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Date: Tue, 10 Apr 2018 22:49:16 +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
Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped
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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