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Date: Wed, 11 Apr 2018 07:09:28 +0800
From: yuankuiz@...eaurora.org
To: Thomas Gleixner <tglx@...utronix.de>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Linux PM <linux-pm@...r.kernel.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Frederic Weisbecker <fweisbec@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Len Brown <len.brown@...el.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-pm-owner@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
++
On 2018-04-10 10:49 PM, yuankuiz@...eaurora.org wrote:
> Typo...
>
> On 2018-04-10 10:08 PM, yuankuiz@...eaurora.org wrote:
>> On 2018-04-10 07:06 PM, Thomas Gleixner wrote:
>>> On Tue, 10 Apr 2018, yuankuiz@...eaurora.org wrote:
>>>> On 2018-04-10 05:10 PM, Thomas Gleixner wrote:
>>>> > On Tue, 10 Apr 2018, yuankuiz@...eaurora.org wrote:
>>>> > > On 2018-04-10 04:00 PM, Rafael J. Wysocki wrote:
>>>> > > > On Tue, Apr 10, 2018 at 9:33 AM, <yuankuiz@...eaurora.org> wrote:
>>>> > > > > From: John Zhao <yuankuiz@...eaurora.org>
>>>> > > > >
>>>> > > > > Variable tick_stopped returned by tick_nohz_tick_stopped
>>>> > > > > can have only true / false values. Since the return type
>>>> > > > > of the tick_nohz_tick_stopped is also bool, variable
>>>> > > > > tick_stopped nice to have data type as bool in place of unsigned int.
>>>> > > > > Moreover, the executed instructions cost could be minimal
>>>> > > > > without potiential data type conversion.
>>>> > > > >
>>>> > > > > Signed-off-by: John Zhao <yuankuiz@...eaurora.org>
>>>> > > > > ---
>>>> > > > > kernel/time/tick-sched.h | 2 +-
>>>> > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
>>>> > > > >
>>>> > > > > diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
>>>> > > > > index 6de959a..4d34309 100644
>>>> > > > > --- a/kernel/time/tick-sched.h
>>>> > > > > +++ b/kernel/time/tick-sched.h
>>>> > > > > @@ -48,8 +48,8 @@ struct tick_sched {
>>>> > > > > unsigned long check_clocks;
>>>> > > > > enum tick_nohz_mode nohz_mode;
>>>> > > > >
>>>> > > > > + bool tick_stopped : 1;
>>>> > > > > unsigned int inidle : 1;
>>>> > > > > - unsigned int tick_stopped : 1;
>>>> > > > > unsigned int idle_active : 1;
>>>> > > > > unsigned int do_timer_last : 1;
>>>> > > > > unsigned int got_idle_tick : 1;
>>>> > > >
>>>> > > > I don't think this is a good idea at all.
>>>> > > >
>>>> > > > Please see https://lkml.org/lkml/2017/11/21/384 for example.
>>>> > > [ZJ] Thanks for this sharing. Looks like, this patch fall into the case of
>>>> > > "Maybe".
>>>> >
>>>> > This patch falls into the case 'pointless' because it adds extra storage
>>>> [ZJ] 1 bit vs 1 bit. no more.
>>>
>>> Groan. No. Care to look at the data structure? You create a new
>>> storage,
>> [ZJ] Say, {unsigned int, unsigned int, unsigned int, unsigned int,
>> unsigned int} becomes
>> {bool , unsigned int, unsigned int, unsigned int,
>> unsigned int}
>> As specified by the rule No.10 at the section 6.7.2.1 of C99 TC2 as:
>> "If enough space remains, a bit-field that immediately follows another
>> bit-field in a
>> structure shall be packed into adjacent bits of the same unit." What
>> is the new storage so far?
>>
>>> which is incidentally merged into the other bitfield by the compiler
>>> at a
>>> different bit position, but there is no guarantee that a compiler
>>> does
>>> that. It's free to use distinct storage for that bool based bit.
>> [ZJ] Per the rule No.10 at section 6.7.2.1 of C99 TC2 as:
>> " If insufficient space remains, whether a bit-field that does
>> not fit is put into
>> the next unit or overlaps adjacent units is
>> implementation-defined."
>> So, implementation is never mind which type will be stored if any.
>>
>>> >> > for no benefit at all.
>>>> [ZJ] tick_stopped is returned by the tick_nohz_tick_stopped() which
>>>> is bool.
>>>> The benefit is no any potiential type conversion could be minded.
>>>
>>> A bit stays a bit. 'bool foo : 1;' or 'unsigned int foo : 1' has to
>>> be
>>> evaluated as a bit. So there is a type conversion from BIT to bool
>>> required
>>> because BIT != bool.
>> [ZJ] Per the rule No.9 at section 6.7.2.1 of C99 TC2 as:
>> "If the value 0 or 1 is stored into a nonzero-width
>> bit-field of types
>> _Bool, the value of the bit-field shall compare equal to the value
>> stored."
>> Obviously, it is nothing related to type conversion actually.
>>>
>>> By chance the evaluation can be done by evaluating the byte in which
>>> the
>>> bit is placed just because the compiler knows that the remaining bits
>>> are
>>> not used. There is no guarantee that this is done, it happens to be
>>> true
>>> for a particular compiler.
>> [ZJ] Actually, such as GCC owe that kind of guarantee to be promised
>> by ABI.
>>>
>>> But that does not make it any more interesting. It just makes the
>>> code
>>> harder to read and eventually leads to bigger storage.
>> [ZJ] To get the benctifit to be profiled, it is given as:
>> number of instructions of function tick_nohz_tick_stopped():
> [ZJ] Here, I used is not the tick_nohz_tick_stopped(), but an
> evaluation() as:
> #include <stdio.h>
> #include <stdbool.h>
>
> struct tick_sched {
> unsigned int inidle : 1;
> unsigned int tick_stopped : 1;
> };
>
> bool get_status()
> {
> struct tick_sched *ts;
> ts->tick_stopped = 1;
> return ts->tick_stopped;
> }
>
> int main()
> {
> if (get_status()) return 0;
> return 0;
> }
>
> [ZJ] Toggle the declaration of tick_stopped in side of the tick_sched
> structure for comparison.
>
>
>> original: 17
>> patched: 14
>> Which was saved is:
>> movzbl %al, %eax
>> testl %eax, %eax
>> setne %al
>> Say, 3 / 17 = 17 % could be gained in the instruction executed
>> for this function can be evaluated.
>>
>> Note:
>> The environment I used is:
>> OS : Ubuntu Desktop 16.04 LTS
>> gcc: 6.3.0 (without optimization
>> for in general purpose)
>>
>>>
>
> Just FYI.
>
> Thanks,
> ZJ
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