Re: [PATCH] driver core: Use unbound workqueue for deferred probes

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Mar 15, 2021 at 04:15:12PM +0530, Yogesh Lal wrote:
> 
> 
> On 2/25/2021 5:14 PM, Greg KH wrote:
> > On Thu, Feb 25, 2021 at 04:03:50PM +0530, Yogesh Lal wrote:
> > > Hi Greg,
> > > 
> > > 
> > > On 2/24/2021 6:13 PM, Greg KH wrote:
> > > > On Wed, Feb 24, 2021 at 05:25:49PM +0530, Yogesh Lal wrote:
> > > > > Queue deferred driver probes on unbounded workqueue, to allow
> > > > > scheduler better manage scheduling of long running probes.
> > > > 
> > > > Really?  What does this change and help?  What is the visable affect of
> > > > this patch?  What problem does it solve?
> > > > 
> > > 
> > > We observed boot up improvement (~400 msec) when the deferred probe work is
> > > made unbound. This is due to scheduler moving the worker running deferred
> > > probe work to big CPUs. without this change, we see the worker is running on
> > > LITTLE CPU due to affinity.
> > 
> > Why is none of this information in the changelog text?  How are we
> > supposed to know this?  And is this 400msec out of 10 seconds or
> 
> We wanted to  first understand the requirement of bounded deferred probe why
> it was really required.
> 
> > something else?  Also, this sounds like your "little" cpus are really
> > bad, you might want to look into fixing them first :)
> > 
> 
> ~600ms (deferred probe bound to little core) and ~200ms (deferred probe
> queued on unbound wq).
> 
> > But if you really want to make this go faster, do not deferr your probe!
> > Why not fix that problem in your drivers instead?
> > 
> 
> Yes, we are exploring in that direction as well but want to get upstream
> opinion and understand the usability of unbounded wq.
> 
> > > Please let us now if there are any concerns/restrictions that deferred probe
> > > work should run only on pinned kworkers. Since this work runs deferred probe
> > > of several devices , the locality may not be that important
> > 
> > Can you prove that it is not important?  I know lots of gyrations are
> > done in some busses to keep probe happening on the same CPU for very
> > good reasons.  Changing that should not be done lightly as you will
> > break this.
> 
> While debugging further and checking if probe are migrating found that init
> thread can potentially migrate, as it has cpu affinity set to all cpus,
> during driver probe (or there is something which prevents it, which I am
> missing?) . Also, async probes use unbounded workqueue.
> So, using unbounded wq for deferred probes looks to be similar to these,
> w.r.t. scheduling behavior.

I do not understand anymore, is this patch still needed or not?

And if so, please resubmit with a lot more description in the changelog
text describing all of this...

thanks,

greg k-h