On Sun, Nov 20, 2016 at 03:27:07PM -0800, Linus Torvalds wrote: > On Sun, Nov 20, 2016 at 2:27 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > > > > Hosts with ~100,000 threads have an issue with /prov/vmallocinfo > > > > It can take about 800 usec to skip over ~100,000 struct vmap_area > > in s_start(), while holding vmap_area_lock spinlock, and therefore > > blocking fork()/pthread_create(). > > > > I presume we can not switch to the rbtree (vmap_area_root) > > for /proc/vmallocinfo, because this file is seek-able, right ? > > Well, the good news is that the file is root-only anyway, which means > that at least it won't have the issue that a lot of other /proc files > have had - namely being opened by random user programs or libraries. > > Which means that the users of it are likely fairly limited. > > Which in turn means that we can probably afford to play more games > with it. Including, for example, possibly marking it non-seekable. > > Or even just limit the maximum entries we are willing to walk. > > Or we could decide that that file shouldn't be a seq_file at all, use > the old "one page buffer" approach that was so common for /proc files, > and make the position encode the vmalloc address in it (make the lower > PAGE_MASK bits be the offset in the line), and then we *could* just > look things up using the btree method. > > Al, do you have any clever ideas?
Umm... One possibility would be something like fs/namespace.c:m_start() - if nothing has changed since the last time, just use a cached pointer. That has sped the damn thing (/proc/mounts et.al.) big way, but it's dependent upon having an event count updated whenever we change the mount tree - doing the same for vma_area list might or might not be a good idea. /proc/mounts and friends get ->poll() on that as well; that probably would _not_ be a good idea in this case.
