Messages in this thread | | | Date | Tue, 10 Apr 2018 22:49:16 +0800 | From | yuankuiz@codeauro ... | Subject | Re: [PATCH] time: tick-sched: use bool for tick_stopped |
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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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