linux-kernel - Re: [PATCH v2 4/4] perf/x86/uncore: modularize Intel uncore driver

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Message-ID: <20140418105354.GA29167@gmail.com>
Date:	Fri, 18 Apr 2014 12:53:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	acme@...radead.org, eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH v2 4/4] perf/x86/uncore: modularize Intel uncore driver


* Yan, Zheng <zheng.z.yan@...el.com> wrote:

> This patch adds support for building Intel uncore driver as module.
> It adds clean-up code and config option for the Intel uncore driver
> 
> Signed-off-by: Yan, Zheng <zheng.z.yan@...el.com>
> ---
> Changes since v1:
>  move config option to "General setup/Kernel Performance Events and Counters"
> 
>  arch/x86/kernel/cpu/Makefile                  |  3 +-
>  arch/x86/kernel/cpu/perf_event_intel_uncore.c | 48 ++++++++++++++++++++++++---
>  init/Kconfig                                  | 10 ++++++
>  3 files changed, 56 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 7fd54f0..5d3da70 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -36,7 +36,8 @@ obj-$(CONFIG_CPU_SUP_AMD)		+= perf_event_amd_iommu.o
>  endif
>  obj-$(CONFIG_CPU_SUP_INTEL)		+= perf_event_p6.o perf_event_knc.o perf_event_p4.o
>  obj-$(CONFIG_CPU_SUP_INTEL)		+= perf_event_intel_lbr.o perf_event_intel_ds.o perf_event_intel.o
> -obj-$(CONFIG_CPU_SUP_INTEL)		+= perf_event_intel_uncore.o perf_event_intel_rapl.o
> +obj-$(CONFIG_CPU_SUP_INTEL)		+= perf_event_intel_rapl.o
> +obj-$(CONFIG_PERF_X86_INTEL_UNCORE)	+= perf_event_intel_uncore.o
>  endif
>  
>  
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_uncore.c b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
> index 618d502..fd5e883 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_uncore.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_uncore.c

So I'm not willing to apply this patch until the documentation of 
perf_event_intel_uncore.c is improved. Right now the file starts 
without a single comment (!). Just lines after lines of code, without 
any explanation what it does, what its scope is, etc.

How should even a knowledgable user know about what it's all about?

That problem percolates to the Kconfig entry as well:

> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1502,6 +1502,16 @@ config PERF_EVENTS
>  
>  	  Say Y if unsure.
>  
> +config PERF_X86_INTEL_UNCORE
> +	default y
> +	tristate "Intel uncore performance monitoring support"
> +	depends on PERF_EVENTS && CPU_SUP_INTEL && PCI
> +	help
> +	  This option allows kernel to access the Uncore performance
> +	  monitoring units of Intel processors.
> +
> +	  Say Y if unsure.

Firstly, it should probably not be 'default y'.

Secondly, the description is essentially non-existent. What does the 
hardware do? Why should users care about uncore PMUs?

This needs to be fixed before we can make this modular.

Thanks,

	Ingo
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