On 10/11/2010 09:42 AM, Daniel Lezcano wrote: > On 10/11/2010 08:48 AM, richard -rw- weinberger wrote: > >> Hi list, >> >> I've a question on freezer.c. >> In freeze_unfreeze() is a while(1) loop which write()s "FROZEN" or >> "THAWED" until read() returns the same. >> Why do we need this loop? >> It looks like a hack to me. >> > > The freezer documentation specifies: > >
Sorry I did an unwanted copy-paste at this place with the end of this email :) > "It's important to note that freezing can be incomplete. In that case we > return EBUSY. This means that some tasks in the cgroup are busy doing > something that prevents us from completely freezing the cgroup at this > time. After EBUSY, the cgroup will remain partially frozen -- reflected > by freezer.state reporting "FREEZING" when read. The state will remain > "FREEZING" until one of these things happens: > > 1) Userspace cancels the freezing operation by writing "THAWED" to the > freezer.state file > > 2) Userspace retries the freezing operation by writing "FROZEN" to > the freezer.state file (writing "FREEZING" is not legal and returns EINVAL) > > 3) The tasks that blocked the cgroup from entering the "FROZEN" state > disappear from the cgroup's set of tasks." > > The loop in the code is for the case (2). > > Let me clarify: > > (1) When we freeze a cgroup, the kernel browse the cgroup's tasks list > and will send a fake signal to each of them. > > But the places where the hooks are in the kernel code (fork and exit), > make possible to send this signal to a task while it is forking and the > child process is not yet in the cgroup's task list. > > The forking task will go to the frozen state right after returning from > the fork but the child task won't because it didn't receive the fake signal. > > If this child process forks again, the new child will automatically go > to the frozen state (because of the cgroup state 'freezing), making the > cgroup to converge to the frozen state. > > When we check the cgroup is frozen, the kernel look at this task list > and find the new child process which is not yet frozen and will return > 'freezing' instead of "frozen'. Freezing the cgroup again will freeze > this missing task. > > > (2) It's possible to freeze a cgroup while it is in important IO, aka > 'uninteruptible state', so we won't be able to freeze immediately and > we have to retry. > > > > These both cases happen rarely, so most of the time, one iteration in > the loop is enough to freeze the container. > > ------------------------------------------------------------------------------ > Beautiful is writing same markup. Internet Explorer 9 supports > standards for HTML5, CSS3, SVG 1.1, ECMAScript5, and DOM L2& L3. > Spend less time writing and rewriting code and more time creating great > experiences on the web. Be a part of the beta today. > http://p.sf.net/sfu/beautyoftheweb > _______________________________________________ > Lxc-devel mailing list > Lxc-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/lxc-devel > > ------------------------------------------------------------------------------ Beautiful is writing same markup. Internet Explorer 9 supports standards for HTML5, CSS3, SVG 1.1, ECMAScript5, and DOM L2 & L3. Spend less time writing and rewriting code and more time creating great experiences on the web. Be a part of the beta today. http://p.sf.net/sfu/beautyoftheweb _______________________________________________ Lxc-devel mailing list Lxc-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lxc-devel
