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

[Mathieu Desnoyers]

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.

One nit below:

[...]
> -	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)

I think you want to surround flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON with
parentheses () to have the expected operator priority.

Thanks!

Mathieu

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