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[94.245.46.61]) by smtp.gmail.com with ESMTPSA id t9sm531397lff.45.2021.01.21.05.35.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 Jan 2021 05:35:12 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 21 Jan 2021 14:35:10 +0100 To: "Paul E. McKenney" Cc: Sebastian Andrzej Siewior , "Uladzislau Rezki (Sony)" , LKML , RCU , Michael Ellerman , Andrew Morton , Daniel Axtens , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Peter Zijlstra , Michal Hocko , Thomas Gleixner , "Theodore Y . Ts'o" , Oleksiy Avramchenko Subject: Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument Message-ID: <20210121133510.GB1872@pc638.lan> References: <20210120162148.1973-1-urezki@gmail.com> <20210120195757.3lgjrpvmzjvb2nce@linutronix.de> <20210120215403.GH2743@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210120215403.GH2743@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 20, 2021 at 01:54:03PM -0800, Paul E. McKenney wrote: > On Wed, Jan 20, 2021 at 08:57:57PM +0100, Sebastian Andrzej Siewior wrote: > > On 2021-01-20 17:21:46 [+0100], Uladzislau Rezki (Sony) wrote: > > > For a single argument we can directly request a page from a caller > > > context when a "carry page block" is run out of free spots. Instead > > > of hitting a slow path we can request an extra page by demand and > > > proceed with a fast path. > > > > > > A single-argument kvfree_rcu() must be invoked in sleepable contexts, > > > and that its fallback is the relatively high latency synchronize_rcu(). > > > Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL > > > to allow limited sleeping within the memory allocator. > > > > > > [ paulmck: Add add_ptr_to_bulk_krc_lock header comment per Michal Hocko. ] > > > Signed-off-by: Uladzislau Rezki (Sony) > > > Signed-off-by: Paul E. McKenney > > > --- > > > kernel/rcu/tree.c | 42 ++++++++++++++++++++++++++---------------- > > > 1 file changed, 26 insertions(+), 16 deletions(-) > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index e04e336bee42..2014fb22644d 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -3465,37 +3465,50 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp) > > > } > > > } > > > > > > +// Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock() > > > +// state specified by flags. If can_alloc is true, the caller must > > > +// be schedulable and not be holding any locks or mutexes that might be > > > +// acquired by the memory allocator or anything that it might invoke. > > > +// Returns true if ptr was successfully recorded, else the caller must > > > +// use a fallback. > > > > The whole RCU department is getting swamped by the // comments. Can't we > > have proper kernel doc and /* */ style comments like the remaining part > > of the kernel? > > Because // comments are easier to type and take up less horizontal space. > Also, this kvfree_call_rcu_add_ptr_to_bulk() function is local to > kvfree_rcu(), and we don't normally docbook-ify such functions. > > > > static inline bool > > > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) > > > +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, > > > + unsigned long *flags, void *ptr, bool can_alloc) > > > { > > > struct kvfree_rcu_bulk_data *bnode; > > > int idx; > > > > > > - if (unlikely(!krcp->initialized)) > > > + *krcp = krc_this_cpu_lock(flags); > > > + if (unlikely(!(*krcp)->initialized)) > > > return false; > > > > > > - lockdep_assert_held(&krcp->lock); > > > idx = !!is_vmalloc_addr(ptr); > > > > > > /* Check if a new block is required. */ > > > - if (!krcp->bkvhead[idx] || > > > - krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) { > > > - bnode = get_cached_bnode(krcp); > > > - /* Switch to emergency path. */ > > > + if (!(*krcp)->bkvhead[idx] || > > > + (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) { > > > + bnode = get_cached_bnode(*krcp); > > > + if (!bnode && can_alloc) { > > > + krc_this_cpu_unlock(*krcp, *flags); > > > + bnode = (struct kvfree_rcu_bulk_data *) > > > > There is no need for this cast. > > Without it, gcc version 7.5.0 says: > > warning: assignment makes pointer from integer without a cast > > > > + __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN); > > > + *krcp = krc_this_cpu_lock(flags); > > > > so if bnode is NULL you could retry get_cached_bnode() since it might > > have been filled (given preemption or CPU migration changed something). > > Judging from patch #3 you think that a CPU migration is a bad thing. But > > why? > > So that the later "(*krcp)->bkvhead[idx] = bnode" assignment associates > it with the correct CPU. > > Though now that you mention it, couldn't the following happen? > > o Task A on CPU 0 notices that allocation is needed, so it > drops the lock disables migration, and sleeps while > allocating. > > o Task B on CPU 0 does the same. > > o The two tasks wake up in some order, and the second one > causes trouble at the "(*krcp)->bkvhead[idx] = bnode" > assignment. > > Uladzislau, do we need to recheck "!(*krcp)->bkvhead[idx]" just after > the migrate_enable()? Along with the KVFREE_BULK_MAX_ENTR check? > Probably i should have mentioned your sequence you described, that two tasks can get a page on same CPU, i was thinking about it :) Yep, it can happen since we drop the lock and a context is fully preemptible, so another one can trigger kvfree_rcu() ending up at the same place - entering a page allocator. I spent some time simulating it, but with no any luck, therefore i did not reflect this case in the commit message, thus did no pay much attention to such scenario. > > Uladzislau, do we need to recheck "!(*krcp)->bkvhead[idx]" just after > the migrate_enable()? Along with the KVFREE_BULK_MAX_ENTR check? > Two woken tasks will be serialized, i.e. an assignment is protected by the our local lock. We do krc_this_cpu_lock(flags); as a first step right after that we do restore a migration. A migration in that case can occur only when krc_this_cpu_unlock(*krcp, *flags); is invoked. The scenario you described can happen, in that case a previous bnode in the drain list can be either empty or partly utilized. But, again i was non able to trigger such scenario. If we should fix it, i think we can go with below "alloc_in_progress" protection: urezki@pc638:~/data/raid0/coding/linux-rcu.git$ git diff diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index cad36074366d..95485ec7267e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3488,12 +3488,19 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, if (!(*krcp)->bkvhead[idx] || (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) { bnode = get_cached_bnode(*krcp); - if (!bnode && can_alloc) { + if (!bnode && can_alloc && !(*krcp)->alloc_in_progress) { migrate_disable(); + + /* Set it before dropping the lock. */ + (*krcp)->alloc_in_progress = true; krc_this_cpu_unlock(*krcp, *flags); + bnode = (struct kvfree_rcu_bulk_data *) __get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN); *krcp = krc_this_cpu_lock(flags); + + /* Clear it, the lock was taken back. */ + (*krcp)->alloc_in_progress = false; migrate_enable(); } urezki@pc638:~/data/raid0/coding/linux-rcu.git$ in that case a second task will follow a fallback path bypassing a page request. I can send it as a separate patch if there are no any objections. -- Vlad Rezki