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Message-ID: <aaa5ae49ff33ce9c83c6cf520cc83c0b@codeaurora.org>
Date: Fri, 20 Apr 2018 09:47:05 +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 2018-04-11 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 4 bytes without the addtional
storage for sure.
Open this and welcome if any other difference behaviour could be
captured.
>>>>
>>>>> 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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