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

[Mathieu Desnoyers]

On 2026-01-19 06:03, Peter Zijlstra wrote:
> On Mon, Jan 19, 2026 at 11:30:53AM +0100, Mathieu Desnoyers wrote:
> > On 2026-01-19 05:21, Peter Zijlstra wrote:
> > > 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.
> >
> > That's a very good step in the right direction. I just wonder how
> > userspace is expected to learn that it runs on a kernel which
> > accepts the RSEQ_FLAG_SLICE_EXT_DEFAULT_ON flag ?
> >
> > I think it could expect it when getauxval for AT_RSEQ_FEATURE_SIZE
> > includes the slice ext field. This gives us a cheap way to know
> > from userspace whether this new flag is supported or not.
>
> struct rseq vs struct rseq_data. I don't think that slice field is
> exposed on the user side of things.

Yes it is. (unless I'm missing something ?)

See the original patch of this thread at https://lore.kernel.org/lkml/20251215155708.669472597@xxxxxxxxxxxxx/

--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
[...]

@@ -142,6 +174,12 @@ struct rseq {
 	__u32 mm_cid;
 
 	/*
+	 * Time slice extension control structure. CPU local updates from
+	 * kernel and user space.
+	 */
+	struct rseq_slice_ctrl slice_ctrl;
+
+	/*
 	 * Flexible array member at end of structure, after last feature field.
 	 */
 	char end[];

> I was thinking it could just try with the flag the firs time, and then
> record if that worked or not and use the 'correct' value for all future
> rseq calls.

That would work too, but would waste a system call on process startup in case
of failure. Not a big deal, but checking with getauxval would be better because
this information is already exported to userspace at program execution and
available without doing any system call.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com