From 3db6b38dfe640207da706b286d4181237391f5bd Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 27 Oct 2025 09:45:19 +0100 Subject: rseq: Switch to fast path processing on exit to user Now that all bits and pieces are in place, hook the RSEQ handling fast path function into exit_to_user_mode_prepare() after the TIF work bits have been handled. If case of fast path failure, TIF_NOTIFY_RESUME has been raised and the caller needs to take another turn through the TIF handling slow path. This only works for architectures which use the generic entry code. Architectures who still have their own incomplete hacks are not supported and won't be. This results in the following improvements: Kernel build Before After Reduction exit to user 80692981 80514451 signal checks: 32581 121 99% slowpath runs: 1201408 1.49% 198 0.00% 100% fastpath runs: 675941 0.84% N/A id updates: 1233989 1.53% 50541 0.06% 96% cs checks: 1125366 1.39% 0 0.00% 100% cs cleared: 1125366 100% 0 100% cs fixup: 0 0% 0 RSEQ selftests Before After Reduction exit to user: 386281778 387373750 signal checks: 35661203 0 100% slowpath runs: 140542396 36.38% 100 0.00% 100% fastpath runs: 9509789 2.51% N/A id updates: 176203599 45.62% 9087994 2.35% 95% cs checks: 175587856 45.46% 4728394 1.22% 98% cs cleared: 172359544 98.16% 1319307 27.90% 99% cs fixup: 3228312 1.84% 3409087 72.10% The 'cs cleared' and 'cs fixup' percentages are not relative to the exit to user invocations, they are relative to the actual 'cs check' invocations. While some of this could have been avoided in the original code, like the obvious clearing of CS when it's already clear, the main problem of going through TIF_NOTIFY_RESUME cannot be solved. In some workloads the RSEQ notify handler is invoked more than once before going out to user space. Doing this once when everything has stabilized is the only solution to avoid this. The initial attempt to completely decouple it from the TIF work turned out to be suboptimal for workloads, which do a lot of quick and short system calls. Even if the fast path decision is only 4 instructions (including a conditional branch), this adds up quickly and becomes measurable when the rate for actually having to handle rseq is in the low single digit percentage range of user/kernel transitions. Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Mathieu Desnoyers Link: https://patch.msgid.link/20251027084307.701201365@linutronix.de --- include/linux/rseq.h | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) (limited to 'include/linux/rseq.h') diff --git a/include/linux/rseq.h b/include/linux/rseq.h index abfbeb42d1a2..ded4baa34586 100644 --- a/include/linux/rseq.h +++ b/include/linux/rseq.h @@ -7,13 +7,19 @@ #include -void __rseq_handle_notify_resume(struct pt_regs *regs); +void __rseq_handle_slowpath(struct pt_regs *regs); -static inline void rseq_handle_notify_resume(struct pt_regs *regs) +/* Invoked from resume_user_mode_work() */ +static inline void rseq_handle_slowpath(struct pt_regs *regs) { - /* '&' is intentional to spare one conditional branch */ - if (current->rseq.event.sched_switch & current->rseq.event.has_rseq) - __rseq_handle_notify_resume(regs); + if (IS_ENABLED(CONFIG_GENERIC_ENTRY)) { + if (current->rseq.event.slowpath) + __rseq_handle_slowpath(regs); + } else { + /* '&' is intentional to spare one conditional branch */ + if (current->rseq.event.sched_switch & current->rseq.event.has_rseq) + __rseq_handle_slowpath(regs); + } } void __rseq_signal_deliver(int sig, struct pt_regs *regs); @@ -152,7 +158,7 @@ static inline void rseq_fork(struct task_struct *t, u64 clone_flags) } #else /* CONFIG_RSEQ */ -static inline void rseq_handle_notify_resume(struct pt_regs *regs) { } +static inline void rseq_handle_slowpath(struct pt_regs *regs) { } static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs) { } static inline void rseq_sched_switch_event(struct task_struct *t) { } static inline void rseq_sched_set_task_cpu(struct task_struct *t, unsigned int cpu) { } -- cgit v1.2.3