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Date:   Sat, 9 May 2020 09:12:10 -0400
From:   Qian Cai <cai@....pw>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        bsegall@...gle.com, Mel Gorman <mgorman@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] sched/cputime: silence a -Wunused-function warning



> On Mar 6, 2020, at 12:13 PM, Nick Desaulniers <ndesaulniers@...gle.com> 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.
>> 
>> Signed-off-by: Qian Cai <cai@....pw>
> 
> Hi Qian, thanks for the patch!
> 
> 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?
> Otherwise,
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Peter, can you take a look at this patch when you have a chance? Having Clang to complain this every time is a bit annoying.

> 
>> ---
>> kernel/sched/cputime.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index cff3e656566d..85da4d6dee24 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -252,7 +252,7 @@ static __always_inline u64 steal_account_process_time(u64 maxtime)
>> /*
>>  * Account how much elapsed time was spent in steal, irq, or softirq time.
>>  */
>> -static inline u64 account_other_time(u64 max)
>> +static inline __maybe_unused u64 account_other_time(u64 max)
>> {
>>        u64 accounted;
>> 
>> --
> 
> -- 
> Thanks,
> ~Nick Desaulniers

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