Re: [PATCH 0/2] rcu: Transform kfree_rcu() into kvfree_rcu()

[Steven Rostedt]

On Wed, 7 Feb 2018 10:10:55 -0800
Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> > For the record, I fully agree with Steve here. 

Thanks, but...

> > 
> > And being a performance "fanatic" I don't like to have the extra branch
> > (and compares) in the free code path... but it's a MM-decision (and
> > sometimes you should not listen to "fanatics" ;-))  
> 
> While free_rcu() is not withut its performance requirements, I think it's
> currently dominated by cache misses and not by branches.  By the time RCU
> gets to run callbacks, memory is certainly L1/L2 cache-cold and probably
> L3 cache-cold.  Also calling the callback functions is utterly impossible
> for the branch predictor.

I agree with Matthew.

This is far from any fast path. A few extra branches isn't going to
hurt anything here as it's mostly just garbage collection. With or
without the Spectre fixes.

-- Steve