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

[Rik van Riel <riel@redhat.com>]

On Thu, 2017-03-23 at 16:55 -0400, Luiz Capitulino wrote:
> When there are two or more tasks executing in user-space and
> taking 100% of a nohz_full CPU, top reports 70% system time
> and 30% user time utilization. Sometimes I'm even able to get
> 100% system time and 0% user time.
> 
> This was reproduced with latest Linus tree (093b995), but I
> don't believe it's a regression (at least not a recent one)
> as I can reproduce it with older kernels. Also, I have
> CONFIG_IRQ_TIME_ACCOUNTING=y and haven't tried to reproduce
> without it yet.
> 
> Below you'll find the steps to reproduce and some initial
> analysis.
> 
> Steps to reproduce
> ------------------
> 
> 1. Set up a CPU for nohz_full with isolcpus= nohz_full=
> 
> 2. Pin two tasks that hog the CPU 100% of the time to that CPU
> 
> 3. Run top -d1 and check system time
> 
> NOTE: When there's only one task hogging a nohz_full CPU, top
>       shows 100% user-time, as expected
> 
> Initial analysis
> ----------------
> 
> When tracing vtime accounting functions and the user-space/kernel
> transitions when the issue is taking place, I see several of the
> following:
> 
> hog-10552 [015]  1132.711104:
> function:             enter_from_user_mode <-- apic_timer_interrupt
> hog-10552 [015]  1132.711105:
> function:             __context_tracking_exit <--
> enter_from_user_mode
> hog-10552 [015]  1132.711105:
> bprint:               __context_tracking_exit.part.4: new state=1 cur
> state=1 active=1
> hog-10552 [015]  1132.711105:
> function:             vtime_account_user <--
> __context_tracking_exit.part.4
> hog-10552 [015]  1132.711105:
> function:             smp_apic_timer_interrupt <--
> apic_timer_interrupt
> hog-10552 [015]  1132.711106: function:             irq_enter <--
> smp_apic_timer_interrupt
> hog-10552 [015]  1132.711106: function:             tick_sched_timer
> <-- __hrtimer_run_queues
> hog-10552 [015]  1132.711108: function:             irq_exit <--
> smp_apic_timer_interrupt
> hog-10552 [015]  1132.711108:
> function:             __context_tracking_enter <--
> prepare_exit_to_usermode
> hog-10552 [015]  1132.711108:
> bprint:               __context_tracking_enter.part.2: new state=1
> cur state=0 active=1
> hog-10552 [015]  1132.711109: function:             vtime_user_enter
> <-- __context_tracking_enter.part.2
> hog-10552 [015]  1132.711109:
> function:             __vtime_account_system <-- vtime_user_enter
> hog-10552 [015]  1132.711109:
> function:             account_system_time <-- __vtime_account_system
> 
> On entering the kernel due to a timer interrupt, vtime_account_user()
> skips user-time accounting. Then later on when returning to user-
> space,
> vtime_user_enter() is probably accounting the whole time (ie. user-
> space
> plus kernel-space) to system time.
> 
> Now, when does vtime_account_user() skips accounting? Well, when the
> time delta is less then one jiffie. This would imply that
> vtime_account_user()
> is being called less than one jiffie since the last accounting, but I
> haven't
> confirmed any of this yet.

Jiffies should be advanced by the timer interrupt, on the
housekeeping CPU, which is not doing context tracking.

Why is the isolated/nohz_full CPU receiving timer interrupts
at all?

I thought it would not, but obviously I am wrong. What is
going on here?