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Date: Fri, 20 Apr 2018 14:44:20 +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,
joe@...ches.com
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
On 2018-04-20 09:47 AM, yuankuiz@...eaurora.org wrote:
> On 2018-04-11 07:20 AM, yuankuiz@...eaurora.org wrote:
>> ++
>> On 2018-04-11 07:09 AM, yuankuiz@...eaurora.org wrote:
>>> ++
>>>
>>> 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?
> [ZJ] Further prototyping has been given based on gcc for both of
> x86_64 and armv8-a,
> unsigned int and bool share the same 1 bytes without the
> addtional storage for sure.
> Open this and welcome if any other difference behaviour could be
> captured.
[ZJ] Typo.. change 4 bytes above to 1 byte actually.
>>>>>
>>>>>> 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.
[ZJ] "-mone-byte-bool" could be used by alpha-linux-gcc to override the
default bool size
to become 1 byte for even Darwin / powerPC from it's manual.
>>>>>>
>>>>>> 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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