Messages in this thread | | | Date | Wed, 11 Apr 2018 07:09:28 +0800 | From | yuankuiz@codeauro ... | Subject | Re: [PATCH] time: tick-sched: use bool for tick_stopped |
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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? >> >>> 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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