* Paul E. McKenney <paul...@linux.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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