[PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Xunlei Pang]

We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
the target memcg doesn't have any reclaimable memory.

It can be easily reproduced as below:
 watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204]
 CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12
 Call Trace:
  shrink_lruvec+0x49f/0x640
  shrink_node+0x2a6/0x6f0
  do_try_to_free_pages+0xe9/0x3e0
  try_to_free_mem_cgroup_pages+0xef/0x1f0
  try_charge+0x2c1/0x750
  mem_cgroup_charge+0xd7/0x240
  __add_to_page_cache_locked+0x2fd/0x370
  add_to_page_cache_lru+0x4a/0xc0
  pagecache_get_page+0x10b/0x2f0
  filemap_fault+0x661/0xad0
  ext4_filemap_fault+0x2c/0x40
  __do_fault+0x4d/0xf9
  handle_mm_fault+0x1080/0x1790

It only happens on our 1-vcpu instances, because there's no chance
for oom reaper to run to reclaim the to-be-killed process.

Add cond_resched() at the upper shrink_node_memcgs() to solve this
issue, and any other possible issue like meomry.min protection.

Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
---
 mm/vmscan.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 99e1796..bbdc38b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2617,6 +2617,8 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 
 		mem_cgroup_calculate_protection(target_memcg, memcg);
 
+		cond_resched();
+
 		if (mem_cgroup_below_min(memcg)) {
 			/*
 			 * Hard protection.
-- 
1.8.3.1

Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Chris Down]

Hi Xunlei,

Xunlei Pang writes:
>Add cond_resched() at the upper shrink_node_memcgs() to solve this
>issue, and any other possible issue like meomry.min protection.

I understand the general intent of the patch, but could you clarify your 
concern around memory protection?

Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Michal Hocko]

On Wed 26-08-20 21:47:02, Xunlei Pang wrote:
> We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
> the target memcg doesn't have any reclaimable memory.
> 
> It can be easily reproduced as below:
>  watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204]
>  CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12
>  Call Trace:
>   shrink_lruvec+0x49f/0x640
>   shrink_node+0x2a6/0x6f0
>   do_try_to_free_pages+0xe9/0x3e0
>   try_to_free_mem_cgroup_pages+0xef/0x1f0
>   try_charge+0x2c1/0x750
>   mem_cgroup_charge+0xd7/0x240
>   __add_to_page_cache_locked+0x2fd/0x370
>   add_to_page_cache_lru+0x4a/0xc0
>   pagecache_get_page+0x10b/0x2f0
>   filemap_fault+0x661/0xad0
>   ext4_filemap_fault+0x2c/0x40
>   __do_fault+0x4d/0xf9
>   handle_mm_fault+0x1080/0x1790
> 
> It only happens on our 1-vcpu instances, because there's no chance
> for oom reaper to run to reclaim the to-be-killed process.
> 
> Add cond_resched() at the upper shrink_node_memcgs() to solve this
> issue, and any other possible issue like meomry.min protection.

I would just add
"
This will mean that we will get a scheduling point for each memcg in the
reclaimed hierarchy without any dependency on the reclaimable memory in
that memcg thus making it more predictable.
" 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/vmscan.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 99e1796..bbdc38b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2617,6 +2617,8 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  
>  		mem_cgroup_calculate_protection(target_memcg, memcg);
>  
> +		cond_resched();
> +
>  		if (mem_cgroup_below_min(memcg)) {
>  			/*
>  			 * Hard protection.
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Johannes Weiner]

On Wed, Aug 26, 2020 at 09:47:02PM +0800, Xunlei Pang wrote:
> We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
> the target memcg doesn't have any reclaimable memory.
> 
> It can be easily reproduced as below:
>  watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204]
>  CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12
>  Call Trace:
>   shrink_lruvec+0x49f/0x640
>   shrink_node+0x2a6/0x6f0
>   do_try_to_free_pages+0xe9/0x3e0
>   try_to_free_mem_cgroup_pages+0xef/0x1f0
>   try_charge+0x2c1/0x750
>   mem_cgroup_charge+0xd7/0x240
>   __add_to_page_cache_locked+0x2fd/0x370
>   add_to_page_cache_lru+0x4a/0xc0
>   pagecache_get_page+0x10b/0x2f0
>   filemap_fault+0x661/0xad0
>   ext4_filemap_fault+0x2c/0x40
>   __do_fault+0x4d/0xf9
>   handle_mm_fault+0x1080/0x1790
> 
> It only happens on our 1-vcpu instances, because there's no chance
> for oom reaper to run to reclaim the to-be-killed process.
> 
> Add cond_resched() at the upper shrink_node_memcgs() to solve this
> issue, and any other possible issue like meomry.min protection.
> 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>

This generally makes sense to me but really should have a comment:

	/*
	 * This loop can become CPU-bound when there are thousands
	 * of cgroups that aren't eligible for reclaim - either
	 * because they don't have any pages, or because their
	 * memory is explicitly protected. Avoid soft lockups.
	 */
	 cond_resched();

The placement in the middle of the multi-part protection checks is a
bit odd too. It would be better to have it either at the top of the
loop, or at the end, by replacing the continues with goto next.

Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Michal Hocko]

On Wed 26-08-20 12:43:32, Johannes Weiner wrote:
> On Wed, Aug 26, 2020 at 09:47:02PM +0800, Xunlei Pang wrote:
> > We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
> > the target memcg doesn't have any reclaimable memory.
> > 
> > It can be easily reproduced as below:
> >  watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204]
> >  CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12
> >  Call Trace:
> >   shrink_lruvec+0x49f/0x640
> >   shrink_node+0x2a6/0x6f0
> >   do_try_to_free_pages+0xe9/0x3e0
> >   try_to_free_mem_cgroup_pages+0xef/0x1f0
> >   try_charge+0x2c1/0x750
> >   mem_cgroup_charge+0xd7/0x240
> >   __add_to_page_cache_locked+0x2fd/0x370
> >   add_to_page_cache_lru+0x4a/0xc0
> >   pagecache_get_page+0x10b/0x2f0
> >   filemap_fault+0x661/0xad0
> >   ext4_filemap_fault+0x2c/0x40
> >   __do_fault+0x4d/0xf9
> >   handle_mm_fault+0x1080/0x1790
> > 
> > It only happens on our 1-vcpu instances, because there's no chance
> > for oom reaper to run to reclaim the to-be-killed process.
> > 
> > Add cond_resched() at the upper shrink_node_memcgs() to solve this
> > issue, and any other possible issue like meomry.min protection.
> > 
> > Suggested-by: Michal Hocko <mhocko@suse.com>
> > Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
> 
> This generally makes sense to me but really should have a comment:
> 
> 	/*
> 	 * This loop can become CPU-bound when there are thousands
> 	 * of cgroups that aren't eligible for reclaim - either
> 	 * because they don't have any pages, or because their
> 	 * memory is explicitly protected. Avoid soft lockups.
> 	 */
> 	 cond_resched();
> 
> The placement in the middle of the multi-part protection checks is a
> bit odd too. It would be better to have it either at the top of the
> loop, or at the end, by replacing the continues with goto next.

Yes makes sense. I would stick it to the begining of the loop to make it
stand out and make it obvious wrt code flow.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Chris Down]

Chris Down writes:
>Xunlei Pang writes:
>>Add cond_resched() at the upper shrink_node_memcgs() to solve this
>>issue, and any other possible issue like meomry.min protection.
>
>I understand the general intent of the patch, but could you clarify 
>your concern around memory protection?

Oh, I see, your concern is just preemption in general rather than a fixing 
anything for the memory protection side. In which case, go for it, but I agree 
with Michael that it would be nice to send v3 with a clarifying comment.

Acked-by: Chris Down <chris@chrisdown.name>