| Message ID | 20241212115754.38f798b3@fangorn (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] memcg: allow exiting tasks to write back data to swap | expand |
On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > A task already in exit can get stuck trying to allocate pages, if its > cgroup is at the memory.max limit, the cgroup is using zswap, but > zswap writeback is enabled, and the remaining memory in the cgroup is > not compressible. > > This seems like an unlikely confluence of events, but it can happen > quite easily if a cgroup is OOM killed due to exceeding its memory.max > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > When this happens, it can sometimes take hours for tasks to exit, > as they are all trying to squeeze things into zswap to bring the group's > memory consumption below memory.max. > > Allowing these exiting programs to push some memory from their own > cgroup into swap allows them to quickly bring the cgroup's memory > consumption below memory.max, and exit in seconds rather than hours. > > Signed-off-by: Rik van Riel <riel@surriel.com> Thanks for sending a v2. I still think maybe this needs to be fixed on the memcg side, at least by not making exiting tasks try really hard to reclaim memory to the point where this becomes a problem. IIUC there could be other reasons why reclaim may take too long, but maybe not as pathological as this case to be fair. I will let the memcg maintainers chime in for this. If there's a fundamental reason why this cannot be fixed on the memcg side, I don't object to this change. Nhat, any objections on your end? I think your fleet workloads were the first users of this interface. Does this break their expectations? > --- > v2: use mm_match_cgroup as suggested by Yosry Ahmed > > mm/memcontrol.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 7b3503d12aaf..ba1cd9c04a02 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -5371,6 +5371,18 @@ bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg) > if (!zswap_is_enabled()) > return true; > > + /* > + * Always allow exiting tasks to push data to swap. A process in > + * the middle of exit cannot get OOM killed, but may need to push > + * uncompressible data to swap in order to get the cgroup memory > + * use below the limit, and make progress with the exit. > + */ > + if (unlikely(current->flags & PF_EXITING)) { > + struct mm_struct *mm = READ_ONCE(current->mm); > + if (mm && mm_match_cgroup(mm, memcg)) > + return true; > + } > + > for (; memcg; memcg = parent_mem_cgroup(memcg)) > if (!READ_ONCE(memcg->zswap_writeback)) > return false; > -- > 2.47.0 > >
On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > > > A task already in exit can get stuck trying to allocate pages, if its > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > zswap writeback is enabled, and the remaining memory in the cgroup is > > not compressible. > > > > This seems like an unlikely confluence of events, but it can happen > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > When this happens, it can sometimes take hours for tasks to exit, > > as they are all trying to squeeze things into zswap to bring the group's > > memory consumption below memory.max. > > > > Allowing these exiting programs to push some memory from their own > > cgroup into swap allows them to quickly bring the cgroup's memory > > consumption below memory.max, and exit in seconds rather than hours. > > > > Signed-off-by: Rik van Riel <riel@surriel.com> > > Thanks for sending a v2. > > I still think maybe this needs to be fixed on the memcg side, at least > by not making exiting tasks try really hard to reclaim memory to the > point where this becomes a problem. IIUC there could be other reasons > why reclaim may take too long, but maybe not as pathological as this > case to be fair. I will let the memcg maintainers chime in for this. > > If there's a fundamental reason why this cannot be fixed on the memcg > side, I don't object to this change. > > Nhat, any objections on your end? I think your fleet workloads were > the first users of this interface. Does this break their expectations? > Let me give my personal take. This seems like a stopgap or a quick hack to resolve the very specific situation happening in real world. I am ok with having this solution but only temporarily. The reason why I think this is short term fix or a quick hack is because it is not specifically solving the fundamental issue here. The same situation can reoccur if let's say the swap storage was slow or stuck or contended. A somewhat similar situation is when there are lot of unreclaimable memory either through pinning or maybe mlock. The fundamental issue is that the exiting process (killed by oomd or simple exit) has to allocated memory but the cgroup is at limit and the reclaim is very very slow. I can see attacking this issue with multiple angles. Some mixture of reusing kernel's oom reaper and some buffer to allow the exiting process to go over the limit. Let's brainstorm and explore this direction. In the meantime, I think we can have this stopgap solution.
On Thu, 2024-12-12 at 09:51 -0800, Shakeel Butt wrote: > > The fundamental issue is that the exiting process (killed by oomd or > simple exit) has to allocated memory but the cgroup is at limit and > the > reclaim is very very slow. > > I can see attacking this issue with multiple angles. Besides your proposed ideas, I suppose we could also limit the gfp_mask of an exiting reclaimer with eg. __GFP_NORETRY, but I do not know how effective that would be, since a single pass through the memory reclaim code was still taking dozens of seconds when I traced the "stuck" workloads.
On Thu, Dec 12, 2024 at 9:07 AM Yosry Ahmed <yosryahmed@google.com> wrote: > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > I still think maybe this needs to be fixed on the memcg side, at least > by not making exiting tasks try really hard to reclaim memory to the > point where this becomes a problem. IIUC there could be other reasons > why reclaim may take too long, but maybe not as pathological as this > case to be fair. I will let the memcg maintainers chime in for this. FWIW, we did have some internal discussions regarding this. We think that for now, this is a good-enough stopgap solution - it remains to be seen whether other more "permanent fixes" are needed, or will not also regress other scenarios. And they are definitely more complicated than the solution Rik is proposing here :) > > If there's a fundamental reason why this cannot be fixed on the memcg > side, I don't object to this change. > > Nhat, any objections on your end? I think your fleet workloads were > the first users of this interface. Does this break their expectations? I had similar concerns as yours, so we rolled the solution to the hosts in trouble. AFAICS: 1. It allowed the pathological workload to make forward progress with the exiting procedure. 2. The other workloads (who also have memory.zswap.writeback disabled) did not observe any regression.
On Thu, Dec 12, 2024 at 10:03 AM Rik van Riel <riel@surriel.com> wrote: > > On Thu, 2024-12-12 at 09:51 -0800, Shakeel Butt wrote: > > > > The fundamental issue is that the exiting process (killed by oomd or > > simple exit) has to allocated memory but the cgroup is at limit and > > the > > reclaim is very very slow. > > > > I can see attacking this issue with multiple angles. > > Besides your proposed ideas, I suppose we could also limit > the gfp_mask of an exiting reclaimer with eg. __GFP_NORETRY, > but I do not know how effective that would be, since a single > pass through the memory reclaim code was still taking dozens > of seconds when I traced the "stuck" workloads. I know we already discussed this, but it'd be nice if we can let the exiting task go ahead with the page fault and bypass the memory limits, if the page fault is crucial for it to make forward progress. Not sure how feasible that is, and how to decide which page fault is really crucial though :) For the pathological memory.zswap.writeback disabling case in particular, another thing we can do here is to make these incompressible pages ineligible for further reclaim attempt, either by putting them on a non-reclaim LRU, or putting them in the zswap LRU to maintain total ordering of the LRUs. That way we can move on to other sources (slab caches for example) quicker, or fail earlier? That said, it remains to be seen what will happen if these incompressible pages are literally all that are left...? I'm biased to this idea though, because they have other benefits. Maybe I'm just looking for excuses to revive the project ;)
On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > > > A task already in exit can get stuck trying to allocate pages, if its > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > zswap writeback is enabled, and the remaining memory in the cgroup is > > not compressible. > > > > This seems like an unlikely confluence of events, but it can happen > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > When this happens, it can sometimes take hours for tasks to exit, > > as they are all trying to squeeze things into zswap to bring the group's > > memory consumption below memory.max. > > > > Allowing these exiting programs to push some memory from their own > > cgroup into swap allows them to quickly bring the cgroup's memory > > consumption below memory.max, and exit in seconds rather than hours. > > > > Signed-off-by: Rik van Riel <riel@surriel.com> > > Thanks for sending a v2. > > I still think maybe this needs to be fixed on the memcg side, at least > by not making exiting tasks try really hard to reclaim memory to the > point where this becomes a problem. IIUC there could be other reasons > why reclaim may take too long, but maybe not as pathological as this > case to be fair. I will let the memcg maintainers chime in for this. > > If there's a fundamental reason why this cannot be fixed on the memcg > side, I don't object to this change. > > Nhat, any objections on your end? I think your fleet workloads were > the first users of this interface. Does this break their expectations? Yes, I don't think we can do this, unfortunately :( There can be a variety of reasons for why a user might want to prohibit disk swap for a certain cgroup, and we can't assume it's okay to make exceptions. There might also not *be* any disk swap to overflow into after Nhat's virtual swap patches. Presumably zram would still have the issue too. So I'm also inclined to think this needs a reclaim/memcg-side fix. We have a somewhat tumultous history of policy in that space: commit 7775face207922ea62a4e96b9cd45abfdc7b9840 Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Date: Tue Mar 5 15:46:47 2019 -0800 memcg: killed threads should not invoke memcg OOM killer allowed dying tasks to simply force all charges and move on. This turned out to be too aggressive; there were instances of exiting, uncontained memcg tasks causing global OOMs. This lead to that: commit a4ebf1b6ca1e011289677239a2a361fde4a88076 Author: Vasily Averin <vasily.averin@linux.dev> Date: Fri Nov 5 13:38:09 2021 -0700 memcg: prohibit unconditional exceeding the limit of dying tasks which reverted the bypass rather thoroughly. Now NO dying tasks, *not even OOM victims*, can force charges. I am not sure this is correct, either: If we return -ENOMEM to an OOM victim in a fault, the fault handler will re-trigger OOM, which will find the existing OOM victim and do nothing, then restart the fault. This is a memory deadlock. The page allocator gives OOM victims access to reserves for that reason. Actually, it looks even worse. For some reason we're not triggering OOM from dying tasks: ret = task_is_dying() || out_of_memory(&oc); Even though dying tasks are in no way privileged or allowed to exit expediently. Why shouldn't they trigger the OOM killer like anybody else trying to allocate memory? As it stands, it seems we have dying tasks getting trapped in an endless fault->reclaim cycle; with no access to the OOM killer and no access to reserves. Presumably this is what's going on here? I think we want something like this: diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 53db98d2c4a1..be6b6e72bde5 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1596,11 +1596,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask, if (mem_cgroup_margin(memcg) >= (1 << order)) goto unlock; - /* - * A few threads which were not waiting at mutex_lock_killable() can - * fail to bail out. Therefore, check again after holding oom_lock. - */ - ret = task_is_dying() || out_of_memory(&oc); + ret = out_of_memory(&oc); unlock: mutex_unlock(&oom_lock); @@ -2198,6 +2194,9 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, if (unlikely(current->flags & PF_MEMALLOC)) goto force; + if (unlikely(tsk_is_oom_victim(current))) + goto force; + if (unlikely(task_in_memcg_oom(current))) goto nomem;
On Thu, Dec 12, 2024 at 11:57:54AM -0500, Rik van Riel wrote: > A task already in exit can get stuck trying to allocate pages, if its > cgroup is at the memory.max limit, the cgroup is using zswap, but > zswap writeback is enabled, and the remaining memory in the cgroup is > not compressible. Is it about a single task or groups of tasks or the entire cgroup? If former, why it's a problem? A tight memcg limit can slow things down in general and I don't see why we should treat the exit() path differently. If it's about the entire cgroup and we have essentially a deadlock, I feel like we need to look into the oom reaper side. Alternatively we can allow to go over the limit in this case, but only assuming all tasks in the cgroup are going to die and no new task can enter it. Thanks!
On Thu, 12 Dec 2024 18:31:57 +0000 Roman Gushchin <roman.gushchin@linux.dev> wrote: > Is it about a single task or groups of tasks or the entire cgroup? > If former, why it's a problem? A tight memcg limit can slow things down > in general and I don't see why we should treat the exit() path differently. > I think the exit path does need to be treated a little differently, since this exit may be the only way such a cgroup can free up memory. > If it's about the entire cgroup and we have essentially a deadlock, > I feel like we need to look into the oom reaper side. You mean something like the below? I have not tested it yet, because we don't have any stuck cgroups right now among the workloads that I'm monitoring. ---8<--- From c0e545fd45bd3ee24cd79b3d3e9b375e968ef460 Mon Sep 17 00:00:00 2001 From: Rik van Riel <riel@surriel.com> Date: Thu, 12 Dec 2024 14:50:49 -0500 Subject: [PATCH] memcg,oom: speed up reclaim for exiting tasks When a memcg reaches its memory limit, and reclaim becomes unavailable or slow for some reason, for example only zswap is available, but zswap writeback is disabled, it can take a long time for tasks to exit, and for the cgroup to get back to normal (or cleaned up). Speed up memcg reclaim for exiting tasks by limiting how much work reclaim does, and by invoking the OOM reaper if reclaim does not free up enough memory to allow the task to make progress. Signed-off-by: Rik van Riel <riel@surriel.com> --- include/linux/oom.h | 8 ++++++++ mm/memcontrol.c | 11 +++++++++++ mm/oom_kill.c | 6 +----- 3 files changed, 20 insertions(+), 5 deletions(-) diff --git a/include/linux/oom.h b/include/linux/oom.h index 1e0fc6931ce9..b2d9cf936664 100644 --- a/include/linux/oom.h +++ b/include/linux/oom.h @@ -111,4 +111,12 @@ extern void oom_killer_enable(void); extern struct task_struct *find_lock_task_mm(struct task_struct *p); +#ifdef CONFIG_MMU +extern void queue_oom_reaper(struct task_struct *tsk); +#else +static intern void queue_oom_reaper(struct task_struct *tsk) +{ +} +#endif + #endif /* _INCLUDE_LINUX_OOM_H */ diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 7b3503d12aaf..21f42758d430 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2231,6 +2231,9 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, if (!gfpflags_allow_blocking(gfp_mask)) goto nomem; + if (unlikely(current->flags & PF_EXITING)) + gfp_mask |= __GFP_NORETRY; + memcg_memory_event(mem_over_limit, MEMCG_MAX); raised_max_event = true; @@ -2284,6 +2287,14 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, goto retry; } nomem: + /* + * We ran out of memory while inside exit. Maybe the OOM + * reaper can help reduce cgroup memory use and get things + * moving again? + */ + if (unlikely(current->flags & PF_EXITING)) + queue_oom_reaper(current); + /* * Memcg doesn't have a dedicated reserve for atomic * allocations. But like the global atomic pool, we need to diff --git a/mm/oom_kill.c b/mm/oom_kill.c index 1c485beb0b93..8d5278e45c63 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -686,7 +686,7 @@ static void wake_oom_reaper(struct timer_list *timer) * before the exit path is able to wake the futex waiters. */ #define OOM_REAPER_DELAY (2*HZ) -static void queue_oom_reaper(struct task_struct *tsk) +void queue_oom_reaper(struct task_struct *tsk) { /* mm is already queued? */ if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags)) @@ -735,10 +735,6 @@ static int __init oom_init(void) return 0; } subsys_initcall(oom_init) -#else -static inline void queue_oom_reaper(struct task_struct *tsk) -{ -} #endif /* CONFIG_MMU */ /**
On Thu, Dec 12, 2024 at 01:30:12PM -0500, Johannes Weiner wrote: > On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > > > > > A task already in exit can get stuck trying to allocate pages, if its > > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > > zswap writeback is enabled, and the remaining memory in the cgroup is > > > not compressible. > > > > > > This seems like an unlikely confluence of events, but it can happen > > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > > > When this happens, it can sometimes take hours for tasks to exit, > > > as they are all trying to squeeze things into zswap to bring the group's > > > memory consumption below memory.max. > > > > > > Allowing these exiting programs to push some memory from their own > > > cgroup into swap allows them to quickly bring the cgroup's memory > > > consumption below memory.max, and exit in seconds rather than hours. > > > > > > Signed-off-by: Rik van Riel <riel@surriel.com> > > > > Thanks for sending a v2. > > > > I still think maybe this needs to be fixed on the memcg side, at least > > by not making exiting tasks try really hard to reclaim memory to the > > point where this becomes a problem. IIUC there could be other reasons > > why reclaim may take too long, but maybe not as pathological as this > > case to be fair. I will let the memcg maintainers chime in for this. > > > > If there's a fundamental reason why this cannot be fixed on the memcg > > side, I don't object to this change. > > > > Nhat, any objections on your end? I think your fleet workloads were > > the first users of this interface. Does this break their expectations? > > Yes, I don't think we can do this, unfortunately :( There can be a > variety of reasons for why a user might want to prohibit disk swap for > a certain cgroup, and we can't assume it's okay to make exceptions. > > There might also not *be* any disk swap to overflow into after Nhat's > virtual swap patches. Presumably zram would still have the issue too. Very good points. > > So I'm also inclined to think this needs a reclaim/memcg-side fix. We > have a somewhat tumultous history of policy in that space: > > commit 7775face207922ea62a4e96b9cd45abfdc7b9840 > Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > Date: Tue Mar 5 15:46:47 2019 -0800 > > memcg: killed threads should not invoke memcg OOM killer > > allowed dying tasks to simply force all charges and move on. This > turned out to be too aggressive; there were instances of exiting, > uncontained memcg tasks causing global OOMs. This lead to that: > > commit a4ebf1b6ca1e011289677239a2a361fde4a88076 > Author: Vasily Averin <vasily.averin@linux.dev> > Date: Fri Nov 5 13:38:09 2021 -0700 > > memcg: prohibit unconditional exceeding the limit of dying tasks > > which reverted the bypass rather thoroughly. Now NO dying tasks, *not > even OOM victims*, can force charges. I am not sure this is correct, > either: > > If we return -ENOMEM to an OOM victim in a fault, the fault handler > will re-trigger OOM, which will find the existing OOM victim and do > nothing, then restart the fault. This is a memory deadlock. The page > allocator gives OOM victims access to reserves for that reason. > > Actually, it looks even worse. For some reason we're not triggering > OOM from dying tasks: > > ret = task_is_dying() || out_of_memory(&oc); > > Even though dying tasks are in no way privileged or allowed to exit > expediently. Why shouldn't they trigger the OOM killer like anybody > else trying to allocate memory? This is a very good point and actually out_of_memory() will mark the dying process as oom victim and put it in the oom reaper's list which should help further in such situation. > > As it stands, it seems we have dying tasks getting trapped in an > endless fault->reclaim cycle; with no access to the OOM killer and no > access to reserves. Presumably this is what's going on here? > > I think we want something like this: The following patch looks good to me. Let's test this out (hopefully Rik will be able to find a live impacted machine) and move forward with this fix. > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 53db98d2c4a1..be6b6e72bde5 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1596,11 +1596,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask, > if (mem_cgroup_margin(memcg) >= (1 << order)) > goto unlock; > > - /* > - * A few threads which were not waiting at mutex_lock_killable() can > - * fail to bail out. Therefore, check again after holding oom_lock. > - */ > - ret = task_is_dying() || out_of_memory(&oc); > + ret = out_of_memory(&oc); > > unlock: > mutex_unlock(&oom_lock); > @@ -2198,6 +2194,9 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, > if (unlikely(current->flags & PF_MEMALLOC)) > goto force; > > + if (unlikely(tsk_is_oom_victim(current))) > + goto force; > + > if (unlikely(task_in_memcg_oom(current))) > goto nomem; >
On Thu, Dec 12, 2024 at 1:35 PM Shakeel Butt <shakeel.butt@linux.dev> wrote: > > On Thu, Dec 12, 2024 at 01:30:12PM -0500, Johannes Weiner wrote: > > On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > > > > > > > A task already in exit can get stuck trying to allocate pages, if its > > > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > > > zswap writeback is enabled, and the remaining memory in the cgroup is > > > > not compressible. > > > > > > > > This seems like an unlikely confluence of events, but it can happen > > > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > > > > > When this happens, it can sometimes take hours for tasks to exit, > > > > as they are all trying to squeeze things into zswap to bring the group's > > > > memory consumption below memory.max. > > > > > > > > Allowing these exiting programs to push some memory from their own > > > > cgroup into swap allows them to quickly bring the cgroup's memory > > > > consumption below memory.max, and exit in seconds rather than hours. > > > > > > > > Signed-off-by: Rik van Riel <riel@surriel.com> > > > > > > Thanks for sending a v2. > > > > > > I still think maybe this needs to be fixed on the memcg side, at least > > > by not making exiting tasks try really hard to reclaim memory to the > > > point where this becomes a problem. IIUC there could be other reasons > > > why reclaim may take too long, but maybe not as pathological as this > > > case to be fair. I will let the memcg maintainers chime in for this. > > > > > > If there's a fundamental reason why this cannot be fixed on the memcg > > > side, I don't object to this change. > > > > > > Nhat, any objections on your end? I think your fleet workloads were > > > the first users of this interface. Does this break their expectations? > > > > Yes, I don't think we can do this, unfortunately :( There can be a > > variety of reasons for why a user might want to prohibit disk swap for > > a certain cgroup, and we can't assume it's okay to make exceptions. > > > > There might also not *be* any disk swap to overflow into after Nhat's > > virtual swap patches. Presumably zram would still have the issue too. > > Very good points. > > > > > So I'm also inclined to think this needs a reclaim/memcg-side fix. We > > have a somewhat tumultous history of policy in that space: > > > > commit 7775face207922ea62a4e96b9cd45abfdc7b9840 > > Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > > Date: Tue Mar 5 15:46:47 2019 -0800 > > > > memcg: killed threads should not invoke memcg OOM killer > > > > allowed dying tasks to simply force all charges and move on. This > > turned out to be too aggressive; there were instances of exiting, > > uncontained memcg tasks causing global OOMs. This lead to that: > > > > commit a4ebf1b6ca1e011289677239a2a361fde4a88076 > > Author: Vasily Averin <vasily.averin@linux.dev> > > Date: Fri Nov 5 13:38:09 2021 -0700 > > > > memcg: prohibit unconditional exceeding the limit of dying tasks > > > > which reverted the bypass rather thoroughly. Now NO dying tasks, *not > > even OOM victims*, can force charges. I am not sure this is correct, > > either: > > > > If we return -ENOMEM to an OOM victim in a fault, the fault handler > > will re-trigger OOM, which will find the existing OOM victim and do > > nothing, then restart the fault. This is a memory deadlock. The page > > allocator gives OOM victims access to reserves for that reason. > > > > Actually, it looks even worse. For some reason we're not triggering > > OOM from dying tasks: > > > > ret = task_is_dying() || out_of_memory(&oc); > > > > Even though dying tasks are in no way privileged or allowed to exit > > expediently. Why shouldn't they trigger the OOM killer like anybody > > else trying to allocate memory? > > This is a very good point and actually out_of_memory() will mark the > dying process as oom victim and put it in the oom reaper's list which > should help further in such situation. > > > > > As it stands, it seems we have dying tasks getting trapped in an > > endless fault->reclaim cycle; with no access to the OOM killer and no > > access to reserves. Presumably this is what's going on here? > > > > I think we want something like this: > > The following patch looks good to me. Let's test this out (hopefully Rik > will be able to find a live impacted machine) and move forward with this > fix. I agree with this too. As Shakeel mentioned, this seemed like a stopgap and not an actual fix for the underlying problem. Johannes further outlined how the stopgap can be problematic. Let's try to fix this on the memcg/reclaim/OOM side, and properly treat dying tasks instead of forcing them into potentially super slow reclaim paths. Hopefully Johannes's patch fixes this.
On Thu, Dec 12, 2024 at 01:30:12PM -0500, Johannes Weiner wrote: > On Thu, Dec 12, 2024 at 09:06:25AM -0800, Yosry Ahmed wrote: > > On Thu, Dec 12, 2024 at 8:58 AM Rik van Riel <riel@surriel.com> wrote: > > > > > > A task already in exit can get stuck trying to allocate pages, if its > > > cgroup is at the memory.max limit, the cgroup is using zswap, but > > > zswap writeback is enabled, and the remaining memory in the cgroup is > > > not compressible. > > > > > > This seems like an unlikely confluence of events, but it can happen > > > quite easily if a cgroup is OOM killed due to exceeding its memory.max > > > limit, and all the tasks in the cgroup are trying to exit simultaneously. > > > > > > When this happens, it can sometimes take hours for tasks to exit, > > > as they are all trying to squeeze things into zswap to bring the group's > > > memory consumption below memory.max. > > > > > > Allowing these exiting programs to push some memory from their own > > > cgroup into swap allows them to quickly bring the cgroup's memory > > > consumption below memory.max, and exit in seconds rather than hours. > > > > > > Signed-off-by: Rik van Riel <riel@surriel.com> > > > > Thanks for sending a v2. > > > > I still think maybe this needs to be fixed on the memcg side, at least > > by not making exiting tasks try really hard to reclaim memory to the > > point where this becomes a problem. IIUC there could be other reasons > > why reclaim may take too long, but maybe not as pathological as this > > case to be fair. I will let the memcg maintainers chime in for this. > > > > If there's a fundamental reason why this cannot be fixed on the memcg > > side, I don't object to this change. > > > > Nhat, any objections on your end? I think your fleet workloads were > > the first users of this interface. Does this break their expectations? > > Yes, I don't think we can do this, unfortunately :( There can be a > variety of reasons for why a user might want to prohibit disk swap for > a certain cgroup, and we can't assume it's okay to make exceptions. > > There might also not *be* any disk swap to overflow into after Nhat's > virtual swap patches. Presumably zram would still have the issue too. > > So I'm also inclined to think this needs a reclaim/memcg-side fix. We > have a somewhat tumultous history of policy in that space: > > commit 7775face207922ea62a4e96b9cd45abfdc7b9840 > Author: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > Date: Tue Mar 5 15:46:47 2019 -0800 > > memcg: killed threads should not invoke memcg OOM killer > > allowed dying tasks to simply force all charges and move on. This > turned out to be too aggressive; there were instances of exiting, > uncontained memcg tasks causing global OOMs. This lead to that: > > commit a4ebf1b6ca1e011289677239a2a361fde4a88076 > Author: Vasily Averin <vasily.averin@linux.dev> > Date: Fri Nov 5 13:38:09 2021 -0700 > > memcg: prohibit unconditional exceeding the limit of dying tasks > > which reverted the bypass rather thoroughly. Now NO dying tasks, *not > even OOM victims*, can force charges. I am not sure this is correct, > either: > > If we return -ENOMEM to an OOM victim in a fault, the fault handler > will re-trigger OOM, which will find the existing OOM victim and do > nothing, then restart the fault. This is a memory deadlock. The page > allocator gives OOM victims access to reserves for that reason. > > Actually, it looks even worse. For some reason we're not triggering > OOM from dying tasks: > > ret = task_is_dying() || out_of_memory(&oc); > > Even though dying tasks are in no way privileged or allowed to exit > expediently. Why shouldn't they trigger the OOM killer like anybody > else trying to allocate memory? > > As it stands, it seems we have dying tasks getting trapped in an > endless fault->reclaim cycle; with no access to the OOM killer and no > access to reserves. Presumably this is what's going on here? > > I think we want something like this: > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 53db98d2c4a1..be6b6e72bde5 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1596,11 +1596,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask, > if (mem_cgroup_margin(memcg) >= (1 << order)) > goto unlock; > > - /* > - * A few threads which were not waiting at mutex_lock_killable() can > - * fail to bail out. Therefore, check again after holding oom_lock. > - */ > - ret = task_is_dying() || out_of_memory(&oc); > + ret = out_of_memory(&oc); I like the idea, but at first glance it might reintroduce the problem fixed by 7775face2079 ("memcg: killed threads should not invoke memcg OOM killer").
On Thu, Dec 12, 2024 at 03:00:03PM -0500, Rik van Riel wrote: > On Thu, 12 Dec 2024 18:31:57 +0000 > Roman Gushchin <roman.gushchin@linux.dev> wrote: > > > Is it about a single task or groups of tasks or the entire cgroup? > > If former, why it's a problem? A tight memcg limit can slow things down > > in general and I don't see why we should treat the exit() path differently. > > > I think the exit path does need to be treated a little differently, > since this exit may be the only way such a cgroup can free up memory. It's true if all tasks in a cgroup are exiting. Otherwise there are other options (at least in theory). > > > If it's about the entire cgroup and we have essentially a deadlock, > > I feel like we need to look into the oom reaper side. > > You mean something like the below? > > I have not tested it yet, because we don't have any stuck > cgroups right now among the workloads that I'm monitoring. Yeah, something like this... Thanks!
On 12/13/24 07:00, Rik van Riel wrote: > On Thu, 12 Dec 2024 18:31:57 +0000 > Roman Gushchin <roman.gushchin@linux.dev> wrote: > >> Is it about a single task or groups of tasks or the entire cgroup? >> If former, why it's a problem? A tight memcg limit can slow things down >> in general and I don't see why we should treat the exit() path differently. >> > I think the exit path does need to be treated a little differently, > since this exit may be the only way such a cgroup can free up memory. > >> If it's about the entire cgroup and we have essentially a deadlock, >> I feel like we need to look into the oom reaper side. > > You mean something like the below? > > I have not tested it yet, because we don't have any stuck > cgroups right now among the workloads that I'm monitoring. > > ---8<--- > > From c0e545fd45bd3ee24cd79b3d3e9b375e968ef460 Mon Sep 17 00:00:00 2001 > From: Rik van Riel <riel@surriel.com> > Date: Thu, 12 Dec 2024 14:50:49 -0500 > Subject: [PATCH] memcg,oom: speed up reclaim for exiting tasks > > When a memcg reaches its memory limit, and reclaim becomes unavailable > or slow for some reason, for example only zswap is available, but zswap > writeback is disabled, it can take a long time for tasks to exit, and > for the cgroup to get back to normal (or cleaned up). > > Speed up memcg reclaim for exiting tasks by limiting how much work > reclaim does, and by invoking the OOM reaper if reclaim does not > free up enough memory to allow the task to make progress. > > Signed-off-by: Rik van Riel <riel@surriel.com> > --- > include/linux/oom.h | 8 ++++++++ > mm/memcontrol.c | 11 +++++++++++ > mm/oom_kill.c | 6 +----- > 3 files changed, 20 insertions(+), 5 deletions(-) > > diff --git a/include/linux/oom.h b/include/linux/oom.h > index 1e0fc6931ce9..b2d9cf936664 100644 > --- a/include/linux/oom.h > +++ b/include/linux/oom.h > @@ -111,4 +111,12 @@ extern void oom_killer_enable(void); > > extern struct task_struct *find_lock_task_mm(struct task_struct *p); > > +#ifdef CONFIG_MMU > +extern void queue_oom_reaper(struct task_struct *tsk); > +#else > +static intern void queue_oom_reaper(struct task_struct *tsk) > +{ > +} > +#endif > + > #endif /* _INCLUDE_LINUX_OOM_H */ > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 7b3503d12aaf..21f42758d430 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2231,6 +2231,9 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, > if (!gfpflags_allow_blocking(gfp_mask)) > goto nomem; > > + if (unlikely(current->flags & PF_EXITING)) > + gfp_mask |= __GFP_NORETRY; > + if (task_is_dying()) gfp_mask |= __GFP_NORETRY > memcg_memory_event(mem_over_limit, MEMCG_MAX); > raised_max_event = true; > > @@ -2284,6 +2287,14 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask, > goto retry; > } > nomem: > + /* > + * We ran out of memory while inside exit. Maybe the OOM > + * reaper can help reduce cgroup memory use and get things > + * moving again? > + */ > + if (unlikely(current->flags & PF_EXITING)) > + queue_oom_reaper(current); > + I am not sure this is helpful without task_will_free_mem(), the dying task shouldn't get sent to the OOM killer when we run out of memory. I did notice that we have heuristics around task_is_dying() and passed_oom, sounds like the end result of your changes would be to ignore the heuristics of passed_oom Balbir Singh.
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 7b3503d12aaf..ba1cd9c04a02 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5371,6 +5371,18 @@ bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg) if (!zswap_is_enabled()) return true; + /* + * Always allow exiting tasks to push data to swap. A process in + * the middle of exit cannot get OOM killed, but may need to push + * uncompressible data to swap in order to get the cgroup memory + * use below the limit, and make progress with the exit. + */ + if (unlikely(current->flags & PF_EXITING)) { + struct mm_struct *mm = READ_ONCE(current->mm); + if (mm && mm_match_cgroup(mm, memcg)) + return true; + } + for (; memcg; memcg = parent_mem_cgroup(memcg)) if (!READ_ONCE(memcg->zswap_writeback)) return false;
A task already in exit can get stuck trying to allocate pages, if its cgroup is at the memory.max limit, the cgroup is using zswap, but zswap writeback is enabled, and the remaining memory in the cgroup is not compressible. This seems like an unlikely confluence of events, but it can happen quite easily if a cgroup is OOM killed due to exceeding its memory.max limit, and all the tasks in the cgroup are trying to exit simultaneously. When this happens, it can sometimes take hours for tasks to exit, as they are all trying to squeeze things into zswap to bring the group's memory consumption below memory.max. Allowing these exiting programs to push some memory from their own cgroup into swap allows them to quickly bring the cgroup's memory consumption below memory.max, and exit in seconds rather than hours. Signed-off-by: Rik van Riel <riel@surriel.com> --- v2: use mm_match_cgroup as suggested by Yosry Ahmed mm/memcontrol.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)