From: Nick Desaulniers <ndesaulniers@google.com>
To: Joe Perches <joe@perches.com>
Cc: Qian Cai <cai@lca.pw>, Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
juri.lelli@redhat.com,
Vincent Guittot <vincent.guittot@linaro.org>,
dietmar.eggemann@arm.com, Steven Rostedt <rostedt@goodmis.org>,
Benjamin Segall <bsegall@google.com>,
mgorman@suse.de, LKML <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] sched/cputime: silence a -Wunused-function warning
Date: Fri, 6 Mar 2020 11:02:11 -0800 [thread overview]
Message-ID: <CAKwvOdmdaDL4bhJc+7Xms=f4YXDw-Rr+WQAknd0Jv6UWOBUcEA@mail.gmail.com> (raw)
In-Reply-To: <442b7ace85a414c6a01040368f05d6d219f86536.camel@perches.com>
On Fri, Mar 6, 2020 at 10:39 AM Joe Perches <joe@perches.com> wrote:
>
> On Fri, 2020-03-06 at 09:25 -0800, Joe Perches wrote:
> > On Fri, 2020-03-06 at 09:13 -0800, Nick Desaulniers wrote:
> > > On Fri, Mar 6, 2020 at 7:42 AM Qian Cai <cai@lca.pw> wrote:
> > > > account_other_time() is only used when CONFIG_IRQ_TIME_ACCOUNTING=y (in
> > > > irqtime_account_process_tick()) or CONFIG_VIRT_CPU_ACCOUNTING_GEN=y (in
> > > > get_vtime_delta()). When both are off, it will generate a compilation
> > > > warning from Clang,
> > > >
> > > > kernel/sched/cputime.c:255:19: warning: unused function
> > > > 'account_other_time' [-Wunused-function]
> > > > static inline u64 account_other_time(u64 max)
> > > >
> > > > Rather than wrapping around this function with a macro expression,
> > > >
> > > > if defined(CONFIG_IRQ_TIME_ACCOUNTING) || \
> > > > defined(CONFIG_VIRT_CPU_ACCOUNTING_GEN)
> > > >
> > > > just use __maybe_unused for this small function which seems like a good
> > > > trade-off.
> > >
> > > Generally, I'm not a fan of __maybe_unused. It is a tool in the
> > > toolbox for solving this issue, but it's my least favorite. Should
> > > the call sites be eliminated, this may mask an unused and entirely
> > > dead function from being removed. Preprocessor guards based on config
> > > give more context into *why* a particular function may be unused.
> > >
> > > So let's take a look at the call sites of account_other_time(). Looks
> > > like irqtime_account_process_tick() (guarded by
> > > CONFIG_IRQ_TIME_ACCOUNTING) and get_vtime_delta() (guarded by
> > > CONFIG_VIRT_CPU_ACCOUNTING_GEN). If it were one config guard, then I
> > > would prefer to move the definition to be within the same guard, just
> > > before the function definition that calls it, but we have a more
> > > complicated case here.
> > >
> > > The next thing I'd check to see is if there's a dependency between
> > > configs. In this case, both CONFIG_IRQ_TIME_ACCOUNTING and
> > > CONFIG_VIRT_CPU_ACCOUNTING_GEN are defined in init/Kconfig. In this
> > > case there's also no dependency between configs, so specifying one
> > > doesn't imply the other; so guarding on one of the two configs is also
> > > not an option.
> > >
> > > So, as much as I'm not a fan of __maybe_unused, it is indeed the
> > > simplest option. Though, I'd still prefer the ifdef you describe
> > > instead, maybe the maintainers can provide guidance/preference?
> >
> > Another option might be to move static inline functions
> > where possible to an #include file (like sched.h) but the
> > same possible dead function issue would still exist.
>
> Turns out there are hundreds of unused static inline
> functions in kernel .h files.
>
> A trivial script to find some of them (with likely
> false positives as some might actually be used via ##
> token pasting mechanisms).
>
> (and there's almost certainly a better way to do this
> too as it takes a _very_ long time to run)
>
> $ grep-2.5.4 -rP --include=*.h '^[ \t]*static\s+inline\s+(?:\w+\s+){1,3}\w+[ \t]*\(' * | \
> grep -P -oh '\w+\s*\(' | \
> sort | uniq -c | sort -n | grep -P '^\s+1\b' | \
> sed -r -e 's/^\s+1\s+//' -e 's/\(//' | \
> while read line ; do \
> echo -n "$line: " ; git grep -w $line | wc -l ; \
> done | \
> grep ": 1$"
Please start sending patches to remove them and I'll review. If this
is a good amount of work, I have newbies that are looking to
contribute and can help.
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2020-03-06 19:02 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-06 15:41 [PATCH] sched/cputime: silence a -Wunused-function warning Qian Cai
2020-03-06 17:13 ` Nick Desaulniers
2020-03-06 17:25 ` Joe Perches
2020-03-06 18:38 ` Joe Perches
2020-03-06 19:02 ` Nick Desaulniers [this message]
2020-03-06 19:10 ` Joe Perches
2020-03-07 0:07 ` linux-kernel: Unused static inline functions Joe Perches
2020-04-23 18:57 ` Nick Desaulniers
2020-04-23 21:22 ` Joe Perches
2020-04-24 16:07 ` Julia Lawall
2020-11-29 23:56 ` Joe Perches
2020-05-09 13:12 ` [PATCH] sched/cputime: silence a -Wunused-function warning Qian Cai
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