linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <alpine.LFD.2.00.0811262156390.3325@localhost.localdomain>
Date:	Wed, 26 Nov 2008 22:18:09 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	eranian@...glemail.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu, x86@...nel.org,
	eranian@...il.com, sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

On Wed, 26 Nov 2008, Andi Kleen wrote:

> On Wed, Nov 26, 2008 at 02:35:18PM +0100, Thomas Gleixner wrote:
> > > +	 * 	  does not work with other types of PMU registers.Thus, no
> > > +	 * 	  address is ever exposed by counters
> > > +	 *
> > > +	 * 	- there is never a dependency between one pmd register and
> > > +	 * 	  another
> > > +	 */
> > > +	for (i = 0; num; i++) {
> > > +		if (likely(pfm_arch_bv_test_bit(i, set->used_pmds))) {
> > > +			pfm_write_pmd(ctx, i, set->pmds[i]);
> > > +			num--;
> > > +		}
> > > +	}
> > 
> >   This loop construct looks scary. It relies on set->nused_pmds >=
> >   bits set in set->used_pmds. I had to look more than once to
> >   understand that. It's used all over the code in variations.
> 
> FWIW this loop style tripped me up during review too.

It's even worse than I thought when looking at it a second time:

All the loops iterate over an array which means in the worst case we
do full array_size iterations. In each iteration we check whether the
corresponding bit in the bitmask is set or not.

What a nonsense. We have a bitmask already. Why not iterate over the
bitmask and be done ?

Thanks,

	tglx
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