Messages in this thread | | | Date | Wed, 11 Apr 2018 07:20:55 +0800 | From | yuankuiz@codeauro ... | Subject | Re: [PATCH] time: tick-sched: use bool for tick_stopped |
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++ 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? >>> >>>> 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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