________________________________________ 发件人: linux-kernel-ow...@vger.kernel.org <linux-kernel-ow...@vger.kernel.org> 代表 Thomas Gleixner <t...@linutronix.de> 发送时间: 2020年8月26日 7:53 收件人: Waiman Long; Zhang, Qiang; el...@google.com 抄送: linux-kernel@vger.kernel.org; a...@linux-foundation.org 主题: Re: 回复: [PATCH] debugobjects: install cpu hotplug callback
On Tue, Aug 25 2020 at 18:26, Waiman Long wrote: > On 8/25/20 12:53 AM, Zhang, Qiang wrote: >> >> When a cpu going offline, we should free objects in "percpu_obj_pool" >> free_objs list which corresponding to this cpu. > > The percpu free object pool is supposed to be accessed only by that > particular cpu without any lock. Trying to access it from another cpu > can cause a race condition unless one can make sure that the offline cpu > won't become online in the mean time. >It is actually safe because CPU hotplug is globally serialized and there >is no way that an offline CPU will come back from death valley >magically. If such a zombie ever surfaces then we have surely more >serious problems than accessing that pool :) > There shouldn't be too many free objects in the percpu pool. Is it > worth the effort to free them? >That's a really good question nevertheless. The only case where this >ever matters is physical hotplug. All other CPU hotplug stuff is >temporarily or in case of a late (post boottime) SMT disable it's going >to be a handful of free objects on that pool. As debugobjects is as the >name says a debug facility the benefit is questionable unless there is a >good reason to do so. I don't know there may not be too many objects in the percpu pool, but that doesn't mean they no need to be free, a CPU may never be online after it is offline. some objects in percpu pool is never free. >Thanks, > tglx
