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[209.85.220.65]) by mx.google.com with SMTPS id f17sor1914389lfc.20.2019.06.09.07.31.35 for (Google Transport Security); Sun, 09 Jun 2019 07:31:36 -0700 (PDT) Received-SPF: pass (google.com: domain of vdavydov.dev@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=EqucCPXT; spf=pass (google.com: domain of vdavydov.dev@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=vdavydov.dev@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=YgVgjJRZpCz9nTLBwfFgFqqvq2EsxYLpfjzYfKQFnJ4=; b=EqucCPXTtVHpMwCdI7LAxvowLp5hWcDsEi0LmmDP0/Tg5JdSC+jX2b4qg5fGYOpeAa TlMjuC9nzjAn4g2NvmPQ6mhM56YfhtgQFd31zrd2+7j9KF8ZKAhgW2fox9lKi+t75hmn d52jseicKOibzH2X/U5xUybKuWrB4xCw+Negyd7DtUDuT5jyQMNI1ZvjQ3YueTzzcZ6P mUKlDMtLzaRvpi7gI3b+0qM2o4a7BJl0sCPGJd1tQVJwTff2XIy/+O888vvCe/7uZiJF 6GJbyqfV47jYV3e6t1Xkl0b9+88NHJbily2qMgf4xQav9vtriI1GwOmj75h7cZYiPobs Rpag== X-Google-Smtp-Source: APXvYqyDJ0YYG87q+UXcO9Am1kPS6on/zrzIe7zNYQwBx1bx6kaIYrPPGTw8xhaSY2e3rT2oZh7HlQ== X-Received: by 2002:ac2:558a:: with SMTP id v10mr33038430lfg.41.1560090695584; Sun, 09 Jun 2019 07:31:35 -0700 (PDT) Received: from esperanza ([176.120.239.149]) by smtp.gmail.com with ESMTPSA id i188sm1388832lji.4.2019.06.09.07.31.34 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 09 Jun 2019 07:31:34 -0700 (PDT) Date: Sun, 9 Jun 2019 17:31:32 +0300 From: Vladimir Davydov To: Roman Gushchin Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Johannes Weiner , Shakeel Butt , Waiman Long Subject: Re: [PATCH v6 07/10] mm: synchronize access to kmem_cache dying flag using a spinlock Message-ID: <20190609143132.cv7b4w5caghuhi53@esperanza> References: <20190605024454.1393507-1-guro@fb.com> <20190605024454.1393507-8-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190605024454.1393507-8-guro@fb.com> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jun 04, 2019 at 07:44:51PM -0700, Roman Gushchin wrote: > Currently the memcg_params.dying flag and the corresponding > workqueue used for the asynchronous deactivation of kmem_caches > is synchronized using the slab_mutex. > > It makes impossible to check this flag from the irq context, > which will be required in order to implement asynchronous release > of kmem_caches. > > So let's switch over to the irq-save flavor of the spinlock-based > synchronization. > > Signed-off-by: Roman Gushchin > --- > mm/slab_common.c | 19 +++++++++++++++---- > 1 file changed, 15 insertions(+), 4 deletions(-) > > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 09b26673b63f..2914a8f0aa85 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -130,6 +130,7 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr, > #ifdef CONFIG_MEMCG_KMEM > > LIST_HEAD(slab_root_caches); > +static DEFINE_SPINLOCK(memcg_kmem_wq_lock); > > void slab_init_memcg_params(struct kmem_cache *s) > { > @@ -629,6 +630,7 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg, > struct memcg_cache_array *arr; > struct kmem_cache *s = NULL; > char *cache_name; > + bool dying; > int idx; > > get_online_cpus(); > @@ -640,7 +642,13 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg, > * The memory cgroup could have been offlined while the cache > * creation work was pending. > */ > - if (memcg->kmem_state != KMEM_ONLINE || root_cache->memcg_params.dying) > + if (memcg->kmem_state != KMEM_ONLINE) > + goto out_unlock; > + > + spin_lock_irq(&memcg_kmem_wq_lock); > + dying = root_cache->memcg_params.dying; > + spin_unlock_irq(&memcg_kmem_wq_lock); > + if (dying) > goto out_unlock; I do understand why we need to sync setting dying flag for a kmem cache about to be destroyed in flush_memcg_workqueue vs checking the flag in kmemcg_cache_deactivate: this is needed so that we don't schedule a new deactivation work after we flush RCU/workqueue. However, I don't think it's necessary to check the dying flag here, in memcg_create_kmem_cache: we can't schedule a new cache creation work after kmem_cache_destroy has started, because one mustn't allocate from a dead kmem cache; since we flush the queue before getting to actual destruction, no cache creation work can be pending. Yeah, it might happen that a cache creation work starts execution while flush_memcg_workqueue is in progress, but I don't see any point in optimizing this case - after all, cache destruction is a very cold path. Since checking the flag in memcg_create_kmem_cache raises question, I suggest to simply drop this check. Anyway, it would be nice to see some comment in the code explaining why we check dying flag under a spin lock in kmemcg_cache_deactivate. > > idx = memcg_cache_id(memcg); > @@ -735,14 +743,17 @@ static void kmemcg_cache_deactivate(struct kmem_cache *s) > > __kmemcg_cache_deactivate(s); > > + spin_lock_irq(&memcg_kmem_wq_lock); > if (s->memcg_params.root_cache->memcg_params.dying) > - return; > + goto unlock; > > /* pin memcg so that @s doesn't get destroyed in the middle */ > css_get(&s->memcg_params.memcg->css); > > s->memcg_params.work_fn = __kmemcg_cache_deactivate_after_rcu; > call_rcu(&s->memcg_params.rcu_head, kmemcg_rcufn); > +unlock: > + spin_unlock_irq(&memcg_kmem_wq_lock); > } > > void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg) > @@ -852,9 +863,9 @@ static int shutdown_memcg_caches(struct kmem_cache *s) > > static void flush_memcg_workqueue(struct kmem_cache *s) > { > - mutex_lock(&slab_mutex); > + spin_lock_irq(&memcg_kmem_wq_lock); > s->memcg_params.dying = true; > - mutex_unlock(&slab_mutex); > + spin_unlock_irq(&memcg_kmem_wq_lock); > > /* > * SLAB and SLUB deactivate the kmem_caches through call_rcu. Make