Re: [RFC PATCH 1/4] rseq: Add sched_state field to struct rseq

[Davidlohr Bueso]

On Wed, 17 May 2023, Mathieu Desnoyers wrote:

> Expose the "on-cpu" state for each thread through struct rseq to allow
> adaptative mutexes to decide more accurately between busy-waiting and
> calling sys_futex() to release the CPU, based on the on-cpu state of the
> mutex owner.

Oh yeah moving the spin stuff out of the kernel is much nicer.

> It is only provided as an optimization hint, because there is no
> guarantee that the page containing this field is in the page cache, and
> therefore the scheduler may very well fail to clear the on-cpu state on
> preemption. This is expected to be rare though, and is resolved as soon
> as the task returns to user-space.
>
> The goal is to improve use-cases where the duration of the critical
> sections for a given lock follows a multi-modal distribution, preventing
> statistical guesses from doing a good job at choosing between busy-wait
> and futex wait behavior.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Cc: Carlos O'Donell <carlos@xxxxxxxxxx>
> Cc: Florian Weimer <fweimer@xxxxxxxxxx>
> Cc: libc-alpha@xxxxxxxxxxxxxx
> ---
> include/linux/sched.h     | 12 ++++++++++++
> include/uapi/linux/rseq.h | 17 +++++++++++++++++
> kernel/rseq.c             | 14 ++++++++++++++
> 3 files changed, 43 insertions(+)

Ie: previous efforts

 kernel/futex.c             |  675 ++++++++++++++++++++++++++++++++++++++------
 kernel/futex.c             |  572 ++++++++++++++++++++++++++++++++++++-------------

Thanks,
Davidlohr