Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless

[Alan Huang <mmpgouride@gmail.com>]

> 2023年6月23日 上午12:42，Qi Zheng <zhengqi.arch@bytedance.com> 写道：
> 
> 
> 
> On 2023/6/22 23:12, Vlastimil Babka wrote:
>> On 6/22/23 10:53, Qi Zheng wrote:
>>> The shrinker_rwsem is a global read-write lock in
>>> shrinkers subsystem, which protects most operations
>>> such as slab shrink, registration and unregistration
>>> of shrinkers, etc. This can easily cause problems in
>>> the following cases.
>>> 
>>> 1) When the memory pressure is high and there are many
>>>    filesystems mounted or unmounted at the same time,
>>>    slab shrink will be affected (down_read_trylock()
>>>    failed).
>>> 
>>>    Such as the real workload mentioned by Kirill Tkhai:
>>> 
>>>    ```
>>>    One of the real workloads from my experience is start
>>>    of an overcommitted node containing many starting
>>>    containers after node crash (or many resuming containers
>>>    after reboot for kernel update). In these cases memory
>>>    pressure is huge, and the node goes round in long reclaim.
>>>    ```
>>> 
>>> 2) If a shrinker is blocked (such as the case mentioned
>>>    in [1]) and a writer comes in (such as mount a fs),
>>>    then this writer will be blocked and cause all
>>>    subsequent shrinker-related operations to be blocked.
>>> 
>>> Even if there is no competitor when shrinking slab, there
>>> may still be a problem. If we have a long shrinker list
>>> and we do not reclaim enough memory with each shrinker,
>>> then the down_read_trylock() may be called with high
>>> frequency. Because of the poor multicore scalability of
>>> atomic operations, this can lead to a significant drop
>>> in IPC (instructions per cycle).
>>> 
>>> We used to implement the lockless slab shrink with
>>> SRCU [1], but then kernel test robot reported -88.8%
>>> regression in stress-ng.ramfs.ops_per_sec test case [2],
>>> so we reverted it [3].
>>> 
>>> This commit uses the refcount+RCU method [4] proposed by
>>> by Dave Chinner to re-implement the lockless global slab
>>> shrink. The memcg slab shrink is handled in the subsequent
>>> patch.
>>> 
>>> Currently, the shrinker instances can be divided into
>>> the following three types:
>>> 
>>> a) global shrinker instance statically defined in the kernel,
>>> such as workingset_shadow_shrinker.
>>> 
>>> b) global shrinker instance statically defined in the kernel
>>> modules, such as mmu_shrinker in x86.
>>> 
>>> c) shrinker instance embedded in other structures.
>>> 
>>> For case a, the memory of shrinker instance is never freed.
>>> For case b, the memory of shrinker instance will be freed
>>> after the module is unloaded. But we will call synchronize_rcu()
>>> in free_module() to wait for RCU read-side critical section to
>>> exit. For case c, the memory of shrinker instance will be
>>> dynamically freed by calling kfree_rcu(). So we can use
>>> rcu_read_{lock,unlock}() to ensure that the shrinker instance
>>> is valid.
>>> 
>>> The shrinker::refcount mechanism ensures that the shrinker
>>> instance will not be run again after unregistration. So the
>>> structure that records the pointer of shrinker instance can be
>>> safely freed without waiting for the RCU read-side critical
>>> section.
>>> 
>>> In this way, while we implement the lockless slab shrink, we
>>> don't need to be blocked in unregister_shrinker() to wait
>>> RCU read-side critical section.
>>> 
>>> The following are the test results:
>>> 
>>> stress-ng --timeout 60 --times --verify --metrics-brief --ramfs 9 &
>>> 
>>> 1) Before applying this patchset:
>>> 
>>>  setting to a 60 second run per stressor
>>>  dispatching hogs: 9 ramfs
>>>  stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
>>>                            (secs)    (secs)    (secs)   (real time) (usr+sys time)
>>>  ramfs            880623     60.02      7.71    226.93     14671.45        3753.09
>>>  ramfs:
>>>           1 System Management Interrupt
>>>  for a 60.03s run time:
>>>     5762.40s available CPU time
>>>        7.71s user time   (  0.13%)
>>>      226.93s system time (  3.94%)
>>>      234.64s total time  (  4.07%)
>>>  load average: 8.54 3.06 2.11
>>>  passed: 9: ramfs (9)
>>>  failed: 0
>>>  skipped: 0
>>>  successful run completed in 60.03s (1 min, 0.03 secs)
>>> 
>>> 2) After applying this patchset:
>>> 
>>>  setting to a 60 second run per stressor
>>>  dispatching hogs: 9 ramfs
>>>  stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
>>>                            (secs)    (secs)    (secs)   (real time) (usr+sys time)
>>>  ramfs            847562     60.02      7.44    230.22     14120.66        3566.23
>>>  ramfs:
>>>           4 System Management Interrupts
>>>  for a 60.12s run time:
>>>     5771.95s available CPU time
>>>        7.44s user time   (  0.13%)
>>>      230.22s system time (  3.99%)
>>>      237.66s total time  (  4.12%)
>>>  load average: 8.18 2.43 0.84
>>>  passed: 9: ramfs (9)
>>>  failed: 0
>>>  skipped: 0
>>>  successful run completed in 60.12s (1 min, 0.12 secs)
>>> 
>>> We can see that the ops/s has hardly changed.
>>> 
>>> [1]. https://lore.kernel.org/lkml/20230313112819.38938-1-zhengqi.arch@bytedance.com/
>>> [2]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@intel.com/
>>> [3]. https://lore.kernel.org/all/20230609081518.3039120-1-qi.zheng@linux.dev/
>>> [4]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/
>>> 
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>> ---
>>>  include/linux/shrinker.h |  6 ++++++
>>>  mm/vmscan.c              | 33 ++++++++++++++-------------------
>>>  2 files changed, 20 insertions(+), 19 deletions(-)
>>> 
>>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>>> index 7bfeb2f25246..b0c6c2df9db8 100644
>>> --- a/include/linux/shrinker.h
>>> +++ b/include/linux/shrinker.h
>>> @@ -74,6 +74,7 @@ struct shrinker {
>>>    	refcount_t refcount;
>>>  	struct completion completion_wait;
>>> +	struct rcu_head rcu;
>>>    	void *private_data;
>>>  @@ -123,6 +124,11 @@ struct shrinker *shrinker_alloc_and_init(count_objects_cb count,
>>>  void shrinker_free(struct shrinker *shrinker);
>>>  void unregister_and_free_shrinker(struct shrinker *shrinker);
>>>  +static inline bool shrinker_try_get(struct shrinker *shrinker)
>>> +{
>>> +	return refcount_inc_not_zero(&shrinker->refcount);
>>> +}
>>> +
>>>  static inline void shrinker_put(struct shrinker *shrinker)
>>>  {
>>>  	if (refcount_dec_and_test(&shrinker->refcount))
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 6f9c4750effa..767569698946 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -57,6 +57,7 @@
>>>  #include <linux/khugepaged.h>
>>>  #include <linux/rculist_nulls.h>
>>>  #include <linux/random.h>
>>> +#include <linux/rculist.h>
>>>    #include <asm/tlbflush.h>
>>>  #include <asm/div64.h>
>>> @@ -742,7 +743,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
>>>  	down_write(&shrinker_rwsem);
>>>  	refcount_set(&shrinker->refcount, 1);
>>>  	init_completion(&shrinker->completion_wait);
>>> -	list_add_tail(&shrinker->list, &shrinker_list);
>>> +	list_add_tail_rcu(&shrinker->list, &shrinker_list);
>>>  	shrinker->flags |= SHRINKER_REGISTERED;
>>>  	shrinker_debugfs_add(shrinker);
>>>  	up_write(&shrinker_rwsem);
>>> @@ -800,7 +801,7 @@ void unregister_shrinker(struct shrinker *shrinker)
>>>  	wait_for_completion(&shrinker->completion_wait);
>>>    	down_write(&shrinker_rwsem);
>>> -	list_del(&shrinker->list);
>>> +	list_del_rcu(&shrinker->list);
>>>  	shrinker->flags &= ~SHRINKER_REGISTERED;
>>>  	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
>>>  		unregister_memcg_shrinker(shrinker);
>>> @@ -845,7 +846,7 @@ EXPORT_SYMBOL(shrinker_free);
>>>  void unregister_and_free_shrinker(struct shrinker *shrinker)
>>>  {
>>>  	unregister_shrinker(shrinker);
>>> -	kfree(shrinker);
>>> +	kfree_rcu(shrinker, rcu);
>>>  }
>>>  EXPORT_SYMBOL(unregister_and_free_shrinker);
>>>  @@ -1067,33 +1068,27 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>>>  	if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
>>>  		return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>>>  -	if (!down_read_trylock(&shrinker_rwsem))
>>> -		goto out;
>>> -
>>> -	list_for_each_entry(shrinker, &shrinker_list, list) {
>>> +	rcu_read_lock();
>>> +	list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
>>>  		struct shrink_control sc = {
>>>  			.gfp_mask = gfp_mask,
>>>  			.nid = nid,
>>>  			.memcg = memcg,
>>>  		};
>>>  +		if (!shrinker_try_get(shrinker))
>>> +			continue;
>>> +		rcu_read_unlock();
>> I don't think you can do this unlock?
>>> +
>>>  		ret = do_shrink_slab(&sc, shrinker, priority);
>>>  		if (ret == SHRINK_EMPTY)
>>>  			ret = 0;
>>>  		freed += ret;
>>> -		/*
>>> -		 * Bail out if someone want to register a new shrinker to
>>> -		 * prevent the registration from being stalled for long periods
>>> -		 * by parallel ongoing shrinking.
>>> -		 */
>>> -		if (rwsem_is_contended(&shrinker_rwsem)) {
>>> -			freed = freed ? : 1;
>>> -			break;
>>> -		}
>>> -	}
>>>  -	up_read(&shrinker_rwsem);
>>> -out:
>>> +		rcu_read_lock();
>> That new rcu_read_lock() won't help AFAIK, the whole
>> list_for_each_entry_rcu() needs to be under the single rcu_read_lock() to be
>> safe.
> 
> In the unregister_shrinker() path, we will wait for the refcount to zero
> before deleting the shrinker from the linked list. Here, we first took
> the rcu lock, and then decrement the refcount of this shrinker.
> 
>    shrink_slab                 unregister_shrinker
>    ===========                 ===================
> 				
> 				/* wait for B */
> 				wait_for_completion()
>  rcu_read_lock()
> 
>  shrinker_put() --> (B)
> 				list_del_rcu()
>                                /* wait for rcu_read_unlock() */
> 				kfree_rcu()
> 
>  /*
>   * so this shrinker will not be freed here,
>   * and can be used to traverse the next node
>   * normally?
>   */
>  list_for_each_entry()
> 
>  shrinker_try_get()
>  rcu_read_unlock()
> 
> Did I miss something?

After calling rcu_read_unlock(), the next shrinker in the list can be freed,
so in the next iteration, use after free might happen?

Is that right?

> 
>> IIUC this is why Dave in [4] suggests unifying shrink_slab() with
>> shrink_slab_memcg(), as the latter doesn't iterate the list but uses IDR.
>>> +		shrinker_put(shrinker);
>>> +	}
>>> +	rcu_read_unlock();
>>>  	cond_resched();
>>>  	return freed;
>>>  }