From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 82013C433DF for ; Thu, 2 Jul 2020 19:45:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 52EC82088E for ; Thu, 2 Jul 2020 19:45:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="FSIlkXh+" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725954AbgGBTpM (ORCPT ); Thu, 2 Jul 2020 15:45:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39832 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725937AbgGBTpL (ORCPT ); Thu, 2 Jul 2020 15:45:11 -0400 Received: from mail-lj1-x243.google.com (mail-lj1-x243.google.com [IPv6:2a00:1450:4864:20::243]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7288CC08C5C1; Thu, 2 Jul 2020 12:45:11 -0700 (PDT) Received: by mail-lj1-x243.google.com with SMTP id h22so26584695lji.9; Thu, 02 Jul 2020 12:45:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=aJWEg+4dTR2cdqHopODN867tmAlEblBzH751/xqR3xQ=; b=FSIlkXh+8AW9zqepLV4jGUaMriIMs+7Nh4U7vB42alqGnqhBJJB2jVCZtrrw6dVL7/ V1Pla3rT8uiTRLvtg3Xa5TMWWTOpnrVAE0eYukUohUJHVtDiXGbZ1AooOhqzbWX/Po1u WWwCkEg9bI0vGrGRSxfowRzAnAw9gMBHONjCcuhZbnLFNuQcp7H6wsQWFDeKP9eTTj8V ktyuiXgeczC1Nidnjdaackgre5tHrBhPjfmY8fwclIiGM//39cQmXuxB8LLr4XEcl2CX gpM6oOmziW0QCLg+Bcl8OIWabBMZZkzomNbO2eGHwzuaDHC5ORwzbfVaPgOyDXRSvvUP ZZCA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:date:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=aJWEg+4dTR2cdqHopODN867tmAlEblBzH751/xqR3xQ=; b=QEn7ePbrq3t1qgiJxQQ5kwpE9gnFye1pa+f5ggP3aS1y+52YVuDpXcCdnolwFcLMcQ /NnJ04sA+qe1NY3CvSrEW/0E6zsuRwpRqHDI0dRJAi764VwLUrUDP5/JQrfxOV1d8VnK nY9LvVPfO4LrpWvNjDEBcFR8nEjABB1ZKH0OnyfauUZRU6uUI8WnnKOfGlFwekCCy1MT 25C3FJHN3uHGteq4Eqb/ZyOg6jMeALD9ABGomtaGBcTJtXQEgXMTjAq6oL4JtOOdbc9d Pmcd8bmiQxV+ewGZCOT9ubh5l2jwR7ieoHrEeBIkLOTAYyd/Ie4KeIpSGHaAU6bs8veo kWSw== X-Gm-Message-State: AOAM531xNZtaGhZ+lOc6/7yJ+piSV+ZnvzxbBWshsZ68dJEWqWXhAc2X LdHFFyT+qPVepz/1bYpp+uY= X-Google-Smtp-Source: ABdhPJybTVroeM644KWkZdaN1dBa+0I4v469LW1KaxWkG3TT2t+FSUTVDMFOzFlufNzneqpli2akIw== X-Received: by 2002:a2e:8882:: with SMTP id k2mr8004718lji.352.1593719109896; Thu, 02 Jul 2020 12:45:09 -0700 (PDT) Received: from pc636 (h5ef52e31.seluork.dyn.perspektivbredband.net. [94.245.46.49]) by smtp.gmail.com with ESMTPSA id n10sm3704165lfb.82.2020.07.02.12.45.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Jul 2020 12:45:08 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Thu, 2 Jul 2020 21:45:06 +0200 To: "Paul E. McKenney" , Sebastian Andrzej Siewior Cc: joel@joelfernandes.org, 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, Uladzislau Rezki Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Message-ID: <20200702194506.GA31478@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200702141216.r4rbt5w3hjzafpgg@linutronix.de> 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 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? > > > > 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. > Another way of fixing it is just dropping the lock letting the page allocator to do an allocation without our "upper/local" lock. I did a proposal like that once upon a time, so maybe it is a time to highlight it again: diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 21c2fa5bd8c3..249f10a89bb9 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3278,9 +3278,11 @@ static void kfree_rcu_monitor(struct work_struct *work) } static inline bool -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) +kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, + void *ptr, unsigned long *flags) { struct kvfree_rcu_bulk_data *bnode; + struct kfree_rcu_cpu *tmp; int idx; if (unlikely(!krcp->initialized)) @@ -3306,6 +3308,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) if (IS_ENABLED(CONFIG_PREEMPT_RT)) return false; + migrate_disable(); + krc_this_cpu_unlock(krcp, *flags); + /* * NOTE: For one argument of kvfree_rcu() we can * drop the lock and get the page in sleepable @@ -3315,6 +3320,12 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr) */ bnode = (struct kvfree_rcu_bulk_data *) __get_free_page(GFP_NOWAIT | __GFP_NOWARN); + + tmp = krc_this_cpu_lock(flags); + migrate_enable(); + + /* Sanity check, just in case. */ + WARN_ON(tmp != krcp); } /* Switch to emergency path. */ @@ -3386,7 +3397,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func) * Under high memory pressure GFP_NOWAIT can fail, * in that case the emergency path is maintained. */ - success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr); + success = kvfree_call_rcu_add_ptr_to_bulk(krcp, ptr, &flags); if (!success) { if (head == NULL) // Inline if kvfree_rcu(one_arg) call. -- Vlad Rezki