Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set

[Frederic Weisbecker]

On Thu, May 21, 2015 at 05:57:59AM -0700, Paul E. McKenney wrote:
> On Thu, May 21, 2015 at 05:42:46PM +0530, Afzal Mohammed wrote:
> > Hi,
> > 
> > On Wed, May 20, 2015 at 02:00:26PM -0700, Paul E. McKenney wrote:
> > 
> > > > > Given that kernel initiated association to isolcpus, a user turning
> > > > > NO_HZ_FULL_ALL on had better not have much generic load to manage.  If
> > > > 
> > > > On a quad-core desktop system with NO_HZ_FULL_ALL, hackbench took 3x
> > > > time as compared to w/o this patch, except boot cpu every one else
> > > > jobless. Though NO_HZ_FULL_ALL (afaik) is not meant for generic load,
> > > > it was working fine, but not after this - it is now like a single core
> > > > system.
> > > 
> > > I have to ask...  What is your use case?  What are you wanting NO_HZ_FULL
> > > to do for you?
> > 
> > I was just playing NO_HZ_FULL with tip-[sched,timers]-* changes.
> > 
> > Thought that shutting down ticks as much as possible would be
> > beneficial to normal loads too, though it has been mentioned to be used
> > for specialized loads. Seems like drawbacks due to it weigh against
> > normal loads, but haven't so far observed any (on a laptop with normal
> > activities) before this change.
> 
> Indeed, NO_HZ_FULL is special purpose.  You normally would select
> NO_HZ_FULL_ALL only on a system intended for heavy compute without
> normal-workload distractions or for some real-time systems.  For mixed
> workloads, you would build with NO_HZ_FULL (but not NO_HZ_FULL_ALL) and
> use the boot parameters to select which CPUs are to be running the
> specialized portion of the workload.
> 
> And you would of course need to lead enough CPUs running normally to
> handle the non-specialized portion of the workload.
> 
> This sort of thing has traditionally required specialized kernels,
> so the cool thing here is that we can make Linux do it.  Though, as
> you noticed, careful configuration is still required.
> 
> Seem reasonable?

That said if he saw a big performance regression after applying these patches,
then there is likely a problem in the patchset. Well it could be due to that mode
which loops on full dynticks before resuming to userspace. Indeed when that is
enabled, I expect real throughput issues on workloads doing lots of kernel <->
userspace roundtrips. We just need to make sure this thing only works when requested.

Anyway, I need to look at the patchset.
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