linux-kernel - Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator

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Date: Tue, 4 Jun 2024 13:31:58 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Paolo Abeni <pabeni@...hat.com>, Mina Almasry <almasrymina@...gle.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator

On Tue, Jun 04, 2024 at 12:15:51PM -0400, Steven Rostedt wrote:
> On Tue, 04 Jun 2024 12:13:15 +0200
> Paolo Abeni <pabeni@...hat.com> wrote:
> 
> > On Thu, 2024-05-30 at 20:16 +0000, Mina Almasry wrote:
> > > diff --git a/net/core/devmem.c b/net/core/devmem.c
> > > index d82f92d7cf9ce..d5fac8edf621d 100644
> > > --- a/net/core/devmem.c
> > > +++ b/net/core/devmem.c
> > > @@ -32,6 +32,14 @@ static void net_devmem_dmabuf_free_chunk_owner(struct gen_pool *genpool,
> > >  	kfree(owner);
> > >  }
> > >  
> > > +static inline dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov)  
> > 
> > Minor nit: please no 'inline' keyword in c files.
> 
> I'm curious. Is this a networking rule? I use 'inline' in my C code all the
> time.

It mostly comes from Documentation/process/coding-style.rst:

15) The inline disease
----------------------

There appears to be a common misperception that gcc has a magic "make me
faster" speedup option called ``inline``. While the use of inlines can be
appropriate (for example as a means of replacing macros, see Chapter 12), it
very often is not. Abundant use of the inline keyword leads to a much bigger
kernel, which in turn slows the system as a whole down, due to a bigger
icache footprint for the CPU and simply because there is less memory
available for the pagecache. Just think about it; a pagecache miss causes a
disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
that can go into these 5 milliseconds.

A reasonable rule of thumb is to not put inline at functions that have more
than 3 lines of code in them. An exception to this rule are the cases where
a parameter is known to be a compiletime constant, and as a result of this
constantness you *know* the compiler will be able to optimize most of your
function away at compile time. For a good example of this later case, see
the kmalloc() inline function.

Often people argue that adding inline to functions that are static and used
only once is always a win since there is no space tradeoff. While this is
technically correct, gcc is capable of inlining these automatically without
help, and the maintenance issue of removing the inline when a second user
appears outweighs the potential value of the hint that tells gcc to do
something it would have done anyway.

Jason