Re: NO_HZ migration of TCP ack timers

[Arun R Bharadwaj <arun@linux.vnet.ibm.com>]

* Andi Kleen <andi@firstfloor.org> [2010-02-18 09:08:35]:

> Anton Blanchard <anton@samba.org> writes:
> 
> > echo 0 > /proc/sys/kernel/timer_migration
> >
> > makes the problem go away.
> >
> > I think the problem is the CPU is most likely to be idle when an rx networking
> > interrupt comes in. It seems the wrong thing to do to migrate any ack timers
> > off the current cpu taking the interrupt, and with enough networks we train
> > wreck transferring everyones ack timers to the nohz load balancer cpu. 
> 
> If the nohz balancer CPU is otherwise idle, shouldn't it have enough
> cycles to handle acks for everyone? Is the problem the cache line
> transfer time?
> 
> But yes if it's non idle the migration might need to spread out 
> to more CPUs.
> 
> >
> > What should we do? Should we use mod_timer_pinned here? Or is this an issue
> 
> Sounds like something that should be controlled by the cpufreq governour's
> idle predictor? Only migrate if predicted idle time is long enough.
> It's essentially the same problem as deciding how deeply idle to put
> a CPU. Heavy measures only pay off if the expected time is long enough.
> 

cpuidle infrastructure hs statistics about the idle times for
all the cpus. Maybe we can look to use this infrastructure to decide
whether to migrate timers or not?

arun

> > other areas might see (eg the block layer) and we should instead avoid
> > migrating timers created out of interrupts.
> 
> -Andi
> 
> -- 
> ak@linux.intel.com -- Speaking for myself only.