Message-ID: <1c8b6d14-7d14-4bd6-0eac-c60fa98aec0b@infradead.org>
Date:   Sat, 7 Nov 2020 16:24:35 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Linuxarm <linuxarm@...wei.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        John Garry <john.garry@...wei.com>
Subject: Re: [PATCH] mm/gup_benchmark: GUP_BENCHMARK depends on DEBUG_FS

On 11/7/20 4:03 PM, John Hubbard wrote:
> On 11/7/20 2:20 PM, Randy Dunlap wrote:
>> On 11/7/20 11:16 AM, John Hubbard wrote:
>>> On 11/7/20 11:05 AM, Song Bao Hua (Barry Song) wrote:
>>>>> -----Original Message-----
>>>>> From: John Hubbard [mailto:jhubbard@...dia.com]
>>> ...
>>>>>>     config GUP_BENCHMARK
>>>>>>         bool "Enable infrastructure for get_user_pages() and related calls
>>>>> benchmarking"
>>>>>> +    depends on DEBUG_FS
>>>>>
>>>>>
>>>>> I think "select DEBUG_FS" is better here. "depends on" has the obnoxious
>>>>> behavior of hiding the choice from you, if the dependencies aren't already met.
>>>>> Whereas what the developer *really* wants is a no-nonsense activation of the
>>>>> choice: "enable GUP_BENCHMARK and the debug fs that it requires".
>>>>>
>>>>
>>>> To some extent, I agree with you. But I still think here it is better to use "depends on".
>>>> According to
>>>> https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt
>>>>
>>>>      select should be used with care. select will force
>>>>      a symbol to a value without visiting the dependencies.
>>>>      By abusing select you are able to select a symbol FOO even
>>>>      if FOO depends on BAR that is not set.
>>>>      In general use select only for non-visible symbols
>>>>      (no prompts anywhere) and for symbols with no dependencies.
>>>>      That will limit the usefulness but on the other hand avoid
>>>>      the illegal configurations all over.
>>>>
>>>> On the other hand, in kernel there are 78 "depends on DEBUG_FS" and
>>>> only 14 "select DEBUG_FS".
>>>>
>>>
>>> You're not looking at the best statistics. Go look at what *already* selects
>>> DEBUG_FS, and you'll find about 50 items.
>>
>> Sorry, I'm not following you. I see the same 14 "select DEBUG_FS" as Barry.
> 
> I ran make menuconfig, and looked at it. Because I want to see the true end result,
> and I didn't trust my grep use, given that the system has interlocking dependencies,
> and I think one select could end up activating others (yes?).
> 
> And sure enough, there are 42 items listed, here they are, cleaned up so that there
> is one per line:
> 
> ZSMALLOC_STAT [=n]
> ZSMALLOC [=m]
> BCACHE_CLOSURES_DEBUG [=n]
> MD [=y]
> BCACHE [=n]
> DVB_C8SECTPFE [=n]
> MEDIA_SUPPORT [=m]
> MEDIA_PLATFORM_SUPPORT [=y]
> DVB_PLATFORM_DRIVERS [=n]
> PINCT
> DRM_I915_DEBUG [=n]
> HAS_IOMEM [=y]
> EXPERT [=y]
> DRM_I915 [=m]
> EDAC_DEBUG [=n]
> EDAC [=y]
> SUNRPC_DEBUG [=n]
> NETWORK_FILESYSTEMS [=y]
> SUNRPC [=m]
> SYSCTL [=y]
> PAGE_OWNER [=n]
> DEBUG_KERNEL [=y]
> STACKTRACE_SUPPORT [=y]
> DEBUG_KMEMLEAK [=n]
> DEBUG_KERNEL [=y]
> HAVE_DEBUG_KMEMLEAK [=y]
> BLK_DEV_IO_TRACE [=n]
> TRACING_SUPPORT [=y]
> FTRACE [=y]
> SYSFS [=y]
> BLOCK [=y]
> PUNIT_ATOM_DEBUG [=n]
> PCI [=y]
> NOTIFIER_ERROR_INJECTION [=n]
> DEBUG_KERNEL [=y]
> FAIL_FUTEX [=n]
> FAULT_INJECTION [=n]
> FUTEX [=y]
> KCOV [=n]
> ARCH_HAS_KCOV [=y]
> CC_HAS_SANCOV_TRACE_PC [=y]
> GCC_PLUGINS
> 

OK, thanks, I see how you get that list now.

JFTR, those are not 42 independent users of DEBUG_FS. There are lots of &&s
and a few ||s in that list.


xconfig shows this for DEBUG_FS: (13 selects for x86_64 allmodconfig)

Selected by [y]:
- PAGE_OWNER [=y] && DEBUG_KERNEL [=y] && STACKTRACE_SUPPORT [=y]
- DEBUG_KMEMLEAK [=y] && DEBUG_KERNEL [=y] && HAVE_DEBUG_KMEMLEAK [=y]
- BLK_DEV_IO_TRACE [=y] && TRACING_SUPPORT [=y] && FTRACE [=y] && SYSFS [=y] && BLOCK [=y]
- FAIL_FUTEX [=y] && FAULT_INJECTION [=y] && FUTEX [=y]
- KCOV [=y] && ARCH_HAS_KCOV [=y] && (CC_HAS_SANCOV_TRACE_PC [=y] || GCC_PLUGINS [=n])
Selected by [m]:
- ZSMALLOC_STAT [=y] && ZSMALLOC [=m]
- BCACHE_CLOSURES_DEBUG [=y] && MD [=y] && BCACHE [=m]
- DVB_C8SECTPFE [=m] && MEDIA_SUPPORT [=m] && MEDIA_PLATFORM_SUPPORT [=y] && DVB_PLATFORM_DRIVERS [=y] && PINCTRL [=y] && DVB_CORE [=m] && I2C [=y] && (ARCH_STI || ARCH_MULTIPLATFORM || COMPILE_TEST [=y])
- DRM_I915_DEBUG [=y] && HAS_IOMEM [=y] && EXPERT [=y] && DRM_I915 [=m]
- EDAC_DEBUG [=y] && EDAC [=m]
- SUNRPC_DEBUG [=y] && NETWORK_FILESYSTEMS [=y] && SUNRPC [=m] && SYSCTL [=y]
- PUNIT_ATOM_DEBUG [=m] && PCI [=y]
- NOTIFIER_ERROR_INJECTION [=m] && DEBUG_KERNEL [=y]

Some other $ARCH could be more...

>>
>> In general we don't want any one large "feature" (or subsystem) to be enabled
>> by one driver. If someone has gone to the trouble to disable DEBUG_FS (or whatever),
>> then a different Kconfig symbol shouldn't undo that.
>>
> 
> I agree with the "in general" point, yes. And my complaint is really 80% due to the
> very unhappy situation with Kconfig, where we seem to get a choice between *hiding*
> a feature, or forcing a dependency break. What we really want is a way to indicate
> a dependency that doesn't hide entire features, unless we want that. (Maybe I should
> attempt to get into the implementation, although I suspect it's harder than I
> realize.)
> 
> But the other 20% of my complaint is, given what we have, I think the appropriate
> adaptation for GUP_BENCHMARK's relationship to DEBUG_FS *in particular*, is: select.
> 
> And 42 other committers have chosen the same thing, for their relationship to
> DEBUG_FS. I'm in good company.
> 
> But if you really disagree, then I'd go with, just drop the patch entirely, because
> it doesn't really make things better as written...IMHO anyway. :)
> 
> thanks,

thanks.
-- 
~Randy

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