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.
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
