Re: [patch V6 07/11] rseq: Implement time slice extension enforcement timer

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 15, 2025 at 05:52:22PM +0100, Thomas Gleixner wrote:

> V5: Document the slice extension range - PeterZ

> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -1228,6 +1228,14 @@ reboot-cmd (SPARC only)
>  ROM/Flash boot loader. Maybe to tell it what to do after
>  rebooting. ???
>  
> +rseq_slice_extension_nsec
> +=========================
> +
> +A task can request to delay its scheduling if it is in a critical section
> +via the prctl(PR_RSEQ_SLICE_EXTENSION_SET) mechanism. This sets the maximum
> +allowed extension in nanoseconds before scheduling of the task is enforced.
> +Default value is 30000ns (30us). The possible range is 10000ns (10us) to
> +50000ns (50us).

The important bit: we're not going to increase these numbers. If
anything, I would like the default to be 10us and taint the kernel if
you up it.

I also think we want some tracing/tool to find the actual length of the
extension used (min/avg/max etc.). That is the time between the kernel
finding the extension bit set and arming the timer and the slice_yield()
syscall.