On Wed, Dec 02, 2020 at 10:27:16AM -0800, Yang Shi wrote: > > 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@xxxxxxxxx/. > 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@xxxxxxxxxxxxx/) > > 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. Hello Yang! The rationale is very well described and makes perfect sense to me. I fully support the idea to make nr_deferred per-memcg. Thank you for such a detailed description! More comments in individual patches. Thanks! > > Yang Shi (9): > mm: vmscan: simplify nr_deferred update code > mm: vmscan: use nid from shrink_control for tracepoint > mm: memcontrol: rename memcg_shrinker_map_mutex to memcg_shrinker_mutex > mm: vmscan: use a new flag to indicate shrinker is registered > mm: memcontrol: add per memcg shrinker nr_deferred > mm: vmscan: use per memcg nr_deferred of shrinker > mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers > mm: memcontrol: reparent nr_deferred when memcg offline > mm: vmscan: shrink deferred objects proportional to priority > > include/linux/memcontrol.h | 9 +++++ > include/linux/shrinker.h | 8 ++++ > mm/memcontrol.c | 148 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---- > mm/vmscan.c | 183 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------------------------------- > 4 files changed, 274 insertions(+), 74 deletions(-) >
