From: Paolo Bonzini <pbonzini@redhat.com>
To: Wanpeng Li <wanpeng.li@hotmail.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc: "Ingo Molnar" <mingo@kernel.org>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
"Rik van Riel" <riel@redhat.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Frederic Weisbecker" <fweisbec@gmail.com>,
"Radim Krčmář" <rkrcmar@redhat.com>
Subject: Re: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug
Date: Tue, 7 Jun 2016 12:34:30 +0200 [thread overview]
Message-ID: <15351b9b-563a-8945-5fb6-2df15107f0d6@redhat.com> (raw)
In-Reply-To: <SG2PR02MB15507073C531BAA82E9C8E67805D0@SG2PR02MB1550.apcprd02.prod.outlook.com>
On 07/06/2016 10:00, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")
> set rq->prev_* to 0 after a cpu hotplug comes back in order to fix the scenario:
>
> | steal is smaller than rq->prev_steal_time we end up with an insane large
> | value which then gets added to rq->prev_steal_time, resulting in a permanent
> | wreckage of the accounting.
>
> However, it is still buggy.
>
> rq->prev_steal_time = 0:
>
> As Rik pointed out:
>
> | setting rq->prev_irq_time to 0 in the guest, and then getting a giant value from
> | the host, could result in a very large of steal_jiffies.
>
> rq->prev_steal_time_rq = 0:
>
> | steal = paravirt_steal_clock(cpu_of(rq));
> | steal -= rq->prev_steal_time_rq;
> |
> | if (unlikely(steal > delta))
> | steal = delta;
> |
> | rq->prev_steal_time_rq += steal;
> | delta -= steal;
> |
> | rq->clock_task += delta;
>
> steal is a giant value and rq->prev_steal_time_rq is 0, rq->prev_steal_time_rq
> grows in delta granularity, rq->clock_task can't ramp up until rq->prev_steal_time_rq
> catches up steal clock since delta value will be 0 after reducing steal time from
> normal execution time. That's why I obersved that cpuhg/1-12 continue running
> until rq->prev_steal_time_rq catches up steal clock timestamp.
>
> I believe rq->prev_irq_time has similar issue. So this patch fix it by setting
> rq->prev_* to current irq time and steal clock timestamp after a cpu hotplug
> comes back.
I'm not sure this patch is necessary. Instead you could just revert
commit e9532e69b8d1. The previous patch obviously makes it unnecessary
to reset rq->prev_steal_time and rq->prev_steal_time_rq, and the reset
of rq->prev_irq_time looks like a no-op to me.
Paolo
> Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")'
> Acked-by: Rik van Riel <riel@redhat.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
> kernel/sched/sched.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 72f1f30..e6758af 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1813,12 +1813,12 @@ static inline void cpufreq_trigger_update(u64 time) {}
> static inline void account_reset_rq(struct rq *rq)
> {
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> - rq->prev_irq_time = 0;
> + rq->prev_irq_time = irq_time_read(cpu_of(rq));
> #endif
> #ifdef CONFIG_PARAVIRT
> - rq->prev_steal_time = 0;
> + rq->prev_steal_time = paravirt_steal_clock(cpu_of(rq));
> #endif
> #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> - rq->prev_steal_time_rq = 0;
> + rq->prev_steal_time_rq = paravirt_steal_clock(cpu_of(rq));
> #endif
> }
>
next parent reply other threads:[~2016-06-07 10:34 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1465661590-4732-1-git-send-email-wanpeng.li@hotmail.com>
[not found] ` <SG2PR02MB15507073C531BAA82E9C8E67805D0@SG2PR02MB1550.apcprd02.prod.outlook.com>
2016-06-07 10:34 ` Paolo Bonzini [this message]
[not found] ` <SG2PR02MB15505ACED7653E515DE21620805D0@SG2PR02MB1550.apcprd02.prod.outlook.com>
2016-06-07 11:50 ` Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug Wanpeng Li
2016-06-07 11:53 ` Paolo Bonzini
[not found] ` <SG2PR02MB15501BB06CAE47E4406FF7AD805D0@SG2PR02MB1550.apcprd02.prod.outlook.com>
2016-06-07 10:47 ` [PATCH v4 3/3] sched/cputime: Add steal time support to full dynticks CPU time accounting Paolo Bonzini
2016-06-07 11:23 ` Wanpeng Li
2016-06-07 12:15 ` Wanpeng Li
2016-06-07 12:34 ` Paolo Bonzini
2016-06-07 8:33 [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug Wanpeng Li
2016-06-07 8:33 ` [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during " Wanpeng Li
2016-06-07 16:18 ` kbuild test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=15351b9b-563a-8945-5fb6-2df15107f0d6@redhat.com \
--to=pbonzini@redhat.com \
--cc=fweisbec@gmail.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=peterz@infradead.org \
--cc=riel@redhat.com \
--cc=rkrcmar@redhat.com \
--cc=tglx@linutronix.de \
--cc=wanpeng.li@hotmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).