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Subject: Re: [PATCH 0/3] perf_events: update extra shared registers
 management (v2)
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, "mingo@elte.hu" <mingo@elte.hu>,
        Andi Kleen <andi@firstfloor.org>, Lin Ming <ming.m.lin@intel.com>
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Date: Mon, 23 May 2011 17:15:00 +0200
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On Mon, 2011-05-23 at 14:00 +0200, Stephane Eranian wrote:
> On Mon, May 23, 2011 at 1:10 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, 2011-05-23 at 12:58 +0200, Stephane Eranian wrote:
> >> There is a major issue as it stands, though. You can
> >> get into an infinite loop bouncing between RSP_0 and RSP_1
> >> in case there is no solution in the group, i.e., you have 3 values
> >> for the extra MSR. I think you need to count the number of times
> >> you've called intel_try_alt_er() with success or maintain some sort
> >> of bitmask of possible alternate choices and when you exhaust that,
> >> you simply fail.
> >
> > That should be sorted by the compare with the initial idx value, no?
> > Once its back where it started out it'll bail.
> >
> Nope.
> 
> Take:
>   - ev1=rsp_0:0x1001
>   - ev2=rsp_0:0x1002
>   - ev3=rsp_1:0x1008
> 
> ev1-> rsp_0
> ev2-> rsp_0, conflict, then try yields rsp_1 -> ok
> ev3 -> rsp_1, conflict, then rsp_0, but fails, try again -> rsp_1,
> fails, and so on
> 
> The issue is that the intel_try() function does not know the
> history of the swaps between rsp_0, rsp1.

But it does, we pass the initial reg->idx in, and return false when that
matches the new idx, so in your example, ev3 will do:

     rsp_1 -> conflict, 
 try rsp_0 -> conflict, 
 try rsp_1 -> bail, return emptyconstraint