linux-kernel - Re: [PATCH] mm/gup_benchmark: GUP_BENCHMARK depends on DEBUG

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Date:   Sat, 7 Nov 2020 16:03:10 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        "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 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


> 
> 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,
-- 
John Hubbard
NVIDIA