Re: 回复: 回复: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
On Wed, Jan 27, 2021 at 09:00:27AM +, Zhang, Qiang wrote: > > > > 发件人: Uladzislau Rezki > 发送时间: 2021年1月26日 22:07 > 收件人: Zhang, Qiang > 抄送: Uladzislau Rezki; Paul E. McKenney; r...@vger.kernel.org; > linux-kernel@vger.kernel.org > 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 +, Zhang, Qiang wrote: > > > > > > > > > > > > 发件人: Uladzislau Rezki > > > 发送时间: 2021年1月22日 4:26 > > > 收件人: Zhang, Qiang > > > 抄送: Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org; > > > ure...@gmail.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.zh...@windriver.com > > > > wrote: > > > > > From: Zqiang > > > > > > > > > > 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 > > > > > > > >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
回复: 回复: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
发件人: Uladzislau Rezki 发送时间: 2021年1月26日 22:07 收件人: Zhang, Qiang 抄送: Uladzislau Rezki; Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org 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 +, Zhang, Qiang wrote: > > > > > > > > 发件人: Uladzislau Rezki > > 发送时间: 2021年1月22日 4:26 > > 收件人: Zhang, Qiang > > 抄送: Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org; > > ure...@gmail.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.zh...@windriver.com wrote: > > > > From: Zqiang > > > > > > > > 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 > > > > > >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. Thanks Qiang > or can be done manually to flush >system caches: > >echo 3 > /proc/sys/vm/drop_caches > >What do you think? > >-- >Vlad Rezki
Re: 回复: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
> > 发件人: Uladzislau Rezki > 发送时间: 2021年1月22日 22:31 > 收件人: Zhang, Qiang > 抄送: Uladzislau Rezki; Paul E. McKenney; r...@vger.kernel.org; > linux-kernel@vger.kernel.org > 主题: Re: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going > offline > > On Fri, Jan 22, 2021 at 01:44:36AM +, Zhang, Qiang wrote: > > > > > > > > 发件人: Uladzislau Rezki > > 发送时间: 2021年1月22日 4:26 > > 收件人: Zhang, Qiang > > 抄送: Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org; > > ure...@gmail.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.zh...@windriver.com wrote: > > > > From: Zqiang > > > > > > > > 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 > > > > > >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 or can be done manually to flush system caches: echo 3 > /proc/sys/vm/drop_caches What do you think? -- Vlad Rezki
Re: 回复: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
On Sun, Jan 24, 2021 at 02:21:07AM +, Zhang, Qiang wrote: > > > > 发件人: Uladzislau Rezki > 发送时间: 2021年1月22日 22:31 > 收件人: Zhang, Qiang > 抄送: Uladzislau Rezki; Paul E. McKenney; r...@vger.kernel.org; > linux-kernel@vger.kernel.org > 主题: Re: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going > offline > > On Fri, Jan 22, 2021 at 01:44:36AM +, Zhang, Qiang wrote: > > > > > > > > 发件人: Uladzislau Rezki > > 发送时间: 2021年1月22日 4:26 > > 收件人: Zhang, Qiang > > 抄送: Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org; > > ure...@gmail.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.zh...@windriver.com wrote: > > > > From: Zqiang > > > > > > > > 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 > > > > > >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. > Exactly. I think that 5 pages are not that important(small amount of memory), so it is not worth to build the logic around it in order to return them back to the page allocator when CPU goes offline, even forever. This is how i see it. If you are aware about some specific systems where it is critical, please let's as know. -- Vlad Rezki
回复: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline
发件人: Uladzislau Rezki 发送时间: 2021年1月22日 22:31 收件人: Zhang, Qiang 抄送: Uladzislau Rezki; Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org 主题: Re: 回复: [PATCH] rcu: Release per-cpu krcp page cache when CPU going offline On Fri, Jan 22, 2021 at 01:44:36AM +, Zhang, Qiang wrote: > > > > 发件人: Uladzislau Rezki > 发送时间: 2021年1月22日 4:26 > 收件人: Zhang, Qiang > 抄送: Paul E. McKenney; r...@vger.kernel.org; linux-kernel@vger.kernel.org; > ure...@gmail.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.zh...@windriver.com wrote: > > > From: Zqiang > > > > > > 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 > > > >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. Thanks Qiang > >-- >Vlad Rezki
