUED_SPINLOCKS_TRACE_CONTENDED_RELEASE (default n). Costs and measurements are in the individual changelogs. Briefly, no throughput or latency change is measurable on either x86_64 or arm64 with QUEUED_SPINLOCKS_TRACE_CONTENDED_RELEASE=y. Tested: x86_64 with PARAVIRT_SPINLOCKS=y and =n, arm64, tracepoint on and off, disassembly checked in both states, locktorture with tracepoint on and off. Not covered: qrwlock, and architectures with fully custom qspinlock implementations (e.g. PowerPC). The stack frame managing instructions on arm64 should be avoidable, but that is not done in this patchset. Patch 1 is Peter's draft from [1] and is missing his Signed-off-by. Peter, please add it if you are happy with the patch. [1]: https://lore.kernel.org/all/[email protected]/ Dmitry Ilvokhin (4): locking: Factor out queued_spin_release() locking/qspinlock: Add contended_release tracepoint tracing/lock: Use TRACE_EVENT_FN() for contended_release x86/paravirt: Trace contended_release on unlock Peter Zijlstra (1): x86/paravirt: Use static_call() for the paravirt spinlock ops arch/mips/include/asm/spinlock.h | 6 +-- arch/x86/hyperv/hv_spinlock.c | 4 +- arch/x86/include/asm/cpufeatures.h | 1 - arch/x86/include/asm/paravirt-spinlock.h | 21 +++++--- arch/x86/kernel/kvm.c | 5 +- arch/x86/kernel/paravirt-spinlocks.c | 63 +++++++++++++++++++++--- arch/x86/kernel/static_call.c | 27 ++++++++++ arch/x86/xen/spinlock.c | 5 +- include/asm-generic/qspinlock.h | 38 ++++++++++++-- include/trace/events/lock.h | 10 +++- kernel/Kconfig.locks | 20 ++++++++ kernel/locking/mutex.c | 4 ++ kernel/locking/qspinlock.c | 22 +++++++++ tools/arch/x86/include/asm/cpufeatures.h | 1 - 14 files changed, 195 insertions(+), 32 deletions(-) base-commit: 5e601ab3615c86be7c4068ce992f94654693a032
For the whole series: Acked-by: Juergen Gross <[email protected]> I'm considering some followup patches replacing the remaining paravirt cases not covered by CONFIG_PARAVIRT_XXL with static_call(), too. This will allow to drop the 32-bit paravirt patching completely. :-) The queued_spin_unlock() hook was the main reason I didn't do that yet. Juergen
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