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    SubjectRe: [PATCH -v3 6/8] fsnotify: add group priorities
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    On Thu, 2008-11-27 at 17:25 +0100, Peter Zijlstra wrote:
    > On Tue, 2008-11-25 at 12:21 -0500, Eric Paris wrote:
    > > In preperation for blocking fsnotify calls group priorities must be added.
    > > When multiple groups request the same event type the lowest priority group
    > > will receive the notification first.
    >
    > > @@ -114,9 +117,26 @@ struct fsnotify_group *fsnotify_find_group(unsigned int group_num, unsigned long
    > >
    > > group->ops = ops;
    > >
    > > - /* add it */
    > > - list_add_rcu(&group->group_list, &fsnotify_groups);
    > > + /* Do we need to be the first entry? */
    > > + if (list_empty(&fsnotify_groups)) {
    > > + list_add_rcu(&group->group_list, &fsnotify_groups);
    > > + goto out;
    > > + }
    > > +
    > > + list_for_each_entry(group_iter, &fsnotify_groups, group_list) {
    > > + /* insert in front of this one? */
    > > + if (priority < group_iter->priority) {
    > > + /* I used list_add_tail() to insert in front of group_iter... */
    > > + list_add_tail_rcu(&group->group_list, &group_iter->group_list);
    > > + break;
    > > + }
    > >
    > > + /* are we at the end? if so insert at end */
    > > + if (list_is_last(&group_iter->group_list, &fsnotify_groups)) {
    > > + list_add_tail_rcu(&group->group_list, &fsnotify_groups);
    > > + break;
    > > + }
    > > + }
    > > out:
    > > mutex_unlock(&fsnotify_grp_mutex);
    > > fsnotify_recalc_global_mask();
    >
    > What priority range do you need to cater for, and how many groups?

    On a typical system I'd expect to see one group for dnotify (rpmidmapd
    uses dnotify so most systems will end up having 1 group I would expect)

    inotify I wouldn't expect more than 3-4 inotify_init() calls

    fsnotify I wouldn't imagine more than 3 groups.

    So total we are talking about maybe 10 groups on a system really making
    use of fs notification?

    > I can
    > imagine for many groups and limit range a priority list might be better
    > suited.

    talking about plist.h? Since I don't allow 2 groups with the same
    priority I'd say a lot of the plist code would just be overhead (the
    prio list and the node list would be the same)

    That's not a big deal since I don't really care about the add/remove
    code paths since they are all notification overhead/setup/teardown. I
    would think that cleaner simpler code would probably be a better idea
    rather than performance for these areas especially since it looks like
    the speed critical parts of plists (list_for_each_entry) would be the
    exact same.

    what I don't see is plists being protected by RCU and looking at
    plist_del it doesn't seem like it would be rcu safe. RCU safe plists
    might be a good idea, but for now I think I should just do my own
    priority listing so I don't have to hold a lock while I walk the group
    list (that path is VERY hot)

    -Eric



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