linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Date:   Thu, 30 Mar 2017 20:27:48 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Luiz Capitulino <lcapitulino@...hat.com>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org, Rik van Riel <riel@...hat.com>
Subject: Re: [BUG nohz]: wrong user and system time accounting

2017-03-30 10:14 GMT+08:00 Luiz Capitulino <lcapitulino@...hat.com>:
> On Thu, 30 Mar 2017 06:46:30 +0800
> Wanpeng Li <kernellwp@...il.com> wrote:
>
>> > So! Now we need to find a proper fix :o)
>> >
>> > Hmm, how bad would it be to revert to sched_clock() instead of jiffies in vtime_delta()?
>> > We could use nanosecond granularity to check deltas but only perform an actual cputime update
>> > when that delta >= TICK_NSEC. That should keep the load ok.
>>
>> Yeah, I mentioned something similar before.
>> https://lkml.org/lkml/2017/3/26/138 However, Rik's commit optimized
>> syscalls by not utilize sched_clock(), so if we should distinguish
>> between syscalls/exceptions and irqs?
>
> Why not use ktime_get()?

I believe ktime_get() is more heavy than local_clock() when sched
clock is stable. So we can cooperate to improve
https://lkml.org/lkml/2017/3/30/456.

Regards,
Wanpeng Li

>
> Here's the solution I was thinking about, it's mostly untested. I'm
> rate limiting below TICK_NSEC because I want to avoid syncing with
> the tick.
>
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index f3778e2b..a8b1e85 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -676,18 +676,20 @@ void thread_group_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
>  static u64 vtime_delta(struct task_struct *tsk)
>  {
> -       unsigned long now = READ_ONCE(jiffies);
> +       return ktime_sub(ktime_get(), tsk->vtime_snap);
> +}
>
> -       if (time_before(now, (unsigned long)tsk->vtime_snap))
> -               return 0;
> +/* A little bit less than the tick period */
> +#define VTIME_RATE_LIMIT (TICK_NSEC - 200000)
>
> -       return jiffies_to_nsecs(now - tsk->vtime_snap);
> +static bool vtime_should_account(struct task_struct *tsk)
> +{
> +       return vtime_delta(tsk) > VTIME_RATE_LIMIT;
>  }
>
>  static u64 get_vtime_delta(struct task_struct *tsk)
>  {
> -       unsigned long now = READ_ONCE(jiffies);
> -       u64 delta, other;
> +       u64 delta, other, now = ktime_get();
>
>         /*
>          * Unlike tick based timing, vtime based timing never has lost
> @@ -696,7 +698,7 @@ static u64 get_vtime_delta(struct task_struct *tsk)
>          * elapsed time. Limit account_other_time to prevent rounding
>          * errors from causing elapsed vtime to go negative.
>          */
> -       delta = jiffies_to_nsecs(now - tsk->vtime_snap);
> +       delta = ktime_sub(now, tsk->vtime_snap);
>         other = account_other_time(delta);
>         WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
>         tsk->vtime_snap = now;
> @@ -711,7 +713,7 @@ static void __vtime_account_system(struct task_struct *tsk)
>
>  void vtime_account_system(struct task_struct *tsk)
>  {
> -       if (!vtime_delta(tsk))
> +       if (!vtime_should_account(tsk))
>                 return;
>
>         write_seqcount_begin(&tsk->vtime_seqcount);
> @@ -723,7 +725,7 @@ void vtime_account_user(struct task_struct *tsk)
>  {
>         write_seqcount_begin(&tsk->vtime_seqcount);
>         tsk->vtime_snap_whence = VTIME_SYS;
> -       if (vtime_delta(tsk))
> +       if (vtime_should_account(tsk))
>                 account_user_time(tsk, get_vtime_delta(tsk));
>         write_seqcount_end(&tsk->vtime_seqcount);
>  }
> @@ -731,7 +733,7 @@ void vtime_account_user(struct task_struct *tsk)
>  void vtime_user_enter(struct task_struct *tsk)
>  {
>         write_seqcount_begin(&tsk->vtime_seqcount);
> -       if (vtime_delta(tsk))
> +       if (vtime_should_account(tsk))
>                 __vtime_account_system(tsk);
>         tsk->vtime_snap_whence = VTIME_USER;
>         write_seqcount_end(&tsk->vtime_seqcount);
> @@ -747,7 +749,7 @@ void vtime_guest_enter(struct task_struct *tsk)
>          * that can thus safely catch up with a tickless delta.
>          */
>         write_seqcount_begin(&tsk->vtime_seqcount);
> -       if (vtime_delta(tsk))
> +       if (vtime_should_account(tsk))
>                 __vtime_account_system(tsk);
>         current->flags |= PF_VCPU;
>         write_seqcount_end(&tsk->vtime_seqcount);
> @@ -776,7 +778,7 @@ void arch_vtime_task_switch(struct task_struct *prev)
>
>         write_seqcount_begin(&current->vtime_seqcount);
>         current->vtime_snap_whence = VTIME_SYS;
> -       current->vtime_snap = jiffies;
> +       current->vtime_snap = ktime_get();
>         write_seqcount_end(&current->vtime_seqcount);
>  }
>