* Mathieu Desnoyers:
> If a thread group has fewer threads than cores, or is limited to run on
> few cores concurrently through sched affinity or cgroup cpusets, the
> virtual cpu ids will be values close to 0, thus allowing efficient use
> of user-space memory for per-cpu data structures.
>From a userspace programmer perspective, what's a good way to obtain a
reasonable upper bound for the possible tg_vcpu_id values?
I believe not all users of cgroup cpusets change the affinity mask.
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 13f6d0419f31..37b43735a400 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -86,10 +86,14 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
> struct rseq __user *rseq = t->rseq;
> u32 cpu_id = raw_smp_processor_id();
> u32 node_id = cpu_to_node(cpu_id);
> + u32 tg_vcpu_id = task_tg_vcpu_id(t);
>
> if (!user_write_access_begin(rseq, t->rseq_len))
> goto efault;
> switch (t->rseq_len) {
> + case offsetofend(struct rseq, tg_vcpu_id):
> + unsafe_put_user(tg_vcpu_id, &rseq->tg_vcpu_id, efault_end);
> + fallthrough;
> case offsetofend(struct rseq, node_id):
> unsafe_put_user(node_id, &rseq->node_id, efault_end);
> fallthrough;
Is the switch really useful? I suspect it's faster to just write as
much as possible all the time. The switch should be well-predictable
if running uniform userspace, but still …
