Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off

[Steven Rostedt]

On Thu, 15 Jan 2015 17:16:34 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> > I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free
> > in CONFIG_PREEMPT. (14.821 ns -> 14.049 ns)
> 
> I'm surprised.  preempt_disable/enable are pretty fast.  I wonder why
> this makes a measurable difference.  Perhaps preempt_enable()'s call
> to preempt_schedule() added pain?

profiling function tracing I discovered that accessing preempt_count
was actually quite expensive, even just to read. But it may not be as
bad since Peter Zijlstra converted preempt_count to a per_cpu variable.
Although, IIRC, the perf profiling showed the access to the %gs
register was where the time consuming was happening, which is what
I believe per_cpu variables still use.

-- Steve
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