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

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Date:	Thu, 2 Jul 2015 09:47:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, josh@...htriplett.org,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging
 normal ones


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> 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?

Thanks,

	Ingo
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