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

From: Jesper Dangaard Brouer
Date: Wed Feb 11 2015 - 19:17:10 EST

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

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
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