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

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Message-ID: <20170323210541.5222eaf6@redhat.com>
Date:   Thu, 23 Mar 2017 21:05:41 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Rik van Riel <riel@...hat.com>
Cc:     fweisbec@...il.com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Thu, 23 Mar 2017 20:56:02 -0400
Rik van Riel <riel@...hat.com> wrote:

> 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.

The hypothesis isn't that it wasn't advanced, but that we stayed in
user-space less than 1ms.

> 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?

There are two runnable SCHED_OTHER tasks on the nohz_full CPU. When
that happens, the tick is re-activated. We're not nohz_full anymore,
but accounting should still work.