linux-kernel - Re: [PATCH V3 13/23] perf/x86/intel/lbr: Factor out intel_pmu_store

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Message-ID: <20200706102557.GA597537@hirez.programming.kicks-ass.net>
Date:   Mon, 6 Jul 2020 12:25:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     mingo@...hat.com, acme@...nel.org, tglx@...utronix.de,
        bp@...en8.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, dave.hansen@...el.com,
        yu-cheng.yu@...el.com, bigeasy@...utronix.de, gorcunov@...il.com,
        hpa@...or.com, alexey.budankov@...ux.intel.com, eranian@...gle.com,
        ak@...ux.intel.com, like.xu@...ux.intel.com,
        yao.jin@...ux.intel.com, wei.w.wang@...el.com
Subject: Re: [PATCH V3 13/23] perf/x86/intel/lbr: Factor out
 intel_pmu_store_lbr

On Fri, Jul 03, 2020 at 04:59:49PM -0400, Liang, Kan wrote:
> On 7/3/2020 3:50 PM, Peter Zijlstra wrote:

> > If I'm not mistaken, this correctly deals with LBR_FORMAT_INFO, so can't
> > we also use the intel_pmu_arch_lbr_read() function for that case?
> 
> But the intel_pmu_arch_lbr_read() doesn't have the optimization
> (LBR_NO_INFO) for the LBR_FORMAT_INFO.
> https://lkml.kernel.org/r/tip-b16a5b52eb90d92b597257778e51e1fdc6423e64@git.kernel.org
> 
> To apply the optimization, we need extra codes as below.

Right, I saw that, but shouldn't we support that for anything with this
format anyway? That is, it's weird and inconsistent to not support
PERF_SAMPLE_BRANCH_NO_{CYCLES,FLAGS} for PEBS/ARCH-LBR output.

Arguably, we should even support NO_CYCLES for FORMAT_TIME. Yes it's
daft, but that's what you get for adding the ABI.