linux-kernel - Re: [PATCH v2] perf, x86: Fix event scheduler for constraints with overlapping counters

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Date:	Thu, 01 Sep 2011 14:56:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robert Richter <robert.richter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] perf, x86: Fix event scheduler for constraints with
 overlapping counters

On Fri, 2011-05-20 at 05:18 +0200, Robert Richter wrote:
> On 19.05.11 14:06:50, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Wed, 2011-05-18 at 23:20 +0200, Ingo Molnar wrote:
> > > > * Peter Zijlstra <peterz@...radead.org> wrote:
> > > > 
> > > > > >                         if (c->weight != w)
> > > > > >                                 continue;
> > > > > >  
> > > > > > -                       for_each_set_bit(j, c->idxmsk, X86_PMC_IDX_MAX) {
> > > > > > -                               if (!test_bit(j, used_mask))
> > > > > > +                       /* for each bit in idxmsk starting from idx */
> > > > > > +                       while (idx < X86_PMC_IDX_MAX) {
> > > > > > +                               idx = find_next_bit(c->idxmsk, X86_PMC_IDX_MAX,
> > > > > > +                                                   idx);
> > > > > 
> > > > > I'd be mighty tempted to ignore that 80 column rule here ;-)
> > > > 
> > > > Please put the body of the loop into a helper function, the function is large 
> > > > and there are countless col80 uglinesses in it!
> > > 
> > > I just tried that, its real ugly due to the amount of state you need to
> > > pass around.
> > 
> > Does it help if you put that state into a helper structure?
> 
> Yes, this is what I have in mind too. We could iterate on such a state
> stucture instead of a couple of single variables. Storing and
> restoring the state will then just copying the structure.

Any word on this work? I just noticed we actually need this for Intel
too, the fixed purpose events have overlapping but non-identical
constraint masks.

Now we could optimize the Intel case by always iterating from the top
down, but it won't cure all cases. For example, suppose one counter
(that could be on a FP reg) previously got scheduled on a GP register
and we take the fast-path, in that case we would still end up
under-utilized.


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