From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756612AbdGXVh0 (ORCPT ); Mon, 24 Jul 2017 17:37:26 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:51514 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756592AbdGXVgd (ORCPT ); Mon, 24 Jul 2017 17:36:33 -0400 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 6/6] doc: Set down RCU's scheduling-clock-interrupt needs Date: Mon, 24 Jul 2017 14:36:28 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20170724213605.GA8766@linux.vnet.ibm.com> References: <20170724213605.GA8766@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 17072421-0040-0000-0000-00000385097E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007419; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000214; SDB=6.00892370; UDB=6.00446020; IPR=6.00672534; BA=6.00005489; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016356; XFM=3.00000015; UTC=2017-07-24 21:36:31 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17072421-0041-0000-0000-0000077926AB Message-Id: <1500932188-9202-6-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-07-24_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1706020000 definitions=main-1707240328 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit documents the situations in which RCU needs the scheduling-clock interrupt to be enabled, along with the consequences of failing to meet RCU's needs in this area. Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 130 +++++++++++++++++++++ 1 file changed, 130 insertions(+) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 95b30fa25d56..62e847bcdcdd 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2080,6 +2080,8 @@ Some of the relevant points of interest are as follows:
  • Scheduler and RCU.
  • Tracing and RCU.
  • Energy Efficiency. +
  • + Scheduling-Clock Interrupts and RCU.
  • Memory Efficiency.
  • Performance, Scalability, Response Time, and Reliability. @@ -2532,6 +2534,134 @@ I learned of many of these requirements via angry phone calls: Flaming me on the Linux-kernel mailing list was apparently not sufficient to fully vent their ire at RCU's energy-efficiency bugs! +

    +Scheduling-Clock Interrupts and RCU

    + +

    +The kernel transitions between in-kernel non-idle execution, userspace +execution, and the idle loop. +Depending on kernel configuration, RCU handles these states differently: + + + + + + + + + + + + + + + + + + +
    HZ KconfigIn-KernelUsermodeIdle
    HZ_PERIODICCan rely on scheduling-clock interrupt.Can rely on scheduling-clock interrupt and its + detection of interrupt from usermode.Can rely on RCU's dyntick-idle detection.
    NO_HZ_IDLECan rely on scheduling-clock interrupt.Can rely on scheduling-clock interrupt and its + detection of interrupt from usermode.Can rely on RCU's dyntick-idle detection.
    NO_HZ_FULLCan only sometimes rely on scheduling-clock interrupt. + In other cases, it is necessary to bound kernel execution + times and/or use IPIs.Can rely on RCU's dyntick-idle detection.Can rely on RCU's dyntick-idle detection.
    + + + + + + + + +
     
    Quick Quiz:
    + Why can't NO_HZ_FULL in-kernel execution rely on the + scheduling-clock interrupt, just like HZ_PERIODIC + and NO_HZ_IDLE do? +
    Answer:
    + Because, as a performance optimization, NO_HZ_FULL + does not necessarily re-enable the scheduling-clock interrupt + on entry to each and every system call. +
     
    + +

    +However, RCU must be reliably informed as to whether any given +CPU is currently in the idle loop, and, for NO_HZ_FULL, +also whether that CPU is executing in usermode, as discussed +earlier. +It also requires that the scheduling-clock interrupt be enabled when +RCU needs it to be: + +

      +
    1. If a CPU is either idle or executing in usermode, and RCU believes + it is non-idle, the scheduling-clock tick had better be running. + Otherwise, you will get RCU CPU stall warnings. Or at best, + very long (11-second) grace periods, with a pointless IPI waking + the CPU from time to time. +
    2. If a CPU is in a portion of the kernel that executes RCU read-side + critical sections, and RCU believes this CPU to be idle, you will get + random memory corruption. DON'T DO THIS!!! + +
      This is one reason to test with lockdep, which will complain + about this sort of thing. +
    3. If a CPU is in a portion of the kernel that is absolutely + positively no-joking guaranteed to never execute any RCU read-side + critical sections, and RCU believes this CPU to to be idle, + no problem. This sort of thing is used by some architectures + for light-weight exception handlers, which can then avoid the + overhead of rcu_irq_enter() and rcu_irq_exit() + at exception entry and exit, respectively. + Some go further and avoid the entireties of irq_enter() + and irq_exit(). + +
      Just make very sure you are running some of your tests with + CONFIG_PROVE_RCU=y, just in case one of your code paths + was in fact joking about not doing RCU read-side critical sections. +
    4. If a CPU is executing in the kernel with the scheduling-clock + interrupt disabled and RCU believes this CPU to be non-idle, + and if the CPU goes idle (from an RCU perspective) every few + jiffies, no problem. It is usually OK for there to be the + occasional gap between idle periods of up to a second or so. + +
      If the gap grows too long, you get RCU CPU stall warnings. +
    5. If a CPU is either idle or executing in usermode, and RCU believes + it to be idle, of course no problem. +
    6. If a CPU is executing in the kernel, the kernel code + path is passing through quiescent states at a reasonable + frequency (preferably about once per few jiffies, but the + occasional excursion to a second or so is usually OK) and the + scheduling-clock interrupt is enabled, of course no problem. + +
      If the gap between a successive pair of quiescent states grows + too long, you get RCU CPU stall warnings. +
    + + + + + + + + +
     
    Quick Quiz:
    + But what if my driver has a hardware interrupt handler + that can run for many seconds? + I cannot invoke schedule() from an hardware + interrupt handler, after all! +
    Answer:
    + One approach is to do rcu_irq_exit();rcu_irq_enter(); + every so often. + But given that long-running interrupt handlers can cause + other problems, not least for response time, shouldn't you + work to keep your interrupt handler's runtime within reasonable + bounds? +
     
    + +

    +But as long as RCU is properly informed of kernel state transitions between +in-kernel execution, usermode execution, and idle, and as long as the +scheduling-clock interrupt is enabled when RCU needs it to be, you +can rest assured that the bugs you encounter will be in some other +part of RCU or some other part of the kernel! +

    Memory Efficiency

    -- 2.5.2