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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 3 Apr 2019 17:25:51 +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 x33GPoF722347998 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 3 Apr 2019 16:25:50 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A0CC8B205F; Wed, 3 Apr 2019 16:25:50 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7D900B2065; Wed, 3 Apr 2019 16:25:50 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.202.55]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 3 Apr 2019 16:25:50 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 602A916C2A41; Wed, 3 Apr 2019 09:25:50 -0700 (PDT) Date: Wed, 3 Apr 2019 09:25:50 -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> <20190402131853.GV4102@linux.ibm.com> <20190403095046.GD4038@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190403095046.GD4038@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040316-0072-0000-0000-00000414F999 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010867; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01183800; UDB=6.00619789; IPR=6.00964551; MB=3.00026279; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-03 16:25:55 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040316-0073-0000-0000-00004BB2CAF9 Message-Id: <20190403162550.GB14111@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-03_10:,, 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=818 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904030111 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 03, 2019 at 11:50:46AM +0200, Peter Zijlstra wrote: > On Tue, Apr 02, 2019 at 06:18:53AM -0700, Paul E. McKenney wrote: > > 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: > > > > > 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. > > Right, there is that. > > > > 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? > > Well, you have that unspecified delay. By forcing the (self) interrupt > you enforce a timely response. Good point! I will give this a go, thank you! Thanx, Paul