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Date: Sat, 18 May 2013 08:14:52 +1000 From: Michael Neuling <mikey@...ling.org> To: Stephane Eranian <eranian@...gle.com> cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>, "ak@...ux.intel.com" <ak@...ux.intel.com>, Michael Ellerman <michael@...erman.id.au>, "benh@...nel.crashing.org" <benh@...nel.crashing.org>, Linux PPC dev <linuxppc-dev@...abs.org> Subject: Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL Stephane Eranian <eranian@...gle.com> wrote: > On Fri, May 17, 2013 at 1:39 PM, Peter Zijlstra <peterz@...radead.org> wrote: > > On Fri, May 17, 2013 at 09:32:08PM +1000, Michael Neuling wrote: > >> Peter Zijlstra <peterz@...radead.org> wrote: > > > >> > Wouldn't it be mostly conditional branches that are the primary control flow > >> > and can get predicted wrong? I mean, I'm sure someone will miss-predict an > >> > unconditional branch but its not like we care about people with such > >> > afflictions do we? > >> > >> You could mispredict the target address of a computed goto. You'd know > >> it was taken but not know target address until later in the pipeline. > > > > Oh right, computed targets could indeed be mis predicted. I was more thinking > > about jumps with immediate values. > > > >> On this, the POWER8 branch history buffer tells us two things about the > >> prediction status. > >> 1) if the branch was predicted taken/not taken correctly > >> 2) if the target address was predicted correctly or not (for computed > >> gotos only) > >> So we'd actually like more prediction bits too :-D > > > > So if I understand this right, 1) maps to the predicted flags we have; 2) > > would be new stuff? > > > > We don't really have anything like that on x86, but I suppose if you make the > > thing optional and present a 'useful' use-case implemented in userspace code > > we could take it :-) > > > >> > Anyway, since PPC people thought it worth baking into hardware, > >> > presumably they have a compelling use case. Mikey could you see if you > >> > can retrieve that from someone in the know? It might be interesting. > >> > >> I don't think we can mispredict a non-conditional non-computed but I'll > >> have to check with the HW folks. > > > > I was mostly wondering about the use-case for the conditional filter. Stephane > > didn't think it useful, clearly your hardware guys thought different :-) > > From my experience talking with compiler people, they care about ALL > the branches and not the conditional so much. They use LBR to do basic > block profiling. OK. I don't have a good handle on what's useful for compilers or JITs right now. I'm just plumbing through what's possible. Mikey -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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