linux-kernel - Re: [PATCH 10/17] netvm: Allow skb allocation to use PFMEMALLOC reserves

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Message-ID: <1336747350.1017.22.camel@twins>
Date:	Fri, 11 May 2012 16:42:30 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mel Gorman <mgorman@...e.de>
Cc:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, neilb@...e.de, michaelc@...wisc.edu,
	emunson@...bm.net
Subject: Re: [PATCH 10/17] netvm: Allow skb allocation to use PFMEMALLOC
 reserves

On Fri, 2012-05-11 at 15:32 +0100, Mel Gorman wrote:
> > > +extern atomic_t memalloc_socks;
> > > +static inline int sk_memalloc_socks(void)
> > > +{
> > > +   return atomic_read(&memalloc_socks);
> > > +}
> > 
> > Please change this to be a static branch.
> > 
> 
> Will do. I renamed memalloc_socks to sk_memalloc_socks, made it a int as
> atomics are unnecessary and I check it directly in a branch instead of a
> static inline. It should be relatively easy for the branch predictor. 

David means you to use include/linux/jump_label.h.

static struct static_key sk_memalloc_socks = STATIC_KEY_INIT_FALSE;

and have your function read:

static inline bool sk_memalloc_socks(void)
{
	return static_key_false(&sk_memalloc_socks);
}

which can be modified using:

  static_key_slow_inc(&sk_memalloc_socks);

or

  static_key_slow_dec(&sk_memalloc_socks);

This magic goo turns the branch into self-modifying code such that the
branch is an unconditional jump at runtime.

  
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