Re: [PATCH] mm/gup_benchmark: GUP_BENCHMARK depends on DEBUG_FS

[John Hubbard]

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@xxxxxxxxxx]
> > ...
> > > > >     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