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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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@linutronix.de/

--- 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