To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Greg KH <greg@kroah.com>, linux-kernel@vger.kernel.org,
       vserver@list.linux-vserver.org, Herbert Poetzl <herbert@13thfloor.at>,
       "Serge E. Hallyn" <serue@us.ibm.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Dave Hansen <haveblue@us.ibm.com>,
       Arjan van de Ven <arjan@infradead.org>,
       Suleiman Souhlal <ssouhlal@FreeBSD.org>,
       Hubertus Franke <frankeh@watson.ibm.com>,
       Cedric Le Goater <clg@fr.ibm.com>, Kyle Moffett <mrmacman_g4@mac.com>
Subject: Re: [PATCH 1/5] pid: Implement task references.
References: <m1psmba4bn.fsf@ebiederm.dsl.xmission.com>
	<m1lkwza479.fsf@ebiederm.dsl.xmission.com>
	<20060129190539.GA26794@kroah.com>
	<m1mzhe7l2c.fsf@ebiederm.dsl.xmission.com>
	<20060130045153.GC13244@kroah.com> <43DDA1E7.5010109@cosmosbay.com>
	<20060130184302.GA17457@kroah.com> <43DE6FE6.40705@cosmosbay.com>
	<m1bqxt5ts3.fsf@ebiederm.dsl.xmission.com>
	<43DE8604.1060109@cosmosbay.com>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Mon, 30 Jan 2006 14:51:09 -0700
In-Reply-To: <43DE8604.1060109@cosmosbay.com> (Eric Dumazet's message of
 "Mon, 30 Jan 2006 22:32:52 +0100")
Message-ID: <m1zmld4c6q.fsf@ebiederm.dsl.xmission.com>
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Eric Dumazet <dada1@cosmosbay.com> writes:

> This function is not inlined.
>
> Adding a test and a branch is a matter of 7 bytes.
>
>   'Bloating the icache' is a litle bit off :)

The size all depends on your architecture.  But I agree it
is not much of a size increase in general.

> Avoiding an atomic is important. This is already done elsewhere in the kernel,
> in a inlined function with *many* call sites :
>
> (See kfree_skb() in include/linux//skbuff.h )
>
> /*
>   * If users == 1, we are the only owner and are can avoid redundant
>   * atomic change.
>   */
>
> /**
>   *      kfree_skb - free an sk_buff
>   *      @skb: buffer to free
>   *
>   *      Drop a reference to the buffer and free it if the usage count has
>   *      hit zero.
>   */
> static inline void kfree_skb(struct sk_buff *skb)
> {
>          if (likely(atomic_read(&skb->users) == 1))
>                  smp_rmb();
>          else if (likely(!atomic_dec_and_test(&skb->users)))
>                  return;
>          __kfree_skb(skb);
> }
>
>
> This is a valid optimization : an atomic_dec_and_test() is very expensive.

It is a valid optimization if you will normally have only one user
like a skb.  For other structures that frequently have many users I'm
not at all certain it is a useful optimization.

Eric
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