On Thu, Jul 02, 2015 at 09:47:19AM +0200, Ingo Molnar wrote:
>
> * 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?
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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