On 12/18/2013 09:43 AM, Frederic Weisbecker wrote: > On Wed, Dec 18, 2013 at 10:04:43AM +0800, Alex Shi wrote: >> On 12/18/2013 06:51 AM, Frederic Weisbecker wrote: >>> So this is what this series brings, more details following: >>> >>> * Some code, naming and whitespace cleanups >>> >>> * Allow all CPUs outside the nohz_full range to handle the timekeeping >>> duty, not just CPU 0. Balancing the timekeeping duty should improve >>> powersavings. >> >> If the system just has one nohz_full cpu running, it will need another >> cpu to do timerkeeper job. Then the system roughly needs 2 cpu living. >> From powersaving POV, that is not good compare to normal nohz idle. > > Sure, but everything has a tradeoff :) > > We could theoretically run with the timekeeper purely idle if the other > CPU in full dynticks mode runs in userspace for a long while and seldom > do syscalls and faults. Timekeeping could be updated on kernel/user > boundaries in this case without much impact on performances. > > But then there is one strict condition for that: it can't read the timeofday > through the vdso but only through a syscall.
Where's your ambition? :) If the vdso timing functions could see that it's been too long since a real timekeeping update, they could fall back to a syscall. Otherwise, they could using rdtsc or whatever is in use. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
