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From: Roman Gushchin <guro@fb.com>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>, <hannes@cmpxchg.org>,
	<vdavydov.dev@gmail.com>, <akpm@linux-foundation.org>,
	<shakeelb@google.com>, <willy@infradead.org>, <alexs@kernel.org>,
	<richard.weiyang@gmail.com>, <songmuchun@bytedance.com>,
	<linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
	<cgroups@vger.kernel.org>
Subject: Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex
Date: Mon, 2 Aug 2021 20:40:04 -0700	[thread overview]
Message-ID: <YQi6lOT6j2DtOGlT@carbon.dhcp.thefacebook.com> (raw)
In-Reply-To: <4a3c23c4-054c-2896-29c5-8cf9a4deee98@huawei.com>

On Sat, Jul 31, 2021 at 10:29:52AM +0800, Miaohe Lin wrote:
> On 2021/7/30 14:50, Michal Hocko wrote:
> > On Thu 29-07-21 20:06:45, Roman Gushchin wrote:
> >> On Thu, Jul 29, 2021 at 08:57:52PM +0800, Miaohe Lin wrote:
> >>> Since percpu_charge_mutex is only used inside drain_all_stock(), we can
> >>> narrow the scope of percpu_charge_mutex by moving it here.
> >>>
> >>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> >>> ---
> >>>  mm/memcontrol.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>> index 6580c2381a3e..a03e24e57cd9 100644
> >>> --- a/mm/memcontrol.c
> >>> +++ b/mm/memcontrol.c
> >>> @@ -2050,7 +2050,6 @@ struct memcg_stock_pcp {
> >>>  #define FLUSHING_CACHED_CHARGE	0
> >>>  };
> >>>  static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> >>> -static DEFINE_MUTEX(percpu_charge_mutex);
> >>>  
> >>>  #ifdef CONFIG_MEMCG_KMEM
> >>>  static void drain_obj_stock(struct obj_stock *stock);
> >>> @@ -2209,6 +2208,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> >>>   */
> >>>  static void drain_all_stock(struct mem_cgroup *root_memcg)
> >>>  {
> >>> +	static DEFINE_MUTEX(percpu_charge_mutex);
> >>>  	int cpu, curcpu;
> >>
> >> It's considered a good practice to protect data instead of code paths. After
> >> the proposed change it becomes obvious that the opposite is done here: the mutex
> >> is used to prevent a simultaneous execution of the code of the drain_all_stock()
> >> function.
> > 
> > The purpose of the lock was indeed to orchestrate callers more than any
> > data structure consistency.
> >  
> >> Actually we don't need a mutex here: nobody ever sleeps on it. So I'd replace
> >> it with a simple atomic variable or even a single bitfield. Then the change will
> >> be better justified, IMO.
> > 
> > Yes, mutex can be replaced by an atomic in a follow up patch.
> > 
> 
> Thanks for both of you. It's a really good suggestion. What do you mean is something like below?
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 616d1a72ece3..508a96e80980 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2208,11 +2208,11 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>   */
>  static void drain_all_stock(struct mem_cgroup *root_memcg)
>  {
> -       static DEFINE_MUTEX(percpu_charge_mutex);
>         int cpu, curcpu;
> +       static atomic_t drain_all_stocks = ATOMIC_INIT(-1);
> 
>         /* If someone's already draining, avoid adding running more workers. */
> -       if (!mutex_trylock(&percpu_charge_mutex))
> +       if (!atomic_inc_not_zero(&drain_all_stocks))
>                 return;

It should work, but why not a simple atomic_cmpxchg(&drain_all_stocks, 0, 1) and
initialize it to 0? Maybe it's just my preference, but IMO (0, 1) is easier
to understand than (-1, 0) here. Not a strong opinion though, up to you.

Thanks!

  parent reply	other threads:[~2021-08-03  3:40 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 12:57 [PATCH 0/5] Cleanups and fixup for memcontrol Miaohe Lin
2021-07-29 12:57 ` [PATCH 1/5] mm, memcg: remove unused functions Miaohe Lin
2021-07-29 14:07   ` Shakeel Butt
2021-07-30  2:39   ` Muchun Song
2021-07-30  2:57   ` Roman Gushchin
2021-07-30  6:45   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex Miaohe Lin
2021-07-30  2:42   ` Muchun Song
2021-07-30  3:06   ` Roman Gushchin
2021-07-30  6:50     ` Michal Hocko
2021-07-31  2:29       ` Miaohe Lin
2021-08-02  6:49         ` Michal Hocko
2021-08-02  9:54           ` Miaohe Lin
2021-08-03  3:40         ` Roman Gushchin [this message]
2021-08-03  6:29           ` Miaohe Lin
2021-08-03  7:11             ` Michal Hocko
2021-08-03  7:13               ` Roman Gushchin
2021-08-03  7:27                 ` Michal Hocko
2021-08-03  9:33             ` Muchun Song
2021-08-03 10:50               ` Miaohe Lin
2021-08-03 14:15       ` Johannes Weiner
2021-08-04  8:20         ` Michal Hocko
2021-08-05  1:44           ` Miaohe Lin
2021-07-30  6:46   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 3/5] mm, memcg: save some atomic ops when flush is already true Miaohe Lin
2021-07-29 14:40   ` Shakeel Butt
2021-07-30  2:37   ` Muchun Song
2021-07-30  3:07   ` Roman Gushchin
2021-07-30  6:51   ` Michal Hocko
2021-07-29 12:57 ` [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup_init() Miaohe Lin
2021-07-29 13:52   ` Matthew Wilcox
2021-07-30  1:50     ` Miaohe Lin
2021-07-30  3:12   ` Roman Gushchin
2021-07-30  6:29     ` Miaohe Lin
2021-07-30  6:44     ` Michal Hocko
2021-07-31  2:05       ` Miaohe Lin
2021-08-02  6:43         ` Michal Hocko
2021-08-02 10:00           ` Miaohe Lin
2021-08-02 10:42             ` Michal Hocko
2021-08-02 11:18               ` Miaohe Lin
2021-07-29 12:57 ` [PATCH 5/5] mm, memcg: always call __mod_node_page_state() with preempt disabled Miaohe Lin
2021-07-29 14:39   ` Shakeel Butt
2021-07-30  1:52     ` Miaohe Lin
2021-07-30  2:33       ` [External] " Muchun Song
2021-07-30  2:46         ` Miaohe Lin

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