linux-kernel - Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

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Date:	Wed, 29 Apr 2009 11:48:29 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"mpm@...enic.com" <mpm@...enic.com>,
	"adobriyan@...il.com" <adobriyan@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Chandra Seetharaman <sekharan@...ibm.com>,
	Nathan Lynch <ntl@...ox.com>, Olof Johansson <olof@...om.net>,
	Helge Deller <deller@...isc-linux.org>
Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

On Wed, Apr 29, 2009 at 10:55:27AM +0800, Andrew Morton wrote:
> On Wed, 29 Apr 2009 10:38:42 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > > > +#define kpf_copy_bit(uflags, kflags, visible, ubit, kbit)		\
> > > > +	do {								\
> > > > +		if (visible || genuine_linus())				\
> > > > +			uflags |= ((kflags >> kbit) & 1) << ubit;	\
> > > > +	} while (0);
> > > 
> > > Did this have to be implemented as a macro?
> > > 
> > > It's bad, because it might or might not reference its argument, so if
> > > someone passes it an expression-with-side-effects, the end result is
> > > unpredictable.  A C function is almost always preferable if possible.
> > 
> > Just tried inline function, the code size is increased slightly:
> > 
> >           text   data    bss     dec    hex   filename
> > macro     1804    128      0    1932    78c   fs/proc/page.o
> > inline    1828    128      0    1956    7a4   fs/proc/page.o
> > 
> 
> hm, I wonder why.  Maybe it fixed a bug ;)
> 
> The code is effectively doing
> 
> 	if (expr1)
> 		something();
> 	if (expr1)
> 		something_else();
> 	if (expr1)
> 		something_else2();
> 
> etc.  Obviously we _hope_ that the compiler turns that into
> 
> 	if (expr1) {
> 		something();
> 		something_else();
> 		something_else2();
> 	}
> 
> for us, but it would be good to check...

By 'expr1', you mean (visible || genuine_linus())?

No, I can confirm the inefficiency does not lie here.

I simplified the kpf_copy_bit() to

        #define kpf_copy_bit(uflags, kflags, ubit, kbit)                     \
                        uflags |= (((kflags) >> (kbit)) & 1) << (ubit);

or

        static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
        {       
                return (((kflags) >> (kbit)) & 1) << (ubit);
        }

and double checked the differences: the gap grows unexpectedly!

              text               data                bss                dec            hex filename
macro         1829                168                  0               1997            7cd fs/proc/page.o
inline        1893                168                  0               2061            80d fs/proc/page.o
              +3.5%

(note: the larger absolute text size is due to some experimental code elsewhere.)

Thanks,
Fengguang
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