Re: [RFC][PATCH 3/3] sched: User Mode Concurency Groups

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 18, 2022 at 10:19:21AM -0800, Peter Oskolkov wrote:
> ============= worker-to-worker context switches
> 
> One example: absl::Mutex (https://abseil.io/about/design/mutex) has
> google-internal extensions that are "fiber aware". More specifically,
> consider this situation:
> 
> - worker W1 acqured the mutex and is doing its work
> - worker W2 calls mutex::lock()
>   mutex::lock(), being aware of workers, understands that W2 is going to sleep;
>   so instead of just doing so, waking the server, and letting
>   the server figure out what to run in place of the sleeping worker,
> mutex::lock()
>   calls into the userspace scheduler in the context of W2 running, and the
>   userspace scheduler then picks W3 to run and does W2->W3 context switch.
> 
> The optimization above replaces W2->Server and Server->W3 context switches
> with a single W2->W3 context switch, which is a material performance gain.

Yes, I've also already reconsidered. Things like pipelines and other
fixed order scheduling policies will greatly benefit from
worker-to-worker switching.

But I think all of them are explicit. That is, we can limit the
::next_tid usage to sys_umcg_wait() and never look at it for implicit
blocks.

> In addition, when W1 calls mutex::unlock(), the scheduling code determines
> that W2 is waiting on the mutex, and thus calls W2::wake() from the context of
> running W1 (you asked earlier why do we need "WAKE_ONLY").

This I'm not at all convinced on. That sounds like it will violate the
1:1 thing.