Messages in this thread | | | Date | Fri, 20 Apr 2018 09:47:05 +0800 | From | yuankuiz@codeauro ... | Subject | Re: [PATCH] time: tick-sched: use bool for tick_stopped |
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On 2018-04-11 07:20 AM, yuankuiz@codeaurora.org wrote: > ++ > On 2018-04-11 07:09 AM, yuankuiz@codeaurora.org wrote: >> ++ >> >> On 2018-04-10 10:49 PM, yuankuiz@codeaurora.org wrote: >>> Typo... >>> >>> On 2018-04-10 10:08 PM, yuankuiz@codeaurora.org wrote: >>>> On 2018-04-10 07:06 PM, Thomas Gleixner wrote: >>>>> On Tue, 10 Apr 2018, yuankuiz@codeaurora.org wrote: >>>>>> On 2018-04-10 05:10 PM, Thomas Gleixner wrote: >>>>>> > On Tue, 10 Apr 2018, yuankuiz@codeaurora.org wrote: >>>>>> > > On 2018-04-10 04:00 PM, Rafael J. Wysocki wrote: >>>>>> > > > On Tue, Apr 10, 2018 at 9:33 AM, <yuankuiz@codeaurora.org> wrote: >>>>>> > > > > From: John Zhao <yuankuiz@codeaurora.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@codeaurora.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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