Date:   Thu, 23 Feb 2023 10:24:47 -0800
From:   Sultan Alsawaf <sultan@kerneltoast.com>
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     akpm@linux-foundation.org, tkhai@ya.ru, hannes@cmpxchg.org,
        shakeelb@google.com, mhocko@kernel.org, roman.gushchin@linux.dev,
        muchun.song@linux.dev, david@redhat.com, shy828301@gmail.com,
        dave@stgolabs.net, penguin-kernel@i-love.sakura.ne.jp,
        paulmck@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
Message-ID: <Y/evb+PBeaahx9Os@sultan-box.localdomain>
References: <20230223132725.11685-1-zhengqi.arch@bytedance.com>
 <20230223132725.11685-3-zhengqi.arch@bytedance.com>
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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@virtuozzo.com/
> [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/
> [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/
> [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/
> [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>  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