On 2025-08-23 12:39, Thomas Gleixner wrote:
> In preparation for rewriting RSEQ exit to user space handling provide
> storage to cache the CPU ID and MM CID values which were written to user
> space. That prepares for a quick check, which avoids the update when
> nothing changed.
What should we do about the numa node_id field ?
On pretty much all arch except powerpc (AFAIK) it's invariant for
the topology, so derived from cpu_id.
On powerpc, we could perhaps reset the cached cpu_id to ~0U for
each thread to trigger an update ? Or just don't care about this ?
Thanks,
Mathieu
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> include/linux/rseq.h | 3 +++
> include/linux/rseq_types.h | 19 +++++++++++++++++++
> include/linux/sched.h | 1 +
> include/trace/events/rseq.h | 4 ++--
> kernel/rseq.c | 4 ++++
> 5 files changed, 29 insertions(+), 2 deletions(-)
>
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -64,11 +64,13 @@ static inline void rseq_fork(struct task
> t->rseq = NULL;
> t->rseq_len = 0;
> t->rseq_sig = 0;
> + t->rseq_ids.cpu_cid = ~0ULL;
> t->rseq_event.all = 0;
> } else {
> t->rseq = current->rseq;
> t->rseq_len = current->rseq_len;
> t->rseq_sig = current->rseq_sig;
> + t->rseq_ids.cpu_cid = ~0ULL;
> t->rseq_event = current->rseq_event;
> }
> }
> @@ -78,6 +80,7 @@ static inline void rseq_execve(struct ta
> t->rseq = NULL;
> t->rseq_len = 0;
> t->rseq_sig = 0;
> + t->rseq_ids.cpu_cid = ~0ULL;
> t->rseq_event.all = 0;
> }
>
> --- a/include/linux/rseq_types.h
> +++ b/include/linux/rseq_types.h
> @@ -27,4 +27,23 @@ struct rseq_event {
> };
> };
>
> +/*
> + * struct rseq_ids - Cache for ids, which need to be updated
> + * @cpu_cid: Compound of @cpu_id and @mm_cid to make the
> + * compiler emit a single compare on 64-bit
> + * @cpu_id: The CPU ID which was written last to user space
> + * @mm_cid: The MM CID which was written last to user space
> + *
> + * @cpu_id and @mm_cid are updated when the data is written to user space.
> + */
> +struct rseq_ids {
> + union {
> + u64 cpu_cid;
> + struct {
> + u32 cpu_id;
> + u32 mm_cid;
> + };
> + };
> +};
> +
> #endif
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1406,6 +1406,7 @@ struct task_struct {
> u32 rseq_len;
> u32 rseq_sig;
> struct rseq_event rseq_event;
> + struct rseq_ids rseq_ids;
> # ifdef CONFIG_DEBUG_RSEQ
> /*
> * This is a place holder to save a copy of the rseq fields for
> --- a/include/trace/events/rseq.h
> +++ b/include/trace/events/rseq.h
> @@ -21,9 +21,9 @@ TRACE_EVENT(rseq_update,
> ),
>
> TP_fast_assign(
> - __entry->cpu_id = raw_smp_processor_id();
> + __entry->cpu_id = t->rseq_ids.cpu_id;
> __entry->node_id = cpu_to_node(__entry->cpu_id);
> - __entry->mm_cid = task_mm_cid(t);
> + __entry->mm_cid = t->rseq_ids.mm_cid;
> ),
>
> TP_printk("cpu_id=%d node_id=%d mm_cid=%d", __entry->cpu_id,
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -184,6 +184,10 @@ static int rseq_update_cpu_node_id(struc
> rseq_unsafe_put_user(t, node_id, node_id, efault_end);
> rseq_unsafe_put_user(t, mm_cid, mm_cid, efault_end);
>
> + /* Cache the user space values */
> + t->rseq_ids.cpu_id = cpu_id;
> + t->rseq_ids.mm_cid = mm_cid;
> +
> /*
> * Additional feature fields added after ORIG_RSEQ_SIZE
> * need to be conditionally updated only if
>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com