RE: [RFC] change non-atomic bitops method

["Wang, Yalin" <Yalin.Wang@sonymobile.com>]

> -----Original Message-----
> From: Rasmus Villemoes [mailto:linux@rasmusvillemoes.dk]
> Sent: Tuesday, February 03, 2015 5:34 PM
> To: Andrew Morton
> Cc: Wang, Yalin; 'Kirill A. Shutemov'; 'arnd@arndb.de'; 'linux-
> arch@vger.kernel.org'; 'linux-kernel@vger.kernel.org';
> 'linux@arm.linux.org.uk'; 'linux-arm-kernel@lists.infradead.org'
> Subject: Re: [RFC] change non-atomic bitops method
> 
> On Tue, Feb 03 2015, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> >
> > You aren't measuring the right thing.  You should compare
> >
> > 	if (p[i] != x)
> > 		p[i] = x;
> >
> > versus
> >
> > 	p[i] = x;
> >
> > and you should do this for two cases:
> >
> > a) p[i] == x
> >
> > b) p[i] != x
> >
> >
> > The first code sequence will be slower when (p[i] != x) and faster when
> > (p[i] == x).
> >
> >
> > Next, we should instrument the kernel to work out the frequency of
> > set_bit on an already-set bit.
> >
> > It is only with both these ratios that we can work out whether the
> > patch is a net gain.  My suspicion is that set_bit on an already-set
> > bit is so rare that the patch will be a loss.
> 
> There's also the code-bloat issue to consider (instruction cache and all
> that); the conditional versions will usually require three extra
> instructions and an extra register. Also, the cache line might already
> be dirty because of something in the surrounding code. Instruction cache
> misses and larger stack footprint (from larger register pressure) won't
> show up in a microbenchmark, so I think this needs a real-world example
> to justify.
> 
> But even if one finds some hot spot that would benefit from the
> conditional, that should simply be added explicitly there, instead of
> pessimizing every other user. (A good example of that is 358eec18243a
> ("vfs: decrapify dput(), fix cache behavior under normal load")).

Oh, thank you, it is really a very nice example.