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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Thu, 20 Jun 2019 23:32:06 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x5KMW6dT38011304 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 20 Jun 2019 22:32:06 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E9A97B2064; Thu, 20 Jun 2019 22:32:05 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BDCB0B2066; Thu, 20 Jun 2019 22:32:05 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 20 Jun 2019 22:32:05 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id CCB2016C2A15; Thu, 20 Jun 2019 15:32:07 -0700 (PDT) Date: Thu, 20 Jun 2019 15:32:07 -0700 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de Subject: Re: Review of RCU-related patches in -rt Reply-To: paulmck@linux.ibm.com References: <20190528205030.GA27149@linux.ibm.com> <20190607160857.nsxkrytedyfroxf3@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190607160857.nsxkrytedyfroxf3@linutronix.de> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19062022-0052-0000-0000-000003D35B5C X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011299; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01220894; UDB=6.00642290; IPR=6.01002032; MB=3.00027398; MTD=3.00000008; XFM=3.00000015; UTC=2019-06-20 22:32:08 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19062022-0053-0000-0000-00006165ACD3 Message-Id: <20190620223207.GC26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-20_15:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906200159 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 07, 2019 at 06:08:57PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-05-28 13:50:30 [-0700], Paul E. McKenney wrote: > > Hello, Sebastian, > Hi Paul, > > > Finally getting around to taking another look: > > > > c7e07056a108 EXP rcu: skip the workqueue path on RT > > > > This one makes sense given the later commit setting the > > rcu_normal_after_boot kernel parameter. Otherwise, it is > > slowing down expedited grace periods for no reason. But > > should the check also include rcu_normal_after_boot and > > rcu_normal? For example: > > > > if ((IS_ENABLED(CONFIG_PREEMPT_RT_FULL) && > > (rcu_normal || rcu_normal_after_boot) || > > !READ_ONCE(rcu_par_gp_wq) || > > rcu_scheduler_active != RCU_SCHEDULER_RUNNING || > > rcu_is_last_leaf_node(rnp)) { > > I recently dropped that patch from the queue because the workqueue > problem vanished. > > > Alternatively, one approach would be to take the kernel > > parameters out in -rt: > > > > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT_FULL); > > #ifndef CONFIG_PREEMPT_RT_FULL > > module_param(rcu_normal_after_boot, int, 0); > > #endif > > > > And similar for rcu_normal and rcu_expedited. > > This makes sense. > > > Or is there some reason to allow run-time expedited grace > > periods in CONFIG_PREEMPT_RT_FULL=y kernels? > > No, I doubt there is any need to use the `expedited' version. The > problem is that it increases latencies. > > > d1f52391bd8a rcu: Disable RCU_FAST_NO_HZ on RT > > > > Looks good. More complexity could be added if too many people > > get themselves in trouble via "select RCU_FAST_NO_HZ". > > That patch disables RCU_FAST_NO_HZ and claims that it has something to > do with a timer_list timer and IRQ-off section. We couldn't schedule > timers from IRQ-off regions but not anymore. Only del_timer_sync() can't > be invoked from IRQ-off regions. > I just booted a box with this enabled together with NO_HZ/ NO_HZ_FULL > and I not complains yet. So I might drop that… > > > 42b346870326 rcu: make RCU_BOOST default on RT > > > > To avoid complaints about this showing up when people don't > > expected, could you please instead "select RCU_BOOST" in > > the Kconfig definition of PREEMPT_RT_FULL? > > > > Or do people really want to be able to disable boosting? > > I have no idea. I guess most people don't know what it does and stay > with the default. It become default on RT once a few people complained > that they run OOM during boot on some "memory contrained systems". That > option avoided it. > So yes, will make it depend on RT. > > > 457c1b0d9c0e sched: Do not account rcu_preempt_depth on RT in might_sleep() > > > > The idea behind this one is to avoid false-positive complaints > > about -rt's sleeping spinlocks, correct? > > Correct. Maybe we could invoke a different schedule() primitiv so RCU is > aware that this is a sleeping spinlock and not a regular sleeping lock. > > > 7ee13e640b01 rbtree: don't include the rcu header > > c9b0c9b87081 rtmutex: annotate sleeping lock context > > > > No specific comments. > > > > 7912d002ebf9 rcu: Eliminate softirq processing from rcutree > > > > This hasn't caused any problems in -rcu from what I can see. > > I am therefore planning to submit the -rcu variant of this to > > mainline during the next merge window. > > wonderful. > > > f06d34ebdbbb srcu: Remove srcu_queue_delayed_work_on() > > > > Looks plausible. I will check more carefully for mainline. > > Hmmm. I though this was already upstream. > That said, we can now schedule work from a preempt_disable() section but > I still like the negative diffstat here :) Right you are! e81baf4cb19a ("srcu: Remove srcu_queue_delayed_work_on()") is in v5.1. > > aeb04e894cc9 srcu: replace local_irqsave() with a locallock > > e48989b033ad irqwork: push most work into softirq context > > > > These look to still be -rt only. > > I might get rid of the local_lock in srcu. Will have to check. > > Thank you Paul. And you! I will check again in a few months, for some definition of "a few". Thanx, Paul