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McKenney" , RCU , linux-mm@kvack.org, Andrew Morton , Steven Rostedt , Oleksiy Avramchenko Subject: Re: [PATCH 1/3] rcu/tree: use more permissive parameters when attaching a head Message-ID: <20200407021118.GB113967@google.com> References: <20200402123253.10382-1-urezki@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200402123253.10382-1-urezki@gmail.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 Thu, Apr 02, 2020 at 02:32:51PM +0200, Uladzislau Rezki (Sony) wrote: > It is documneted that a headless object can be reclaimed from > might_sleep() context only. Because of that when a head is > dynamically attached it makes sense to drop the lock and do > an allocation with much more permissve flags comparing if it > is done from atomic context. > > That is why use GFP_KERNEL flag plus some extra ones which > would make an allocation most likely to be succeed. The big > advantage of doing so is a direct reclaim process. > > Tested such approach on my local tiny system with 145MB of > ram(the minimum amount the KVM system is capable of booting) > and 4xCPUs. For stressing the rcuperf module was used. During > tests with difference combinations i did not observe any hit > of our last emergency case, when synchronize_rcu() is involved. > > Please note, the "dynamically attaching" path was enabled only, > apart of that all types of objects were considered as headless > variant during testing. > > Signed-off-by: Uladzislau Rezki (Sony) > Suggested-by: Joel Fernandes (Google) > --- > kernel/rcu/tree.c | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 6172e6296dd7..24f620a06219 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3148,13 +3148,10 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj) > { > unsigned long *ptr; > > + /* Try hard to get the memory. */ > ptr = kmalloc(sizeof(unsigned long *) + > - sizeof(struct rcu_head), GFP_NOWAIT | __GFP_NOWARN); > - > - if (!ptr) > - ptr = kmalloc(sizeof(unsigned long *) + > - sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN); > - > + sizeof(struct rcu_head), GFP_KERNEL | > + __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL); On thing here though, you removed the NOWARN. Was there a reason? It would now warn even when synchronously waiting right? I will fixup your commit to add it back for now but let me know if you had some other reason to remove it. thanks, - Joel > if (!ptr) > return NULL; > > @@ -3222,9 +3219,20 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) > if (!success) { > /* Is headless object? */ > if (head == NULL) { > + /* Drop the lock. */ > + if (krcp->initialized) > + spin_unlock(&krcp->lock); > + local_irq_restore(flags); > + > head = attach_rcu_head_to_object(ptr); > if (head == NULL) > - goto unlock_return; > + goto inline_return; > + > + /* Take it back. */ > + local_irq_save(flags); > + krcp = this_cpu_ptr(&krc); > + if (krcp->initialized) > + spin_lock(&krcp->lock); > > /* > * Tag the headless object. Such objects have a back-pointer > @@ -3263,6 +3271,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) > spin_unlock(&krcp->lock); > local_irq_restore(flags); > > +inline_return: > /* > * High memory pressure, so inline kvfree() after > * synchronize_rcu(). We can do it from might_sleep() > -- > 2.20.1 >