On 06/24/2016 03:41 PM, Arnd Bergmann wrote: > On Friday, June 24, 2016 9:00:48 AM CEST David Laight wrote: >> The intent of the >> 10 was probably to avoid an expensive 64bit divide. >> So maybe something like: >> diff = time_end - time_start; >> if (diff >= INT_MAX/2) >> diff_32 = INT_MAX/2/1000; >> else >> diff_32 = diff; >> diff_32 += diff_32 >> 6; >> diff_32 >>= 10; >> } >> >> Adding an extra 1/32 makes the division by be something slightly below 1000. > > Why not change the definition of the time to nanoseconds and update > the users accordingly? > > I see that cpuidle_enter_state() writes to the last_residency value, > and it is read in three places: ladder_select_state(), menu_update() > and show_state_time() (for sysfs). >
Updating show_state_time() to convert ns to us gets bit messy since show_state_##_name which is used for disable, usage and time simply returns state_usage->_name. And state_usage->time updation happens in cpuidle_enter_state() itself. > 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? --Shreyas _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev