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

[Peter Zijlstra]

On Fri, Jan 14, 2022 at 03:09:55PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 21, 2021 at 05:19:00PM +0000, Peter Oskolkov wrote:

> > > +	/*
> > > +	 * Workers will still block in umcg_notify_resume() before they can
> > > +	 * consume their error, servers however need to get the error asap.
> > > +	 *
> > > +	 * Still, things might be unrecoverably screwy after this. Not our
> > > +	 * problem.
> > 
> > I think we should explicitly document the unrecoverable screwiness
> > of errors here, so that the userspace proactively kills itself
> > to avoid badness. The only reason that returning an error here is
> > mildly preferable to just killing the task (we already do that
> > in other places) is to give the userspace an opportunity to
> > log an error, with more state/info than we can do here.
> 
> Bah, I should've written a better comment, because I can't quite
> remember the case I had in mind. Also, again from the LAZY patch, I
> think we can actually do better in some of the cases here.
> 
> Specifically, currently we'll enqueue on ::runnable_workers_ptr and fail
> waking ::next_tid and leave it at that. While I think waking
> ::server_tid in that case makes sense.
> 
> I'll go prod at this.

Is anybody actually planning to use ::next_tid for workers?

My current thinking is that much of the problems here stem from that.

Let me ponder more..