linux-kernel - Re: [PATCH] sched/cputime: silence a -Wunused-function warning

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Date:   Fri, 06 Mar 2020 09:25:29 -0800
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>, Qian Cai <cai@....pw>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        bsegall@...gle.com, mgorman@...e.de,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] sched/cputime: silence a -Wunused-function warning

On Fri, 2020-03-06 at 09:13 -0800, Nick Desaulniers wrote:
> On Fri, Mar 6, 2020 at 7:42 AM Qian Cai <cai@....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.