linux-kernel - Re: [PATCH] sched/fair: Fix the nohz.next

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Message-ID: <CANRm+Cw7fWay=N7YAst3-0kNZ6frs=7vgQZBn3on0Sth5Xx=Rw@mail.gmail.com>
Date:   Tue, 7 Feb 2017 06:06:06 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched/fair: Fix the nohz.next_balance update mess

2017-02-06 21:23 GMT+08:00 Vincent Guittot <vincent.guittot@...aro.org>:
> On 6 February 2017 at 09:33, Wanpeng Li <kernellwp@...il.com> wrote:
>> Hi Vincent,
>> 2017-02-06 16:07 GMT+08:00 Vincent Guittot <vincent.guittot@...aro.org>:
>>> Hi Wanpeng
>>>
>>> On 5 February 2017 at 10:57, Wanpeng Li <kernellwp@...il.com> wrote:
>>>> From: Wanpeng Li <wanpeng.li@...mail.com>
>>>>
>>>> The commit:
>>>>   c5afb6a87f2 ("sched/fair: Fix nohz.next_balance update")
>>>>
>>>> intends to update nohz.next_balance in two steps.
>>>>
>>>> 1) The ILB CPU utilizes next_balance variable in nohz_idle_balance()
>>>>    to gather the shortest next balance of other idle CPUs before
>>>>    updating nohz.next_balance.
>>>> 2) The ILB CPU updates the nohz.next_balance according to its own
>>>>    next_balance after load balance on behalf of other idle CPUs.
>>>>
>>>> However, there is a mess which breaks the original intention of the
>>>
>>> Have you got details of the mess that this generates ?
>>>
>>>> first step, every idle CPUs update nohz.next_balance during ILB CPU
>>>> on behalf of them to do load balance, and then the ILB CPU utilizes
>>>> next_balance variable in nohz_idle_balance() to gather the shortest
>>>> next balance of other idle CPUs before updating nohz.next_balance.
>>>>
>>>> This patch fixes it by don't update nohz.next_balance for other idle
>>>> CPUs when ILB CPU on behalf of them to do load balance.
>>>
>>> But how do you take into account the next balance of other idle CPUs ?
>>
>> The step 1) which I describe above for your original commit takes it
>> into account. In addition, please refers to the comments which you
>> added(rebalance_domains()) in the original commit:
>
> yes sorry I mixed rebalance_domains and  nohz_idle_balance code
>
> But are you sure that this additional condition will change anything ?
>
> When an ILB is triggered, it means that nohz.next_balance is before jiffies.
> Then, for all Idle CPUs (except the ILB CPU), the rq->next_balance
> will be for sure after nohz.next_balance once we have finished the
> for_each_domain loop of rebalance_domain() so it can't trig
> nohz.next_balance = rq->next_balance  and the current condition if
> fine.

Oh, I miss it. Thanks for your pointing out.

Regards,
Wanpeng Li