From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752035AbdLAThJ (ORCPT ); Fri, 1 Dec 2017 14:37:09 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:55938 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751713AbdLATg6 (ORCPT ); Fri, 1 Dec 2017 14:36:58 -0500 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: 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, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 16/17] doc: Update dyntick-idle design documentation for NMI/irq consolidation Date: Fri, 1 Dec 2017 11:36:47 -0800 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20171201193625.GA20681@linux.vnet.ibm.com> References: <20171201193625.GA20681@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 17120119-0036-0000-0000-00000294E786 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00008137; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000243; SDB=6.00953995; UDB=6.00482082; IPR=6.00734070; BA=6.00005726; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00018295; XFM=3.00000015; UTC=2017-12-01 19:36:55 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17120119-0037-0000-0000-0000428B9D44 Message-Id: <1512157008-22355-16-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-12-01_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1712010228 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Signed-off-by: Paul E. McKenney --- .../Design/Data-Structures/Data-Structures.html | 46 +++++++++++++++------- 1 file changed, 32 insertions(+), 14 deletions(-) diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html index 38d6d800761f..1ac011de606e 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -1182,8 +1182,8 @@ CPU (and from tracing) unless otherwise stated. Its fields are as follows: 
-  1   int dynticks_nesting;
-  2   int dynticks_nmi_nesting;
+  1   long dynticks_nesting;
+  2   long dynticks_nmi_nesting;
   3   atomic_t dynticks;
   4   bool rcu_need_heavy_qs;
   5   unsigned long rcu_qs_ctr;
@@ -1191,15 +1191,31 @@ Its fields are as follows:

The ->dynticks_nesting field counts the -nesting depth of normal interrupts. -In addition, this counter is incremented when exiting dyntick-idle -mode and decremented when entering it. +nesting depth of process execution, so that in normal circumstances +this counter has value zero or one. +NMIs, irqs, and tracers are counted by the ->dynticks_nmi_nesting +field. +Because NMIs cannot be masked, changes to this variable have to be +undertaken carefully using an algorithm provided by Andy Lutomirski. +The initial transition from idle adds one, and nested transitions +add two, so that a nesting level of five is represented by a +->dynticks_nmi_nesting value of nine. This counter can therefore be thought of as counting the number of reasons why this CPU cannot be permitted to enter dyntick-idle -mode, aside from non-maskable interrupts (NMIs). -NMIs are counted by the ->dynticks_nmi_nesting -field, except that NMIs that interrupt non-dyntick-idle execution -are not counted. +mode, aside from process-level transitions. + +

However, it turns out that when running in non-idle kernel context, +the Linux kernel is fully capable of entering interrupt handlers that +never exit and perhaps also vice versa. +Therefore, whenever the ->dynticks_nesting field is +incremented up from zero, the ->dynticks_nmi_nesting field +is set to a large positive number, and whenever the +->dynticks_nesting field is decremented down to zero, +the the ->dynticks_nmi_nesting field is set to zero. +Assuming that the number of misnested interrupts is not sufficient +to overflow the counter, this approach corrects the +->dynticks_nmi_nesting field every time the corresponding +CPU enters the idle loop from process context.

The ->dynticks field counts the corresponding CPU's transitions to and from dyntick-idle mode, so that this counter @@ -1231,14 +1247,16 @@ in response.   Quick Quiz: - Why not just count all NMIs? - Wouldn't that be simpler and less error prone? + Why not simply combine the ->dynticks_nesting + and ->dynticks_nmi_nesting counters into a + single counter that just counts the number of reasons that + the corresponding CPU is non-idle? Answer: - It seems simpler only until you think hard about how to go about - updating the rcu_dynticks structure's - ->dynticks field. + Because this would fail in the presence of interrupts whose + handlers never return and of handlers that manage to return + from a made-up interrupt.   -- 2.5.2