linux-kernel - Re: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline

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Message-ID: <20210122143129.GB1873@pc638.lan>
Date:   Fri, 22 Jan 2021 15:31:29 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Zhang, Qiang" <Qiang.Zhang@...driver.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 回复: [PATCH] rcu: Release
 per-cpu krcp page cache when CPU going offline

On Fri, Jan 22, 2021 at 01:44:36AM +0000, Zhang, Qiang wrote:
> 
> 
> ________________________________________
> 发件人: Uladzislau Rezki <urezki@...il.com>
> 发送时间: 2021年1月22日 4:26
> 收件人: Zhang, Qiang
> 抄送: Paul E. McKenney; rcu@...r.kernel.org; linux-kernel@...r.kernel.org; urezki@...il.com
> 主题: Re: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
> >Hello, Qiang,
> 
> > On Thu, Jan 21, 2021 at 02:49:49PM +0800, qiang.zhang@...driver.com wrote:
> > > From: Zqiang <qiang.zhang@...driver.com>
> > >
> > > If CPUs go offline, the corresponding krcp's page cache can
> > > not be use util the CPU come back online, or maybe the CPU
> > > will never go online again, this commit therefore free krcp's
> > > page cache when CPUs go offline.
> > >
> > > Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> >
> >Do you consider it as an issue? We have 5 pages per CPU, that is 20480 bytes.
> >
> 
> Hello Rezki 
> 
> In a multi CPUs system, more than one CPUs may be offline, there are more than 5 pages,  and these offline CPUs may never go online again  or  in the process of CPUs online, there are errors, which lead to the failure of online, these scenarios will lead to the per-cpu krc page cache will never be released.
> 
Thanks for your answer. I was thinking more about if you knew some platforms
which suffer from such extra page usage when CPU goes offline. Any issues
your platforms or devices run into because of that.

So i understand that if CPU goes offline the 5 pages associated with it are
unused until it goes online back.

--
Vlad Rezki