Re: [patch V4 28/36] rseq: Switch to fast path processing on exit to user

Mathieu Desnoyers
2025-09-11T14:45:03Z

On 2025-09-08 17:32, Thomas Gleixner wrote:
> 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.

Remove comma after "architectures"

> Architectures, who still have their own incomplete hacks are not supported

Remove comma after "Architectures"

> 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' percentanges are not relative to the exit

percentages

> 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 <tglx@linutronix.de>
> ---
> V4: Move the rseq handling into a separate loop to avoid gotos later on
> ---
> include/linux/irq-entry-common.h | 7 ++-----
> include/linux/resume_user_mode.h | 2 +-
> include/linux/rseq.h | 23 +++++++++++++++++------
> init/Kconfig | 2 +-
> kernel/entry/common.c | 26 +++++++++++++++++++-------
> kernel/rseq.c | 8 ++++++--
> 6 files changed, 46 insertions(+), 22 deletions(-)
>
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -197,11 +197,8 @@ static __always_inline void arch_exit_to
> */
> void arch_do_signal_or_restart(struct pt_regs *regs);
>
> -/**
> - * exit_to_user_mode_loop - do any pending work before leaving to user space
> - */
> -unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> - unsigned long ti_work);
> +/* Handle pending TIF work */
> +unsigned long exit_to_user_mode_loop(struct pt_regs *regs, unsigned long ti_work);
>
> /**
> * exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
> --- a/include/linux/resume_user_mode.h
> +++ b/include/linux/resume_user_mode.h
> @@ -59,7 +59,7 @@ static inline void resume_user_mode_work
> mem_cgroup_handle_over_high(GFP_KERNEL);
> blkcg_maybe_throttle_current();
>
> - rseq_handle_notify_resume(regs);
> + rseq_handle_slowpath(regs);
> }
>
> #endif /* LINUX_RESUME_USER_MODE_H */
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -5,13 +5,19 @@
> #ifdef CONFIG_RSEQ
> #include <linux/sched.h>
>
> -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)

Ref. to earlier comment about has_rseq check perhaps redundant.

> + __rseq_handle_slowpath(regs);
> + }
> }
>
> void __rseq_signal_deliver(int sig, struct pt_regs *regs);
> @@ -142,11 +148,16 @@ static inline void rseq_fork(struct task
> } else {
> t->rseq = current->rseq;
> t->rseq.ids.cpu_cid = ~0ULL;

As discussed earlier, do we really want to clear cpu_cid here, or
copy from parent ? If we keep the parent's cached values, I suspect
we can skip the page fault on return from fork in many cases.

> + /*
> + * If it has rseq, force it into the slow path right away
> + * because it is guaranteed to fault.
> + */
> + t->rseq.event.slowpath = t->rseq.event.has_rseq;

I think we can do better here. It's only guaranteed to fault if:

- has_rseq is set, AND
- cpu or cid has changed compared to the cached value OR
- rseq_cs user pointer is non-NULL.

Otherwise we should be able to handle the return from fork from the fast
path just with loads from the rseq area, or am I missing something ?

Thanks,

Mathieu

> }
> }
>
> #else /* CONFIG_RSEQ */
> -static inline void rseq_handle_notify_resume(struct ksignal *ksig, 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) { }
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1911,7 +1911,7 @@ config RSEQ_DEBUG_DEFAULT_ENABLE
> config DEBUG_RSEQ
> default n
> bool "Enable debugging of rseq() system call" if EXPERT
> - depends on RSEQ && DEBUG_KERNEL
> + depends on RSEQ && DEBUG_KERNEL && !GENERIC_ENTRY

I'm confused about this hunk. Perhaps this belongs to a different
commit ?

Thanks,

Mathieu

> select RSEQ_DEBUG_DEFAULT_ENABLE
> help
> Enable extra debugging checks for the rseq system call.
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -11,13 +11,8 @@
> /* Workaround to allow gradual conversion of architecture code */
> void __weak arch_do_signal_or_restart(struct pt_regs *regs) { }
>
> -/**
> - * exit_to_user_mode_loop - do any pending work before leaving to user space
> - * @regs: Pointer to pt_regs on entry stack
> - * @ti_work: TIF work flags as read by the caller
> - */
> -__always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> - unsigned long ti_work)
> +static __always_inline unsigned long __exit_to_user_mode_loop(struct pt_regs *regs,
> + unsigned long ti_work)
> {
> /*
> * Before returning to user space ensure that all pending work
> @@ -62,6 +57,23 @@ void __weak arch_do_signal_or_restart(st
> return ti_work;
> }
>
> +/**
> + * exit_to_user_mode_loop - do any pending work before leaving to user space
> + * @regs: Pointer to pt_regs on entry stack
> + * @ti_work: TIF work flags as read by the caller
> + */
> +__always_inline unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
> + unsigned long ti_work)
> +{
> + for (;;) {
> + ti_work = __exit_to_user_mode_loop(regs, ti_work);
> +
> + if (likely(!rseq_exit_to_user_mode_restart(regs)))
> + return ti_work;
> + ti_work = read_thread_flags();
> + }
> +}
> +
> noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
> {
> irqentry_state_t ret = {
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -234,7 +234,11 @@ static bool rseq_handle_cs(struct task_s
>
> static void rseq_slowpath_update_usr(struct pt_regs *regs)
> {
> - /* Preserve rseq state and user_irq state for exit to user */
> + /*
> + * Preserve rseq state and user_irq state. The generic entry code
> + * clears user_irq on the way out, the non-generic entry
> + * architectures are not having user_irq.
> + */
> const struct rseq_event evt_mask = { .has_rseq = true, .user_irq = true, };
> struct task_struct *t = current;
> struct rseq_ids ids;
> @@ -286,7 +290,7 @@ static void rseq_slowpath_update_usr(str
> }
> }
>
> -void __rseq_handle_notify_resume(struct pt_regs *regs)
> +void __rseq_handle_slowpath(struct pt_regs *regs)
> {
> /*
> * If invoked from hypervisors before entering the guest via
>

--
Mathieu Desnoyers
EfficiOS Inc.