Date: Fri, 24 Apr 2015 13:13:40 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Hagen Paul Pfeifer <hagen@jauu.net>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>, x86@kernel.org
Subject: Re: [PATCH] enforce function inlining for hot functions
Message-ID: <20150424201340.GD5561@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <1429825201-2825-1-git-send-email-hagen@jauu.net>
 <20150424124905.07e29a3b1392513144cd1568@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150424124905.07e29a3b1392513144cd1568@linux-foundation.org>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1968
Lines: 47

On Fri, Apr 24, 2015 at 12:49:05PM -0700, Andrew Morton wrote:
> On Thu, 23 Apr 2015 23:40:01 +0200 Hagen Paul Pfeifer <hagen@jauu.net> wrote:
> 
> > GCC inlining heuristics are sometimes quizzical. Especially with inline
> > assembler constructs GCC seems to have issues. A allyesconfig show a rather
> > long list of functions where GCC inlining decisions are questionable (not
> > inlined).
> 
> I can't reproduce this with either gcc-4.8.2 or gcc-4.4.4.  The patch
> makes zero difference to `size vmlinux' and a bit of poking around with
> nm doesn't show any out-of-lined versions of the functions you
> identify.
> 
> So.  More details, please.  How to demonstrate this, gcc versions, etc.
> 
> > Furthermore, because the functions are declared with static
> > linkage each function is copied n times - and n can be rather high:
> > 
> >               atomic_inc: 544 duplicates
> >          rcu_read_unlock: 453 duplicates
> >            rcu_read_lock: 383 duplicates

Hmmm...  allyesconfig would have PROVE_RCU=y, which would mean that the
above two would contain lockdep calls that might in some cases defeat
inlining.  With the more typical production choice of PROVE_RCU=n, I would
expect these to just be a call instruction, which should get inlined.

							Thanx, Paul

> >              get_dma_ops: 271 duplicates
> >   arch_local_irq_restore: 258 duplicates
> >               atomic_dec: 215 duplicates
> >                  kzalloc: 185 duplicates
> >            cpumask_check: 157 duplicates
> >         test_and_set_bit: 156 duplicates
> >             cpumask_next: 146 duplicates
> >                 list_del: 131 duplicates
> >                 kref_get: 126 duplicates
> 
> That's pretty pathetic.
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/