Re: [PATCH] sched: smpnice work around for active_load_balance()

[Peter Williams <pwil3058@bigpond.net.au>]

Siddha, Suresh B wrote:
> On Wed, Mar 29, 2006 at 02:42:45PM +1100, Peter Williams wrote:
> > I meant that it doesn't explicitly address your problem.  What it does 
> > is ASSUME that failure of load balancing to move tasks is because there 
> > was exactly one task on the source run queue and that this makes it a 
> > suitable candidate to have that single task moved elsewhere in the blind 
> > hope that it may fix an HT/MC imbalance that may or may not exist.  In 
> > my mind this is very close to random.  
>
> That so called assumption happens only when load balancing has
> failed for more than the domain specific cache_nice_tries. Only reason
> why it can fail so many times is because of all pinned tasks or only a single
> task is running on that particular CPU. load balancing code takes care of both
> these scenarios..
>
> sched groups cpu_power controls the mechanism of implementing HT/MC
> optimizations in addition to active balance code... There is no randomness
> in this.

The above explanation just increases my belief in the randomness of this 
solution.  This code is mostly done without locks and is therefore very 
racy and any assumptions made based on the number of times load 
balancing has failed etc. are highly speculative.

And even if there is only one task on the CPU there's no guarantee that
that CPU is in a package that meets the other requirements to make the 
move desirable.  So there's a good probability that you'll be moving 
tasks unnecessarily.

It's a poor solution and it's being inflicted on architectures that 
don't need it.  Even if cache_nice_tries is used to suppress this 
behaviour on architectures that don't need it they have to carry the 
code in their kernel.

> > Also back to front and inefficient.
>
> HT/MC imbalance is detected in a normal way.. A lightly loaded group
> finds an imbalance and tries to pull some load from a busy group (which
> is inline with normal load balance)... pull fails because the only task
> on that cpu is busy running and needs to go off the cpu (which is triggered
> by active load balance)... Scheduler load balance is generally done by a 
> pull mechansim and here (HT/MC) it is still a pull mechanism(triggering a 
> final push only because of the single running task) 
>
> If you have any better generic and simple method, please let us know.

I gave an example in a previous e-mail.  Basically, at the end of 
scheduler_tick() if rebalance_tick() doesn't move any tasks (it would be 
foolish to contemplate moving tasks of the queue just after you've moved 
some there) and the run queue has exactly one running task and it's time 
for a HT/MC rebalance check on the package that this run queue belongs 
to then check that package to to see if it meets the rest of criteria 
for needing to lose some tasks.  If it does look for a package that is a 
suitable recipient for the moved task and if you find one then mark this 
run queue as needing active load balancing and arrange for its migration 
thread to be started.

Simple, direct and amenable to being only built on architectures that 
need the functionality.

Another (more complex) solution that would also allow improvements to 
other HT related code (e.g. the sleeping dependent code) would be to 
modify the load balancing code so that all CPUs in a package share a run 
queue and load balancing is then done between packages.  As long as the 
number of CPUs in a package is small this shouldn't have scalability 
issues.  The big disadvantage of this approach is its complexity which 
is probably too great to contemplate doing it in 2.6.X kernels.


Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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