Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless

From: Qi Zheng
Date: Thu Feb 23 2023 - 23:17:16 EST



On 2023/2/24 12:00, Qi Zheng wrote:


On 2023/2/24 02:24, Sultan Alsawaf wrote:
On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote:
The shrinker_rwsem is a global lock in shrinkers subsystem,
it is easy to cause blocking in the following cases:

a. the write lock of shrinker_rwsem was held for too long.
    For example, there are many memcgs in the system, which
    causes some paths to hold locks and traverse it for too
    long. (e.g. expand_shrinker_info())
b. the read lock of shrinker_rwsem was held for too long,
    and a writer came at this time. Then this writer will be
    forced to wait and block all subsequent readers.
    For example:
    - be scheduled when the read lock of shrinker_rwsem is
      held in do_shrink_slab()
    - some shrinker are blocked for too long. Like the case
      mentioned in the patchset[1].

Therefore, many times in history ([2],[3],[4],[5]), some
people wanted to replace shrinker_rwsem reader with SRCU,
but they all gave up because SRCU was not unconditionally
enabled.

But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"),
the SRCU is unconditionally enabled. So it's time to use
SRCU to protect readers who previously held shrinker_rwsem.

[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@xxxxxxxxxxxxx/
[2]. https://lore.kernel.org/all/1437080113.3596.2.camel@xxxxxxxxxxxx/
[3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
[4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
[5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@xxxxxxxxxxxxxxx/

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
  mm/vmscan.c | 27 +++++++++++----------------
  1 file changed, 11 insertions(+), 16 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f895ca6216c..02987a6f95d1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task,
  LIST_HEAD(shrinker_list);
  DECLARE_RWSEM(shrinker_rwsem);
+DEFINE_SRCU(shrinker_srcu);
  #ifdef CONFIG_MEMCG
  static int shrinker_nr_max;
@@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
  void register_shrinker_prepared(struct shrinker *shrinker)
  {
      down_write(&shrinker_rwsem);
-    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);
@@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker)
          return;
      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);
      debugfs_entry = shrinker_debugfs_remove(shrinker);
      up_write(&shrinker_rwsem);
+    synchronize_srcu(&shrinker_srcu);
+
      debugfs_remove_recursive(debugfs_entry);
      kfree(shrinker->nr_deferred);
@@ -786,6 +789,7 @@ void synchronize_shrinkers(void)
  {
      down_write(&shrinker_rwsem);
      up_write(&shrinker_rwsem);
+    synchronize_srcu(&shrinker_srcu);
  }
  EXPORT_SYMBOL(synchronize_shrinkers);
@@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
  {
      unsigned long ret, freed = 0;
      struct shrinker *shrinker;
+    int srcu_idx;
      /*
       * The root memcg might be allocated even though memcg is disabled
@@ -1007,10 +1012,10 @@ 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;
+    srcu_idx = srcu_read_lock(&shrinker_srcu);
-    list_for_each_entry(shrinker, &shrinker_list, list) {
+    list_for_each_entry_srcu(shrinker, &shrinker_list, list,
+                 srcu_read_lock_held(&shrinker_srcu)) {
          struct shrink_control sc = {
              .gfp_mask = gfp_mask,
              .nid = nid,
@@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
          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:
+    srcu_read_unlock(&shrinker_srcu, srcu_idx);
      cond_resched();
      return freed;
  }
--
2.20.1



Hi Qi,

A different problem I realized after my old attempt to use SRCU was that the
unregister_shrinker() path became quite slow due to the heavy synchronize_srcu()
call. Both register_shrinker() *and* unregister_shrinker() are called frequently
these days, and SRCU is too unfair to the unregister path IMO.

Hi Sultan,

IIUC, for unregister_shrinker(), the wait time is hardly longer with
SRCU than with shrinker_rwsem before.

And I just did a simple test. After using the script in cover letter to
increase the shrink_slab hotspot, I did umount 1k times at the same
time, and then I used bpftrace to measure the time consumption of
unregister_shrinker() as follows:

bpftrace -e 'kprobe:unregister_shrinker { @start[tid] = nsecs; } kretprobe:unregister_shrinker /@start[tid]/ { @ns[comm] = hist(nsecs - @start[tid]); delete(@start[tid]); }'

@ns[umount]:
[16K, 32K)             3 |      |
[32K, 64K)            66 |@@@@@@@@@@      |
[64K, 128K)           32 |@@@@@      |
[128K, 256K)          22 |@@@      |
[256K, 512K)          48 |@@@@@@@      |
[512K, 1M)            19 |@@@      |
[1M, 2M)             131 |@@@@@@@@@@@@@@@@@@@@@      |
[2M, 4M)             313 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4M, 8M)             302 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  |
[8M, 16M)             55 |@@@@@@@@@

I see that the highest time-consuming of unregister_shrinker() is between 8ms and 16ms, which feels tolerable?

And when I use the synchronize_srcu_expedited() mentioned by Paul,
the measured time consumption has a more obvious decrease:

@ns[umount]:
[16K, 32K) 105 |@@@@@@@@@@ |
[32K, 64K) 521 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[64K, 128K) 119 |@@@@@@@@@@@ |
[128K, 256K) 32 |@@@ |
[256K, 512K) 70 |@@@@@@ |
[512K, 1M) 49 |@@@@ |
[1M, 2M) 34 |@@@ |
[2M, 4M) 18 |@ |
[4M, 8M) 4 |


Thanks,
Qi


Although I never got around to submitting it, I made a non-SRCU solution [1]
that uses fine-grained locking instead, which is fair to both the register path
and unregister path. (The patch I've linked is a version of this adapted to an
older 4.14 kernel FYI, but it can be reworked for the current kernel.)

What do you think about the fine-grained locking approach?

Thanks,
Sultan

[1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a



--
Thanks,
Qi