On Tue, Jun 24, 2014 at 12:02:34PM +0800, Rui Xiang wrote:
> Hi Greg or Thomas,
>
> 0e576acbc1d9600cf2d9b4a141a2554639959d50
> nohz: Fix another inconsistency between CONFIG_NO_HZ=n and nohz=off
>
> This fixed an inconsistency about nohz.
>
> Without this patch, When we boot with nohz=off and CONFIG_NO_HZ=Y,
> and the system is idle, powertop shows the cpu is always in polling
> state. It can affect the performance in some cases.
>
> *without patch*
> PowerTOP version 1.13 (C) 2007 Intel Corporation
>
> Cn Avg residency P-states (frequencies)
> C0 (cpu running) ( 0.0%) Turbo Mode 0.0%
> polling 70.3ms (100.3%) 2.40 Ghz 0.0%
> C1 mwait 0.0ms ( 0.0%) 2.27 Ghz 0.0%
> C2 mwait 0.0ms ( 0.0%) 2.14 Ghz 0.0%
> C3 mwait 0.0ms ( 0.0%) 1.60 Ghz 100.0%
>
> *with patch*
> PowerTOP version 1.13 (C) 2007 Intel Corporation
>
> Cn Avg residency P-states (frequencies)
> C0 (cpu running) ( 0.3%) Turbo Mode 0.0%
> polling 0.0ms ( 0.0%) 2.40 Ghz 0.0%
> C1 mwait 0.0ms ( 0.0%) 2.27 Ghz 0.0%
> C2 mwait 0.0ms ( 0.0%) 2.14 Ghz 0.0%
> C3 mwait 3.9ms (99.7%) 1.60 Ghz 100.0%
>
>
> This patch looks applicable to stable-3.4 and stable 3.10. It can be
> applyed to 3.10 cleanly, but should be added some adjustment for 3.4.
>
> And what do you think?
>
> Thanks,
> Rui
Thank you Rui, I'll queue it for the 3.11 kernel as well.
Cheers,
--
Luís
>
> -----------------------
>
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> commit 0e576acbc1d9600cf2d9b4a141a2554639959d50 upstream
>
> If CONFIG_NO_HZ=n tick_nohz_get_sleep_length() returns NSEC_PER_SEC/HZ.
>
> If CONFIG_NO_HZ=y and the nohz functionality is disabled via the
> command line option "nohz=off" or not enabled due to missing hardware
> support, then tick_nohz_get_sleep_length() returns 0. That happens
> because ts->sleep_length is never set in that case.
>
> Set it to NSEC_PER_SEC/HZ when the NOHZ mode is inactive.
>
> Reported-by: Michal Hocko <mhocko@xxxxxxx>
> Reported-by: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> [xr: Backported to 3.4:
> - applied the change to tick_nohz_stop_sched_tick() instead of
> can_stop_idle_tick]
> Signed-off-by: Rui Xiang <rui.xiang@xxxxxxxxxx>
> ---
> kernel/time/tick-sched.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 638dadf..67106f8 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -296,8 +296,10 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
> tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> }
>
> - if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
> + if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) {
> + ts->sleep_length = (ktime_t) { .tv64 = NSEC_PER_SEC/HZ };
> return;
> + }
>
> if (need_resched())
> return;
> --
>
>
> --
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