On Wed, Jan 27, 2021 at 09:00:27AM +0000, Zhang, Qiang wrote: > > > ________________________________________ > 发件人: Uladzislau Rezki <[email protected]> > 发送时间: 2021年1月26日 22:07 > 收件人: Zhang, Qiang > 抄送: Uladzislau Rezki; Paul E. McKenney; [email protected]; > [email protected] > 主题: Re: 回复: 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 <[email protected]> > > > 发送时间: 2021年1月22日 4:26 > > > 收件人: Zhang, Qiang > > > 抄送: Paul E. McKenney; [email protected]; [email protected]; > > > [email protected] > > > 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, [email protected] > > > > wrote: > > > > > From: Zqiang <[email protected]> > > > > > > > > > > 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 <[email protected]> > > > > > > > >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. > > > > I agree with you, But I still want to talk about what I think > > > > My understanding is that when the CPU is offline, the pages is not > > accessible, beacuse we don't know when this CPU will > > go online again, so we best to return these page to the buddy system, > > when the CPU goes online again, we can allocate page from the buddy > > system to fill krcp's page cache. maybe you may think that this memory > > is small and don't need to. > > > >BTW, we can release the caches via shrinker path instead, what is more makes > >sense to me. We already have a callback, that frees pages when a page > >allocator > >asks for it. I think in that case it would be fair to return it to the buddy > >system. It happens under low memory condition > > I agree. it can be done in shrink callback, can release the currently > existing per-cpu > page cache. > Would not you mind to send a patch? If you need some input, i am happy to participate.
Thanks! -- Vlad Rezki
