On 06/27/2016 02:29 PM, David Laight wrote: > From: Arnd Bergmann >> Sent: 24 June 2016 20:43 >> On Friday, June 24, 2016 9:31:35 PM CEST Shreyas B Prabhu wrote: >>>> If those functions are called less often than cpuidle_enter_state(), >>>> we could just move the division there. Since the divisor is constant, >>>> do_div() can convert it into a multiply and shift, or we could use >>>> your the code you suggest above, or use a 32-bit division most of >>>> the time: >>>> >>>> if (diff <= UINT_MAX) >>>> diff_32 = (u32)diff / NSECS_PER_USEC; >>>> else >>>> diff_32 = div_u64(diff, NSECS_PER_USEC; >>>> >>>> which gcc itself will turn into a multiplication or series of >>>> shifts on CPUs on which that is faster. >>>> >>> I'm not sure which division method of the three suggested here to use. >>> Does anyone have a strong preference? >>> >> >> It depends on how accurate we want it and how long we expect >> the times to be. The optimization for the 4.2 second cutoff >> for doing a 32-bit division only makes sense if the majority >> of the sleep times are below that. > > It also depends if the code actually cares about the length of 'long' sleeps. > I'd guess that for cpu idle 4.2 seconds is 'a long time', so the div_u64() > result could be treated as 4.2 seconds without causing grief. > > Actually the cost of a 64bit divide after a 4 second sleep will be noise. > OTOH a 64bit divide after a sleep that lasted a few ns will be significant. > Agreed. I'll use the code you suggested, with a small change- Using diff_32 += diff_32 >> 5 instead of diff_32 += diff_32 >> 6 since I want to err on the side of last_residency being more than actual.
And for long sleep cases, I'll use div_u64(). Thanks, Shreyas _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
