Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

[Paul E. McKenney]

On Thu, Jul 02, 2015 at 09:47:19AM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, Jul 01, 2015 at 07:02:42PM +0200, Peter Zijlstra wrote:
> > > On Wed, Jul 01, 2015 at 09:17:05AM -0700, Paul E. McKenney wrote:
> > > > On Wed, Jul 01, 2015 at 04:17:10PM +0200, Peter Zijlstra wrote:
> > > 
> > > > > 74b51ee152b6 ("ACPI / osl: speedup grace period in acpi_os_map_cleanup")
> > > > 
> > > > Really???
> > > > 
> > > > I am not concerned about this one.  After all, one of the first things that 
> > > > people do for OS-jitter-sensitive workloads is to get rid of binary blobs.  
> > > > And any runtime use of ACPI as well.  And let's face it, if your 
> > > > latency-sensitive workload is using either binary blobs or ACPI, you have 
> > > > already completely lost.  Therefore, an additional expedited grace period 
> > > > cannot possibly cause you to lose any more.
> > > 
> > > This isn't solely about rt etc.. this call is a generic facility used by 
> > > however many consumers. A normal workstation/server could run into it at 
> > > relatively high frequency depending on its workload.
> > > 
> > > Even on not latency sensitive workloads I think hammering all active CPUs is 
> > > bad behaviour. Remember that a typical server class machine easily has more 
> > > than 32 CPUs these days.
> > 
> > Well, that certainly is one reason for the funnel locking, sequence counters, 
> > etc., keeping the overhead bounded despite large numbers of CPUs.  So I don't 
> > believe that a non-RT/non-HPC workload is going to notice.
> 
> So I think Peter's concern is that we should not be offering/promoting APIs that 
> are easy to add, hard to remove/convert - especially if we _know_ they eventually 
> have to be converted. That model does not scale, it piles up increasing amounts of 
> crud.
> 
> Also, there will be a threshold over which it will be increasingly harder to make 
> hard-rt promises, because so much seemingly mundane functionality will be using 
> these APIs. The big plus of -rt is that it's out of the box hard RT - if people 
> are able to control their environment carefully they can use RTAI or so. I.e. it 
> directly cuts into the usability of Linux in certain segments.
> 
> Death by a thousand cuts and such.
> 
> And it's not like it's that hard to stem the flow of algorithmic sloppiness at the 
> source, right?

OK, first let me make sure that I understand what you are asking for:

1.	Completely eliminate synchronize_rcu_expedited() and
	synchronize_sched_expedited(), replacing all uses with their
	unexpedited counterparts.  (Note that synchronize_srcu_expedited()
	does not wake up CPUs, courtesy of its read-side memory barriers.)
	The fast-boot guys are probably going to complain, along with
	the networking guys.

2.	Keep synchronize_rcu_expedited() and synchronize_sched_expedited(),
	but push back hard on any new uses and question any existing uses.

3.	Revert 74b51ee152b6 ("ACPI / osl: speedup grace period in
	acpi_os_map_cleanup").

4.	Something else?

							Thanx, Paul

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