Re: [patch V6 01/11] rseq: Add fields and constants for time slice extension

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Jan 17, 2026 at 05:16:16PM +0100, Mathieu Desnoyers wrote:

> My main concern is about the overhead of added system calls at thread
> creation. I recall that doing an additional rseq system call at thread
> creation was analyzed thoroughly for performance regressions at the
> libc level. I would not want to start requiring libc to issue a
> handful of additional prctl system calls per thread creation for no good
> reason.

A wee something like so?

That would allow registering rseq with RSEQ_FLAG_SLICE_EXT_DEFAULT_ON
set and if all the stars align, it will then have it on at the end.

---

--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -424,7 +424,7 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 		return 0;
 	}
 
-	if (unlikely(flags))
+	if (unlikely(flags & ~(RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)))
 		return -EINVAL;
 
 	if (current->rseq.usrptr) {
@@ -459,8 +459,12 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 	if (!access_ok(rseq, rseq_len))
 		return -EFAULT;
 
-	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
+	if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
 		rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
+		if (rseq_slice_extension_enabled() &&
+		    flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)
+			rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
+	}
 
 	scoped_user_write_access(rseq, efault) {
 		/*
@@ -488,6 +492,10 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 	current->rseq.len = rseq_len;
 	current->rseq.sig = sig;
 
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+	current->rseq.slice.state.enabled = !!(rseqfl & RSEQ_CS_FLAG_SLICE_EXT_ENABLED);
+#endif
+
 	/*
 	 * If rseq was previously inactive, and has just been
 	 * registered, ensure the cpu_id_start and cpu_id fields
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -19,7 +19,8 @@ enum rseq_cpu_id_state {
 };
 
 enum rseq_flags {
-	RSEQ_FLAG_UNREGISTER = (1 << 0),
+	RSEQ_FLAG_UNREGISTER			= (1 << 0),
+	RSEQ_FLAG_SLICE_EXT_DEFAULT_ON		= (1 << 1),
 };
 
 enum rseq_cs_flags_bit {