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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Tue, 2 Apr 2019 14:18:50 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x32DImvi18481378 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 2 Apr 2019 13:18:49 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C2A38B205F; Tue, 2 Apr 2019 13:18:48 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 92F34B2065; Tue, 2 Apr 2019 13:18:48 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Tue, 2 Apr 2019 13:18:48 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 94EFA16C1B24; Tue, 2 Apr 2019 06:18:53 -0700 (PDT) Date: Tue, 2 Apr 2019 06:18:53 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Sebastian Andrzej Siewior Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Check for wakeup-safe conditions in rcu_read_unlock_special() Reply-To: paulmck@linux.ibm.com References: <20190329182608.GA23877@linux.ibm.com> <20190329182634.24994-2-paulmck@linux.ibm.com> <20190401083211.GD11158@hirez.programming.kicks-ass.net> <20190401172257.GN4102@linux.ibm.com> <20190402070953.GG12232@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190402070953.GG12232@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040213-0064-0000-0000-000003C56614 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010860; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01183261; UDB=6.00619463; IPR=6.00964009; MB=3.00026260; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-02 13:18:54 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040213-0065-0000-0000-00003CEC4613 Message-Id: <20190402131853.GV4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-02_05:,, 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=953 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904020090 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 02, 2019 at 09:09:53AM +0200, Peter Zijlstra wrote: > On Mon, Apr 01, 2019 at 10:22:57AM -0700, Paul E. McKenney wrote: > > > > The initial solution to this problem was to use set_tsk_need_resched() and > > > > set_preempt_need_resched() to force a future context switch, which allows > > > > rcu_preempt_note_context_switch() to report the deferred quiescent state > > > > to RCU's core processing. Unfortunately for expedited grace periods, > > > > there can be a significant delay between the call for a context switch > > > > and the actual context switch. > > > > > > This is all PREEMPT=y kernels, right? Where is the latency coming from? > > > Because PREEMPT=y _should_ react quite quickly. > > > > Yes, PREEMPT=y. It happens like this: > > > > 1. rcu_read_lock() with everything enabled. > > > > 2. Preemption then resumption. > > > > 3. local_irq_disable(). > > > > 4. rcu_read_unlock(). > > > > 5. local_irq_enable(). > > > > From what I know, the scheduler doesn't see anything until the next > > interrupt, local_bh_enable(), return to userspace, etc. Because this > > is PREEMPT=y, preempt_enable() and cond_resched() do nothing. So > > it could be some time (milliseconds, depending on HZ, NO_HZ_FULL, and > > so on) until the scheduler responds. With NO_HZ_FULL, last I knew, > > the delay can be extremely long. > > > > Or am I missing something that gets the scheduler on the job faster? > > Oh urgh, yah. So normally we only twiddle with the need_resched state: > > - while preempt_disabl(), such that preempt_enable() will reschedule > - from interrupt context, such that interrupt return will reschedule > > But the usage here 'violates' those rules and then there is an > unspecified latency between setting the state and it getting observed, > but no longer than 1 tick I would think. In general, yes, which is fine (famous last words) for normal grace periods but not so good for expedited grace periods. > I don't think we can go NOHZ with need_resched set, because the moment > we hit the idle loop with that set, we _will_ reschedule. Agreed, and I believe that transitioning to usermode execution also gives the scheduler a chance to take action. The one exception to this is when a nohz_full CPU running in nohz_full mode does a system call that decides to execute for a very long time. Last I checked, the scheduling-clock interrupt did -not- get retriggered in this case, and the delay could be indefinite, as in bad even for normal grace periods. > So in that respect the irq_work suggestion I made would fix things > properly. But wouldn't the current use of set_tsk_need_resched(current) followed by set_preempt_need_resched() work just as well in that case? The scheduler would react to these at the next scheduler-clock interrupt on their own, right? Or am I being scheduler-naive again? Thanx, Paul