On Tue, May 3, 2011 at 4:18 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > Hi, > > On Mon, May 02, 2011 at 03:07:29PM -0500, James Bottomley wrote: > > The fatal livelock in kswapd, reported in this thread: > > > > http://marc.info/?t=130392066000001 > > > > Is mitigateable if we prevent the cgroups code being so aggressive in > > its zone shrinking (by reducing it's default shrink from 0 [everything] > > to DEF_PRIORITY [some things]). This will have an obvious knock on > > effect to cgroup accounting, but it's better than hanging systems. > > Actually, it's not that obvious. At least not to me. I added Balbir, > who added said comment and code in the first place, to CC: Here is the > comment in full quote: > > I missed this email in my inbox, just saw it and responding > /* > * NOTE: Although we can get the priority field, using it > * here is not a good idea, since it limits the pages we can scan. > * if we don't reclaim here, the shrink_zone from balance_pgdat > * will pick up pages from other mem cgroup's as well. We hack > * the priority and make it zero. > */ > > The idea is that if one memcg is above its softlimit, we prefer > reducing pages from this memcg over reclaiming random other pages, > including those of other memcgs. > > My comment and code were based on the observations I saw during my tests. With DEF_PRIORITY we see scan >> priority in get_scan_count(), since we know how much exactly we are over the soft limit, it makes sense to go after the pages, so that normal balancing can be restored. > But the code flow looks like this: > > balance_pgdat > mem_cgroup_soft_limit_reclaim > mem_cgroup_shrink_node_zone > shrink_zone(0, zone, &sc) > shrink_zone(prio, zone, &sc) > > so the success of the inner memcg shrink_zone does at least not > explicitely result in the outer, global shrink_zone steering clear of > other memcgs' pages. Yes, but it allows soft reclaim to know what to target first for success > It just tries to move the pressure of balancing > the zones to the memcg with the biggest soft limit excess. That can > only really work if the memcg is a large enough contributor to the > zone's total number of lru pages, though, and looks very likely to hit > the exceeding memcg too hard in other cases. > > I am very much for removing this hack. There is still more scan > pressure applied to memcgs in excess of their soft limit even if the > extra scan is happening at a sane priority level. And the fact that > global reclaim operates completely unaware of memcgs is a different > story. > > However, this code came into place with v2.6.31-8387-g4e41695. Why is > it only now showing up? > > You also wrote in that thread that this happens on a standard F15 > installation. On the F15 I am running here, systemd does not > configure memcgs, however. Did you manually configure memcgs and set > soft limits? Because I wonder how it ended up in soft limit reclaim > in the first place. > > I am running F15 as well, but never hit the problem so far. I am surprised to see the stack posted on the thread, it seemed like you never explicitly enabled anything to wake up the memcg beast :) Balbir _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
