Re: [PATCH RFC 0/8] dcache: increase poison resistance

[Junxiao Bi]

Hi Konstantin,

We tested this patch set recently and found it limiting negative dentry 
to a small part of total memory. And also we don't see any performance 
regression on it. Do you have any plan to integrate it into mainline? It 
will help a lot on memory fragmentation issue causing by dentry slab, 
there were a lot of customer cases where sys% was very high since most 
cpu were doing memory compaction, dentry slab was taking too much memory 
and nearly all dentry there were negative.

The following is test result we run on two types of servers, one is 256G 
memory with 24 CPUS and another is 3T memory with 384 CPUS. The test 
case is using a lot of processes to generate negative dentry in 
parallel, the following is the test result after 72 hours, the negative 
dentry number is stable around that number even running longer time. If 
without the patch set, in less than half an hour 197G was took by 
negative dentry on 256G system, in 1 day 2.4T was took on 3T system.

                neg-dentry-number        neg-dentry-mem-usage

256G 55259084 10.6G

3T 202306756 38.8G

For perf test, we run the following, and no regression found.

- create 1M negative dentry and then touch them to convert them to 
positive dentry

- create 10K/100K/1M files

- remove 10K/100K/1M files

- kernel compile

To verify the fsnotify fix, we used inotifywait to watch file 
create/open in some directory where there is a lot of negative dentry, 
without the patch set, the system will run into soft lockup, with it, no 
soft lockup.

We also try to defeat the limitation by making different processes 
generating negative dentry with the same naming way, that will make one 
negative dentry being accessed couple times around same time, 
DCACHE_REFERENCED will be set on it and then it can't be trimmed easily. 
We do see negative dentry will take all the memory slowly from one of 
our system with 120G memory, for above two system, we see the memory 
usage were increased, but still a small part of total memory. This looks 
ok, since the common negative dentry user case will be create some temp 
files and then remove it, it will be rare to access same negative dentry 
around same time.

Thanks,

Junxiao.


On 5/8/20 5:23 AM, Konstantin Khlebnikov wrote:
> For most filesystems result of every negative lookup is cached, content of
> directories is usually cached too. Production of negative dentries isn't
> limited with disk speed. It's really easy to generate millions of them if
> system has enough memory.
>
> Getting this memory back ins't that easy because slab frees pages only when
> all related objects are gone. While dcache shrinker works in LRU order.
>
> Typical scenario is an idle system where some process periodically creates
> temporary files and removes them. After some time, memory will be filled
> with negative dentries for these random file names.
>
> Simple lookup of random names also generates negative dentries very fast.
> Constant flow of such negative denries drains all other inactive caches.
>
> Negative dentries are linked into siblings list along with normal positive
> dentries. Some operations walks dcache tree but looks only for positive
> dentries: most important is fsnotify/inotify. Hordes of negative dentries
> slow down these operations significantly.
>
> Time of dentry lookup is usually unaffected because hash table grows along
> with size of memory. Unless somebody especially crafts hash collisions.
>
> This patch set solves all of these problems:
>
> Move negative denries to the end of sliblings list, thus walkers could
> skip them at first sight (patches 3-6).
>
> Keep in dcache at most three unreferenced negative denties in row in each
> hash bucket (patches 7-8).
>
> ---
>
> Konstantin Khlebnikov (8):
>        dcache: show count of hash buckets in sysctl fs.dentry-state
>        selftests: add stress testing tool for dcache
>        dcache: sweep cached negative dentries to the end of list of siblings
>        fsnotify: stop walking child dentries if remaining tail is negative
>        dcache: add action D_WALK_SKIP_SIBLINGS to d_walk()
>        dcache: stop walking siblings if remaining dentries all negative
>        dcache: push releasing dentry lock into sweep_negative
>        dcache: prevent flooding with negative dentries
>
>
>   fs/dcache.c                                   | 144 +++++++++++-
>   fs/libfs.c                                    |  10 +-
>   fs/notify/fsnotify.c                          |   6 +-
>   include/linux/dcache.h                        |   6 +
>   tools/testing/selftests/filesystems/Makefile  |   1 +
>   .../selftests/filesystems/dcache_stress.c     | 210 ++++++++++++++++++
>   6 files changed, 370 insertions(+), 7 deletions(-)
>   create mode 100644 tools/testing/selftests/filesystems/dcache_stress.c
>
> --
> Signature