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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id f5sm148644lja.17.2020.07.08.11.48.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Jul 2020 11:48:36 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 8 Jul 2020 20:48:34 +0200 To: Joel Fernandes Cc: Uladzislau Rezki , "Paul E. McKenney" , Sebastian Andrzej Siewior , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Message-ID: <20200708184834.GA3141@pc636> References: <20200624201200.GA28901@paulmck-ThinkPad-P72> <20200624201226.21197-3-paulmck@kernel.org> <20200630164543.4mdcf6zb4zfclhln@linutronix.de> <20200630183534.GG9247@paulmck-ThinkPad-P72> <20200702141216.r4rbt5w3hjzafpgg@linutronix.de> <20200702164826.GQ9247@paulmck-ThinkPad-P72> <20200702201908.jfiacgvion6a4nmj@linutronix.de> <20200706210645.GJ9247@paulmck-ThinkPad-P72> <20200707173441.GA28267@pc636> <20200707184531.GC233429@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200707184531.GC233429@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Jul 07, 2020 at 02:45:31PM -0400, Joel Fernandes wrote: > On Tue, Jul 07, 2020 at 07:34:41PM +0200, Uladzislau Rezki wrote: > > On Mon, Jul 06, 2020 at 02:06:45PM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 02, 2020 at 10:19:08PM +0200, Sebastian Andrzej Siewior wrote: > > > > On 2020-07-02 09:48:26 [-0700], Paul E. McKenney wrote: > > > > > On Thu, Jul 02, 2020 at 04:12:16PM +0200, Sebastian Andrzej Siewior wrote: > > > > > > On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote: > > > > > > > > This is not going to work together with the "wait context validator" > > > > > > > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about > > > > > > > > printk() which is why it is still disabled by default. > > > > > > > > > > > > > > Fixing that should be "interesting". In particular, RCU CPU stall > > > > > > > warnings rely on the raw spin lock to reduce false positives due > > > > > > > to race conditions. Some thought will be required here. > > > > > > > > > > > > I don't get this part. Can you explain/give me an example where to look > > > > > > at? > > > > > > > > > > Starting from the scheduler-clock interrupt's call into RCU, > > > > > we have rcu_sched_clock_irq() which calls rcu_pending() which > > > > > calls check_cpu_stall() which calls either print_cpu_stall() or > > > > > print_other_cpu_stall(), depending on whether the stall is happening on > > > > > the current CPU or on some other CPU, respectively. > > > > > > > > > > Both of these last functions acquire the rcu_node structure's raw ->lock > > > > > and expect to do printk()s while holding it. > > > > > > > > … > > > > > Thoughts? > > > > > > > > Okay. So in the RT queue there is a printk() rewrite which fixes this > > > > kind of things. Upstream the printk() interface is still broken in this > > > > regard and therefore CONFIG_PROVE_RAW_LOCK_NESTING is disabled. > > > > [Earlier the workqueue would also trigger a warning but this has been > > > > fixed as of v5.8-rc1.] > > > > This was just me explaining why this bad, what debug function would > > > > report it and why it is not enabled by default. > > > > > > Whew!!! ;-) > > > > > > > > > > > So assume that this is fixed and enabled then on !PREEMPT_RT it will > > > > > > > > complain that you have a raw_spinlock_t acquired (the one from patch > > > > > > > > 02/17) and attempt to acquire a spinlock_t in the memory allocator. > > > > > > > > > > > > > > Given that the slab allocator doesn't acquire any locks until it gets > > > > > > > a fair way in, wouldn't it make sense to allow a "shallow" allocation > > > > > > > while a raw spinlock is held? This would require yet another GFP_ flag, > > > > > > > but that won't make all that much of a difference in the total. ;-) > > > > > > > > > > > > That would be one way of dealing with. But we could go back to > > > > > > spinlock_t and keep the memory allocation even for RT as is. I don't see > > > > > > a downside of this. And we would worry about kfree_rcu() from real > > > > > > IRQ-off region once we get to it. > > > > > > > > > > Once we get to it, your thought would be to do per-CPU queuing of > > > > > memory from IRQ-off kfree_rcu(), and have IRQ work or some such clean > > > > > up after it? Or did you have some other trick in mind? > > > > > > > > So for now I would very much like to revert the raw_spinlock_t back to > > > > the spinlock_t and add a migrate_disable() just avoid the tiny > > > > possible migration between obtaining the CPU-ptr and acquiring the lock > > > > (I think Joel was afraid of performance hit). > > > > > > Performance is indeed a concern here. > > > > > > > Should we get to a *real* use case where someone must invoke kfree_rcu() > > > > from a hard-IRQ-off region then we can think what makes sense. per-CPU > > > > queues and IRQ-work would be one way of dealing with it. > > > > > > It looks like workqueues can also be used, at least in their current > > > form. And timers. > > > > > > Vlad, Joel, thoughts? > > > > > Some high level thoughts: > > > > Currently everything is done in workqueue context, it means all freeing > > happens there. For RT kernel we can invoke a page allocator only for single > > kfree_rcu() argument(though we skip it). As for double one, it is impossible, > > that is why a simple path is used by linking rcu_head among each other for > > further reclaim in wq context. As of now, for RT, everything is already > > deferred. > > > > If we revert to spinlock_t then calling of kfree_rcu() from hard IRQ > > context is broken, even though we think that for RT kernel it will > > never happen. Therefore i do not see a clear motivation and benefits > > why we should revert to spinlock_t. > > > > IMHO, if we can avoid of such drawback i would go with that way, i.e. > > i would not like to think what to do with that when it becomes broken. > > I am also of Vlad's opinion. It seems to me that doing the migrate_disable() > before dropping the raw spinlock should suffice. > > Note that in the future, we may have other users of this path such as > a potential kmem_cache_free_rcu(). It seems burdensome to make these all > callable only from sleepable-context. > > Is there a drawback of doing migrate_disable before dropping the raw internal > lock, that I'm missing? > I used migrate_disable() just to underline that we want to stay on the same CPU. We can go with preempt_enable()/preempt_disable() what is exactly the same as migrate_disable() for regular kernel. -- Vlad Rezki