Messages in this thread | | | Date | Tue, 6 Sep 2011 09:36:41 -0400 | From | Jason Baron <> | Subject | Re: [PATCH RFC] epoll: limit paths |
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Hi,
Thanks for your review!
On Fri, Sep 02, 2011 at 02:49:26PM -0700, Andrew Morton wrote: > On Fri, 2 Sep 2011 14:59:23 -0400 > Jason Baron <jbaron@redhat.com> wrote: > > > The current epoll code can be tickled to run basically indefinitely in both > > loop detection path check (on ep_insert()), and in the wakeup paths. The > > programs that tickle this behavior set up deeply linked networks of epoll > > file descriptors that cause the epoll algorithms to traverse them indefinitely. > > A couple of these sample programs have been previously posted in this thread: > > https://lkml.org/lkml/2011/2/25/297. > > > > To fix the loop detection path check algorithms, I simply keep track of the > > epoll nodes that have been already visited. Thus, the loop detection > > becomes proportional to the number of epoll file descriptor and links. This > > dramatically decreases the run-time of the loop check algorithm. In one > > diabolical case I tried it reduced the run-time from 15 mintues > > (all in kernel time) to .3 seconds. > > > > Fixing the wakeup paths could be done at wakeup time in a similar manner by > > keeping track of nodes that have already been visited, but the complexity is > > harder, since there can be multiple wakeups on different cpus...Thus, I've > > opted to limit the number of possible wakeup paths when the paths are created. > > > > This is accomplished, by noting that the end file descriptor points that are > > found during the loop detection pass (from the newly added link), are actually > > the sources for wakeup events. I keep a list of these file descriptors and > > limit the number and length of these paths that emanate from these 'source file > > descriptors'. In the current implemetation I allow 1000 paths of length 1, > > 500 of length 2, 100 of length 3, 50 of length 4 and 10 of length 5. Note that > > it is sufficient to check the 'source file descriptors' reachable from the newly > > added link, since no other 'source file descriptors' will have newly added > > links. This allows us to check only the wakeup paths that may have gotten too > > long, and not re-check all possible wakeup paths on the system. > > > > In terms of the path limit selection, I think its first worth noting that the > > most common case for epoll, is probably the model where you have 1 epoll file > > descriptor that is monitoring n number of 'source file descriptors'. In this > > case, each 'source file descriptor' has a 1 path of length 1. Thus, I believe > > that the limits I'm proposing are quite reasonable and in fact may be too > > generous. Thus, I'm hoping that the proposed limits will not prevent any > > workloads that currently work to fail. > > > > In terms of locking, I have extended the use of the 'epmutex' to all epoll_ctl > > add and remove operations. Currently its only used in a subset of the add paths. > > I need to hold the epmutex, so that we can correctly traverse a coherent graph, > > to check the number of paths. I believe that this additional locking is > > probably ok, since its in the setup/teardown paths, and doesn't affect the > > running paths, but it certainly is going to add some extra overhead. Also, > > worth noting is that the epmuex was recently added to the ep_ctl add operations > > in the initial path loop detection code using the argument that it was > > not on a critical path. > > > > Another thing to note here, is the length of epoll chains that is allowed. > > Currently, eventpoll.c defines: > > > > /* Maximum number of nesting allowed inside epoll sets */ > > #define EP_MAX_NESTS 4 > > > > This basically means that I am limited to a graph depth of 5 (EP_MAX_NESTS + 1). > > However, this limit is currently only enforced during the loop check detection > > code, and only when the epoll file descriptors are added in a certain order. > > Thus, this limit is currently easily bypassed. The newly added check for wakeup > > paths, stricly limits the wakeup paths to a length of 5, regardless of the order > > in which ep's are linked together. Thus, a side-effect of the new code is a more > > consistent enforcement of the graph depth. > > > > Thus far, I've tested this, using the sample programs previously mentioned, > > which now either return quickly or return -EINVAL. I've also testing using > > the piptest.c epoll tester, which showed no difference in performance. I've > > also created a number of different epoll networks and tested that they behave > > as expectdd. > > > > I believe this solves the original diabolical test cases, while still preserving > > the sane epoll nesting. > > > > Cool, thanks for working on this - it is rather a stinker. > > I don't think we have any maintained public test code for epoll? And I > trust you have some? It would be good if you could merge whatever you > have into the main kernel. Then each time we fix bugs or add features, > I can harrass people to update the test harness to track the changes. >
ok. The tests I've used to test this were:
-pipetest.c: http://www.gelato.unsw.edu.au/archives/linux-ia64/0405/9684.html -test I posted in https://lkml.org/lkml/2011/2/25/297 -test Nelson posted in https://lkml.org/lkml/2011/2/25/297 -some variations on the above tests
I can clean these up, and try and propose them for merge...also where would they live?
> A number of minor things: > > > --- a/fs/eventpoll.c > > +++ b/fs/eventpoll.c > > @@ -188,6 +188,12 @@ struct eventpoll { > > > > /* The user that created the eventpoll descriptor */ > > struct user_struct *user; > > + > > + struct file *file; > > + > > + /* used to optimize loop detection check */ > > + int visited; > > + struct list_head visitedllink; > > Strange name. Can we improve this? Something like visited_loop_link? > > > };
sure.
> > > > /* Wait structure used by the poll hooks */ > > @@ -246,6 +252,12 @@ static struct kmem_cache *epi_cache __read_mostly; > > /* Slab cache used to allocate "struct eppoll_entry" */ > > static struct kmem_cache *pwq_cache __read_mostly; > > > > +/* Visited nodes during ep_loop_check(), so we can unset them when we finish */ > > +LIST_HEAD(visited_list); > > static > > > +/* Files with newly added links, which need a limit on emanating paths */ > > +LIST_HEAD(tfile_check_list); > > static > > Add a comment describing the locking for this. > > That locking will need to be kernel-wide, which might have scalability > issues? >
The 'tfile_check_list' is created and checked only during an epoll_ctl(..., EPOLL_CTL_ADD, ...) operation. After the add finishes, the list is cleared. I am using the 'epmutex' to protect the list. The possible contention is:
1) another epoll add 2) another epoll remove 3) if any of the files in the list are closed with no more references - unlink.
Case #1 was already taking the epmutex in some case, I've extened it to all case.
case #2 was not using the epmutex, I've added it.
case #3 was already using the epmutex. no changes with this path. see: eventpoll_release().
So, I believe any scalability issues, would be within epoll itself. That said, it is limited to teardown and setup, which in theory are not the hot-paths. I would think that the wakeups and polling paths are the more critical paths, which aren't affected by this path. But certainly we need to test this more...
> > > > ... > > > > @@ -915,6 +927,96 @@ static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi) > > rb_insert_color(&epi->rbn, &ep->rbr); > > } > > > > + > > + > > +#define PATH_ARR_SIZE 5 > > +/* These are the number paths of length 1 to 5, that we are allowing to emanate > > The conventional comment layout is > > /* > * These are the number paths of length 1 to 5, that we are allowing to emanate > > > + * from a single file of interest. For example, we allow 1000 paths of length > > + * 1, to emanate from each file of interest. This essentially represents the > > + * potential wakeup paths, which need to be limited in order to avoid massive > > + * uncontrolled wakeup storms. The common use case should be a single ep which > > + * is connected to n file sources. In this case each file source has 1 path > > + * of length 1. Thus, the numbers below should be more than sufficient. > > + */ > > +int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 }; > > static const > > > +int path_count[PATH_ARR_SIZE]; > > static > > Add a comment describing the locking which protects this. > > That lock will necessarily be kernel-wide. Seems nasty. What are the > implications of this? >
see previous comments.
> > > > ... > > > > @@ -1264,18 +1379,35 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests) > > int error = 0; > > struct file *file = priv; > > struct eventpoll *ep = file->private_data; > > + struct eventpoll *ep_tovisit; > > struct rb_node *rbp; > > struct epitem *epi; > > > > mutex_lock(&ep->mtx); > > + ep->visited = 1; > > + list_add(&ep->visitedllink, &visited_list); > > for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) { > > epi = rb_entry(rbp, struct epitem, rbn); > > if (unlikely(is_file_epoll(epi->ffd.file))) { > > + ep_tovisit = epi->ffd.file->private_data; > > + if (ep_tovisit->visited) > > + continue; > > error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS, > > ep_loop_check_proc, epi->ffd.file, > > - epi->ffd.file->private_data, current); > > + ep_tovisit, current); > > if (error != 0) > > break; > > + } else { > > + /* if we've reached a file that is not associated with > > /* > * If > > > + * an ep, then then we need to check if the newly added > > s/then // > > > + * links are going to add too many wakeup paths. We do > > + * this by adding it to the tfile_check_list, if it's > > + * not already there, and calling reverse_path_check() > > + * during ep_insert() > > + */ > > + if (list_empty(&epi->ffd.file->f_tfile_llink)) > > + list_add(&epi->ffd.file->f_tfile_llink, > > + &tfile_check_list); > > I guess that tfile_check_list is protected by the big epmutex lock. >
yes.
> I assume you've verified that all paths that lead to manipulation of > all these new globals reliably take that lock. >
right, as mentioned the tfile_check_list is only manipulated directly on an epoll add path.
> > } > > } > > mutex_unlock(&ep->mtx); > > > > ... > > >
I'll probably look to re-spin this next week with your comments, since I'll be at plumbers the rest of the week...
Thanks,
-Jason
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