Re: [PATCH 2/3] slub: Support for array operations

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Wed, 11 Feb 2015 16:06:50 -0600 (CST)
Christoph Lameter <cl@linux.com> wrote:

> On Thu, 12 Feb 2015, Jesper Dangaard Brouer wrote:
> 
> > > > This is quite an expensive lock with irqsave.
[...]
> > > We can require that interrupt are off when the functions are called. Then
> > > we can avoid the "save" part?
> >
> > Yes, we could also do so with an "_irqoff" variant of the func call,
> > but given we are defining the API we can just require this from the
> > start.
> 
> Allright. Lets do that then.

Okay. Some measurements to guide this choice.

Measured on my laptop CPU i7-2620M CPU @ 2.70GHz:

 * 12.775 ns - "clean" spin_lock_unlock
 * 21.099 ns - irqsave variant spinlock
 * 22.808 ns - "manual" irqsave before spin_lock
 * 14.618 ns - "manual" local_irq_disable + spin_lock

Reproducible via my github repo:
 https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c

The clean spin_lock_unlock is 8.324 ns faster than irqsave variant.
The irqsave variant is actually faster than expected, as the measurement
of an isolated local_irq_save_restore were 13.256 ns. 

The difference to the "manual" irqsave is only 1.709 ns, which is approx
the cost of an extra function call.

If one can use the non-flags-save version of local_irq_disable, then one
can save 6.481 ns (on this specific CPU and kernel config 3.17.8-200.fc20.x86_64).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

https://github.com/netoptimizer/prototype-kernel/commit/1471ac60