On Mon, Jan 7, 2013 at 8:34 PM, Tejun Heo [off-list ref] wrote:

Hello, Viresh.

On Mon, Jan 07, 2013 at 03:28:33PM +0530, Viresh Kumar wrote:

quoted

Firstly the root cause of this patchset.

Myself and some others in Linaro are working on ARM future cores:
big.LITTLE systems.
Here we have few very powerful, high power consuming cores (big,
currently A15's) and
few very efficient ones (LITTLE, currently A7's).

The ultimate goal is to save as much power as possible without compromising
much with performance. For, that we wanted most of the stuff to run on LITTLE
cores and some performance-demanding stuff on big Cores. There are
multiple things
going around in this direction. Now, we thought A15's or big cores
shouldn't be used
for running small tasks like timers/workqueues and hence this patch is
an attempt
towards reaching that goal.

I see.  My understanding of big.little is very superficial so there
are very good chances that I'm utterly wrong, but to me it seems like
more of a stop-gap solution than something which will have staying
power in the sense that the complexity doesn't seem to have any
inherent reason other than the failure to make the big ones power
efficient enough while idle or under minor load.

The two processors use different manufacturing processes - one
optimised for performance, the other for really low power. So the
reason is physics at this point. Other architectures are known to be
playing with similar schemes. ARM's big.LITTLE is just the first one
to the market.

Maybe this isn't specific to big.little and heterogeneous cores will
be the way of future with big.little being just a forefront of the
trend.  And even if that's not the case, it definitely doesn't mean
that we can't accomodate big.little, but, at this point, it does make
me a bit more reluctant to make wholesale changes specifically for
big.little.

The patches aren't targeted to benefit only b.L systems though it was
definitely the trigger for our investigations. Our hope is that after
presenting more analysis results from our side we'll be able to
convince the community of the general usefulness of this feature.

Here are a few short introductions to big.LITTLE in case you're
interested.[1][2]

[1] http://www.arm.com/files/downloads/big.LITTLE_Final.pdf
[2] http://lwn.net/Articles/481055/