Messages in this thread | | | Date | Fri, 9 Dec 2011 16:30:09 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2 |
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On 12/08, Andrew Morton wrote: > > On Fri, 9 Dec 2011 01:28:53 +0400 > Cyrill Gorcunov <gorcunov@gmail.com> wrote: > > > On Thu, Dec 08, 2011 at 05:35:35PM +0100, Oleg Nesterov wrote: > > ... > > > > > > However, ->children list is not rcu-safe, this means that even > > > list_for_each() itself is not safe. Either you need tasklist or > > > we can probably make it rcu-safe... > > > > > > > Andrew, Oleg, does the below one look more less fine? Note the > > tasklist_lock is back and it worries me a bit since I imagine > > one could be endlessly reading some /proc/<pid>/children file > > increasing contention over this lock on the whole system > > (regardless the fact that it's take for read only). > > It is a potential problem, from the lock-hold point of view and > also it can cause large scheduling latencies. What's involved in > making ->children an rcu-protected list?
At first glance, this doesn't look trivial... forget_original_parent() abuses ->sibling.
But yes, it is not really nice to hold tasklist_lock here. May be we can change this code so that every iteration records the reported task_struct and then tries to continue. This means we should verify that ->real_parent is still the same under tasklist, but at least this way we do not hold it throughout.
> > From: Cyrill Gorcunov <gorcunov@openvz.org> > > Subject: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v4 > > > > There is no easy way to make a reverse parent->children chain > > from arbitrary <pid> (while parent pid is provided in "PPid" > > field of /proc/<pid>/status). > > > > So instead of walking over all pids in the system to figure out which > > children a task have -- we add explicit /proc/<pid>/children entry, > > because kernel already has this kind of information but it is not > > yet exported. This is a first level children, not the whole process > > tree, neither the process threads are identified with this interface. > > The changelog doesn't explain why we want the patch, so there's no > reason to merge it! Something to do with c/r, yes? > > If so, I guess the feature could/should be configurable. Probably with > a CONFIG_PROC_CHILDREN which is selected by CONFIG_CHECKPOINT_RESTORE. > Which is all getting a bit over the top, but I suppose we must do it.
Heh. This is the rare case when I personally like the new feature ;) I mean, it looks "obviously useful" to me. If nothing else, it can help to debug the problems. Probably the tools like pstree can use it.
Personally I'd even prefer /proc/pid/children/ directory (like /proc/pid/task), but I guess this needs much more complications.
Oleg.
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