From: Yang Shi shy828301@gmail.com
mainline inclusion from mainline-v5.13-rc1 commit 8efb4b596df05f004e847d6bfadad3492b766ab3 category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/I48N0H CVE: NA
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Patch series "Make shrinker's nr_deferred memcg aware", v10.
Recently huge amount one-off slab drop was seen on some vfs metadata heavy workloads, it turned out there were huge amount accumulated nr_deferred objects seen by the shrinker.
On our production machine, I saw absurd number of nr_deferred shown as the below tracing result:
<...>-48776 [032] .... 27970562.458916: mm_shrink_slab_start: super_cache_scan+0x0/0x1a0 ffff9a83046f3458: nid: 0 objects to shrink 2531805877005 gfp_flags GFP_HIGHUSER_MOVABLE pgs_scanned 32 lru_pgs 9300 cache items 1667 delta 11 total_scan 833
There are 2.5 trillion deferred objects on one node, assuming all of them are dentry (192 bytes per object), so the total size of deferred on one node is ~480TB. It is definitely ridiculous.
I managed to reproduce this problem with kernel build workload plus negative dentry generator.
First step, run the below kernel build test script:
NR_CPUS=`cat /proc/cpuinfo | grep -e processor | wc -l`
cd /root/Buildarea/linux-stable
for i in `seq 1500`; do cgcreate -g memory:kern_build echo 4G > /sys/fs/cgroup/memory/kern_build/memory.limit_in_bytes
echo 3 > /proc/sys/vm/drop_caches cgexec -g memory:kern_build make clean > /dev/null 2>&1 cgexec -g memory:kern_build make -j$NR_CPUS > /dev/null 2>&1
cgdelete -g memory:kern_build done
Then run the below negative dentry generator script:
NR_CPUS=`cat /proc/cpuinfo | grep -e processor | wc -l`
mkdir /sys/fs/cgroup/memory/test echo $$ > /sys/fs/cgroup/memory/test/tasks
for i in `seq $NR_CPUS`; do while true; do FILE=`head /dev/urandom | tr -dc A-Za-z0-9 | head -c 64` cat $FILE 2>/dev/null done & done
Then kswapd will shrink half of dentry cache in just one loop as the below tracing result showed:
kswapd0-475 [028] .... 305968.252561: mm_shrink_slab_start: super_cache_scan+0x0/0x190 0000000024acf00c: nid: 0 objects to shrink 4994376020 gfp_flags GFP_KERNEL cache items 93689873 delta 45746 total_scan 46844936 priority 12 kswapd0-475 [021] .... 306013.099399: mm_shrink_slab_end: super_cache_scan+0x0/0x190 0000000024acf00c: nid: 0 unused scan count 4994376020 new scan count 4947576838 total_scan 8 last shrinker return val 46844928
There were huge number of deferred objects before the shrinker was called, the behavior does match the code but it might be not desirable from the user's stand of point.
The excessive amount of nr_deferred might be accumulated due to various reasons, for example:
* GFP_NOFS allocation
* Significant times of small amount scan (< scan_batch, 1024 for vfs metadata)
However the LRUs of slabs are per memcg (memcg-aware shrinkers) but the deferred objects is per shrinker, this may have some bad effects:
* Poor isolation among memcgs. Some memcgs which happen to have frequent limit reclaim may get nr_deferred accumulated to a huge number, then other innocent memcgs may take the fall. In our case the main workload was hit.
* Unbounded deferred objects. There is no cap for deferred objects, it can outgrow ridiculously as the tracing result showed.
* Easy to get out of control. Although shrinkers take into account deferred objects, but it can go out of control easily. One misconfigured memcg could incur absurd amount of deferred objects in a period of time.
* Sort of reclaim problems, i.e. over reclaim, long reclaim latency, etc. There may be hundred GB slab caches for vfe metadata heavy workload, shrink half of them may take minutes. We observed latency spike due to the prolonged reclaim.
These issues also have been discussed in https://lore.kernel.org/linux-mm/20200916185823.5347-1-shy828301@gmail.com/. The patchset is the outcome of that discussion.
So this patchset makes nr_deferred per-memcg to tackle the problem. It does:
* Have memcg_shrinker_deferred per memcg per node, just like what shrinker_map does. Instead it is an atomic_long_t array, each element represent one shrinker even though the shrinker is not memcg aware, this simplifies the implementation. For memcg aware shrinkers, the deferred objects are just accumulated to its own memcg. The shrinkers just see nr_deferred from its own memcg. Non memcg aware shrinkers still use global nr_deferred from struct shrinker.
* Once the memcg is offlined, its nr_deferred will be reparented to its parent along with LRUs.
* The root memcg has memcg_shrinker_deferred array too. It simplifies the handling of reparenting to root memcg.
* Cap nr_deferred to 2x of the length of lru. The idea is borrowed from Dave Chinner's series (https://lore.kernel.org/linux-xfs/20191031234618.15403-1-david@fromorbit.com...)
The downside is each memcg has to allocate extra memory to store the nr_deferred array. On our production environment, there are typically around 40 shrinkers, so each memcg needs ~320 bytes. 10K memcgs would need ~3.2MB memory. It seems fine.
We have been running the patched kernel on some hosts of our fleet (test and production) for months, it works very well. The monitor data shows the working set is sustained as expected.
This patch (of 13):
The tracepoint's nid should show what node the shrink happens on, the start tracepoint uses nid from shrinkctl, but the nid might be set to 0 before end tracepoint if the shrinker is not NUMA aware, so the tracing log may show the shrink happens on one node but end up on the other node. It seems confusing. And the following patch will remove using nid directly in do_shrink_slab(), this patch also helps cleanup the code.
Link: https://lkml.kernel.org/r/20210311190845.9708-1-shy828301@gmail.com Link: https://lkml.kernel.org/r/20210311190845.9708-2-shy828301@gmail.com Signed-off-by: Yang Shi shy828301@gmail.com Acked-by: Vlastimil Babka vbabka@suse.cz Acked-by: Kirill Tkhai ktkhai@virtuozzo.com Reviewed-by: Shakeel Butt shakeelb@google.com Acked-by: Roman Gushchin guro@fb.com Cc: Dave Chinner david@fromorbit.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@suse.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Chen Wandun chenwandun@huawei.com Reviewed-by: Tong Tiangen tongtiangen@huawei.com Signed-off-by: Zheng Zengkai zhengzengkai@huawei.com --- mm/vmscan.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c index 244953ea73a8..66a553e90046 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -538,7 +538,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, else new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
- trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan); + trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan); return freed; }