linux-kernel - Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

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Date:	Mon, 16 Sep 2013 08:07:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

On Fri, Sep 13, 2013 at 11:18:21PM -0700, Andi Kleen wrote:
> tip-bot for Peter Zijlstra <tipbot@...or.com> writes:
> > +
> > +	at  = (struct pebs_record_nhm *)(unsigned long)ds->pebs_buffer_base;
> > +	top = (struct pebs_record_nhm *)(unsigned long)ds->pebs_index;
> >  
> >  	ds->pebs_index = ds->pebs_buffer_base;
> >  
> > +	n = (top - at) / x86_pmu.pebs_record_size;
> 
> 
> This adds a full slow division to the PEBS hot path.
> 
> Does not seem like a improvement to me.

There already was an implicit division there, and
sizeof(pebs_record_hsw) = 176, can it really optimize that constant
division?

I suppose we could go and introduce CONFIG_PERF_DEBUG and stuff sanity
checks under that.. :/
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