linux-kernel - Re: [PATCH -rt] timer: delay waking softirqs from the jiffy tick

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Message-ID: <520f0cf10908210512n7e426d1es4b5b0e0d953af7c5@mail.gmail.com>
Date:	Fri, 21 Aug 2009 14:12:08 +0200
From:	John Kacur <jkacur@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>, dinogun@...il.com
Subject: Re: [PATCH -rt] timer: delay waking softirqs from the jiffy tick

On Fri, Aug 21, 2009 at 11:56 AM, Peter Zijlstra<peterz@...radead.org> wrote:
> Hi,
>
> most people were complaining about broken balancing with the recent -rt
> series.
>
> A look at /proc/sched_debug yielded:
>
> cpu#0, 2393.874 MHz
>  .nr_running                    : 0
>  .load                          : 0
>  .cpu_load[0]                   : 177522
>  .cpu_load[1]                   : 177522
>  .cpu_load[2]                   : 177522
>  .cpu_load[3]                   : 177522
>  .cpu_load[4]                   : 177522
> cpu#1, 2393.874 MHz
>  .nr_running                    : 4
>  .load                          : 4096
>  .cpu_load[0]                   : 181618
>  .cpu_load[1]                   : 180850
>  .cpu_load[2]                   : 180274
>  .cpu_load[3]                   : 179938
>  .cpu_load[4]                   : 179758
>
>
> Which indicated the cpu_load computation was hosed, the 177522 value
> indicates that there is one RT task runnable. Initially I thought the
> old problem of calculating the cpu_load from a softirq had re-surfaced,
> however looking at the code shows its being done from scheduler_tick().
>
> [ we really should fix this RT/cfs interaction some day... ]
>
> A few trace_printk()s later:
>
>    sirq-timer/1-19    [001]   174.289744:     19: 50:S ==> [001]     0:140:R <idle>
>          <idle>-0     [001]   174.290724: enqueue_task_rt: adding task: 19/sirq-timer/1 with load: 177522
>          <idle>-0     [001]   174.290725:      0:140:R   + [001]    19: 50:S sirq-timer/1
>          <idle>-0     [001]   174.290730: scheduler_tick: current load: 177522
>          <idle>-0     [001]   174.290732: scheduler_tick: current: 0/swapper
>          <idle>-0     [001]   174.290736:      0:140:R ==> [001]    19: 50:R sirq-timer/1
>    sirq-timer/1-19    [001]   174.290741: dequeue_task_rt: removing task: 19/sirq-timer/1 with load: 177522
>    sirq-timer/1-19    [001]   174.290743:     19: 50:S ==> [001]     0:140:R <idle>
>
> We see that we always raise the timer softirq before doing the load
> calculation. Avoid this by re-ordering the scheduler_tick() call in
> update_process_times() to occur before we deal with timers.
>
> This lowers the load back to sanity and restores regular load-balancing
> behaviour.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  kernel/timer.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 8137cce..96ac1b4 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -1221,10 +1221,10 @@ void update_process_times(int user_tick)
>
>        /* Note: this timer irq context must be accounted for as well. */
>        account_process_tick(p, user_tick);
> +       scheduler_tick();
>        run_local_timers();
>        if (rcu_pending(cpu))
>                rcu_check_callbacks(cpu, user_tick);
> -       scheduler_tick();
>        run_posix_cpu_timers(p);
>  }
>
>
> --

Cool! I applied this to the v2.6.31-rc6-rt5 tree and with the
following results from /proc/sched_debug

Before applying the patch:

cpu#0, 2792.838 MHz
  .cpu_load[0]                   : 180594
  .cpu_load[1]                   : 192061
  .cpu_load[2]                   : 205170
  .cpu_load[3]                   : 204449
  .cpu_load[4]                   : 199281

cpu#1, 2792.838 MHz
  .cpu_load[0]                   : 177522
  .cpu_load[1]                   : 178378
  .cpu_load[2]                   : 178932
  .cpu_load[3]                   : 178808

After applying the patch:

cpu#0, 2792.847 MHz
  .cpu_load[0]                   : 1024
  .cpu_load[1]                   : 960
  .cpu_load[2]                   : 700
  .cpu_load[3]                   : 430
  .cpu_load[4]                   : 260

cpu#1, 2792.847 MHz
  .cpu_load[0]                   : 1024
  .cpu_load[1]                   : 1280
  .cpu_load[2]                   : 1393
  .cpu_load[3]                   : 1390
  .cpu_load[4]                   : 1871
--
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