Re: [RFC PATCH v4 01/13] ktask: add documentation

[Peter Zijlstra]

On Tue, Nov 06, 2018 at 12:34:11PM -0800, Daniel Jordan wrote:
> On Tue, Nov 06, 2018 at 09:49:11AM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 05, 2018 at 11:55:46AM -0500, Daniel Jordan wrote:
> > > +Concept
> > > +=======
> > > +
> > > +ktask is built on unbound workqueues to take advantage of the thread management
> > > +facilities it provides: creation, destruction, flushing, priority setting, and
> > > +NUMA affinity.
> > > +
> > > +A little terminology up front:  A 'task' is the total work there is to do and a
> > > +'chunk' is a unit of work given to a thread.
> > 
> > So I hate on the task naming. We already have a task, lets not overload
> > that name.
> 
> Ok, agreed, it's a crowded field with 'task', 'work', 'thread'...
> 
> Maybe 'job', since nothing seems to have taken that in kernel/.

Do we want to somehow convey the fundamentally parallel nature of the
thing?

> > I see no mention of padata anywhere; I also don't see mention of the
> > async init stuff. Both appear to me to share, at least in part, the same
> > reason for existence.
> 
> padata is news to me.  From reading its doc, it comes with some special
> requirements of its own, like softirqs disabled during the parallel callback,
> and some ktask users need to sleep.  I'll check whether it could be reworked to
> handle this.

Right, padata is something that came from the network stack I think.
It's a bit of an odd thing, but it would be nice if we can fold it into
something larger.

> And yes, async shares the same basic infrastructure, but ktask callers need to
> wait, so the two seem fundamentally at odds.  I'll add this explanation in.

Why does ktask have to be fundamentally async?

> > > +Scheduler Interaction
> > > +=====================
> ...
> > > +It is possible for a helper thread to start running and then be forced off-CPU
> > > +by a higher priority thread.  With the helper's CPU time curtailed by MAX_NICE,
> > > +the main thread may wait longer for the task to finish than it would have had
> > > +it not started any helpers, so to ensure forward progress at a single-threaded
> > > +pace, once the main thread is finished with all outstanding work in the task,
> > > +the main thread wills its priority to one helper thread at a time.  At least
> > > +one thread will then always be running at the priority of the calling thread.
> > 
> > What isn't clear is if this calling thread is waiting or not. Only do
> > this inheritance trick if it is actually waiting on the work. If it is
> > not, nobody cares.
> 
> The calling thread waits.  Even if it didn't though, the inheritance trick
> would still be desirable for timely completion of the job.

No, if nobody is waiting on it, it really doesn't matter.

> > > +Power Management
> > > +================
> > > +
> > > +Starting additional helper threads may cause the system to consume more energy,
> > > +which is undesirable on energy-conscious devices.  Therefore ktask needs to be
> > > +aware of cpufreq policies and scaling governors.
> > > +
> > > +If an energy-conscious policy is in use (e.g. powersave, conservative) on any
> > > +part of the system, that is a signal that the user has strong power management
> > > +preferences, in which case ktask is disabled.
> > > +
> > > +TODO: Implement this.
> > 
> > No, don't do that, its broken. Also, we're trying to move to a single
> > cpufreq governor for all.
> >
> > Sure we'll retain 'performance', but powersave and conservative and all
> > that nonsense should go away eventually.
> 
> Ok, good to know.
> 
> > That's not saying you don't need a knob for this; but don't look at
> > cpufreq for this.
> 
> Ok, I'll dig through power management to see what else is there.  Maybe there's
> some way to ask "is this machine energy conscious?"

IIRC you're presenting at LPC, drop by the Power Management and
Energy-awareness MC.