linux-kernel - Re: [PATCH 1/5] sched,time: Count actually elapsed irq & softirq time

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Date:	Wed, 10 Aug 2016 14:33:24 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 1/5] sched,time: Count actually elapsed irq & softirq time

2016-08-10 13:07 GMT+08:00 Rik van Riel <riel@...hat.com>:
> On Wed, 2016-08-10 at 07:39 +0800, Wanpeng Li wrote:
>> 2016-08-10 7:25 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
>> > 2016-08-09 22:06 GMT+08:00 Rik van Riel <riel@...hat.com>:
>> > > On Tue, 2016-08-09 at 11:59 +0800, Wanpeng Li wrote:
>> > > > Hi Rik,
>> > > > 2016-07-13 22:50 GMT+08:00 Frederic Weisbecker <fweisbec@...il.
>> > > > com>:
>> > > > > From: Rik van Riel <riel@...hat.com>
>> > > > >
>> > > > > Currently, if there was any irq or softirq time during
>> > > > > 'ticks'
>> > > > > jiffies, the entire period will be accounted as irq or
>> > > > > softirq
>> > > > > time.
>> > > > >
>> > > > > This is inaccurate if only a subset of the time was actually
>> > > > > spent
>> > > > > handling irqs, and could conceivably mis-count all of the
>> > > > > ticks
>> > > > > during
>> > > > > a period as irq time, when there was some irq and some
>> > > > > softirq
>> > > > > time.
>> > > > >
>> > > > > This can actually happen when irqtime_account_process_tick is
>> > > > > called
>> > > > > from account_idle_ticks, which can pass a larger number of
>> > > > > ticks
>> > > > > down
>> > > > > all at once.
>> > > > >
>> > > > > Fix this by changing irqtime_account_hi_update,
>> > > > > irqtime_account_si_update,
>> > > > > and steal_account_process_ticks to work with cputime_t time
>> > > > > units,
>> > > > > and
>> > > > > return the amount of time spent in each mode.
>> > > >
>> > > > Do we need to minus st cputime from idle cputime in
>> > > > account_idle_ticks() when noirqtime is true? I try to add this
>> > > > logic
>> > > > w/ noirqtime and idle=poll boot parameter for a full dynticks
>> > > > guest,
>> > > > however, there is no difference, where I miss?
>> > >
>> > > Yes, you are right. The code in account_idle_ticks()
>> > > could use the same treatment.
>> > >
>> > > I am not sure why it would not work, though...
>> >
>> > Actually I observed a regression caused by this patch. I use a i5
>>
>> The regression is caused by your commit "sched,time: Count actually
>> elapsed irq & softirq time".
>
> Wanpeng and I discussed this issue, and discovered
> that this bug is triggered by my patch, specifically
> this bit:
>
> -       if (steal_account_process_tick(ULONG_MAX))
> +       other = account_other_time(cputime);
> +       if (other >= cputime)
>                 return;
>
> Replacing "cputime" with "ULONG_MAX" as the argument
> to account_other_time makes the bug disappear.
>
> However, this is not the cause of the bug.
>
> The cause of the bug appears to be that the values
> used to figure out how much steal time has passed
> are never initialized.
>
>                 steal = paravirt_steal_clock(smp_processor_id());
>                 steal -= this_rq()->prev_steal_time;
>
> The first of the two may be initialized by the host
> (I did not verify that), but the second one does not
> have any explicit initializers anywhere in the kernel
> tree.
>
> This can lead to an arbitrarily large difference between
> paravirt_steal_clock(smp_processor_id()) and
> this_rq()->prev_steal_time, which results in nothing but
> steal time getting accounted for a potentially a very
> long amount of time.
>
> Previously we carried this patch to initialize the
> various rq->prev_* values at CPU hotplug time:
>
> https://patchwork.codeaurora.org/patch/27699/
>
> Which got reverted by Paolo here:
>
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=sche
> d/core&id=3d89e5478bf550a50c99e93adf659369798263b0
>
> Which leads me to this question:
>
> Paulo, how would you like us to fix this bug?
>
> It seems like the host and guest steal time CAN get out
> of sync, sometimes by a ridiculous amount, and we need
> some way to get the excessive difference out of the way,
> without it getting accounted as steal time (not immediately,
> and not over the next 17 hours, or months).

I can be the volunteer to fix it if you guys have an idea. :)

Regards,
Wanpeng Li