From mboxrd@z Thu Jan 1 00:00:00 1970 From: yuankuiz@codeaurora.org Subject: Re: [PATCH] time: tick-sched: use bool for tick_stopped Date: Tue, 10 Apr 2018 22:08:07 +0800 Message-ID: <2f7755fae34bb65ef0a4b5a11c67f431@codeaurora.org> References: <891d4f632fbff5052e11f2d0b6fac35d@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Thomas Gleixner Cc: "Rafael J. Wysocki" , Linux PM , "Rafael J. Wysocki" , Frederic Weisbecker , Peter Zijlstra , Ingo Molnar , Len Brown , Linux Kernel Mailing List , linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org 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, wrote: >> > > > > From: John Zhao >> > > > > >> > > > > 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 >> > > > > --- >> > > > > 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(): 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) > > Thanks, > > tglx