From owner-freebsd-hackers@freebsd.org Mon Mar 13 08:21:24 2017 Return-Path: Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 00CF0D07968 for ; Mon, 13 Mar 2017 08:21:24 +0000 (UTC) (envelope-from pho@holm.cc) Received: from relay01.pair.com (relay01.pair.com [209.68.5.15]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id CA4C01BB6; Mon, 13 Mar 2017 08:21:23 +0000 (UTC) (envelope-from pho@holm.cc) Received: from x2.osted.lan (87-58-223-204-dynamic.dk.customer.tdc.net [87.58.223.204]) by relay01.pair.com (Postfix) with ESMTP id 05D22D010C9; Mon, 13 Mar 2017 04:21:21 -0400 (EDT) Received: from x2.osted.lan (localhost [127.0.0.1]) by x2.osted.lan (8.14.9/8.14.9) with ESMTP id v2D8LKlt045645 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Mon, 13 Mar 2017 09:21:20 +0100 (CET) (envelope-from pho@x2.osted.lan) Received: (from pho@localhost) by x2.osted.lan (8.14.9/8.14.9/Submit) id v2D8LKmb045644; Mon, 13 Mar 2017 09:21:20 +0100 (CET) (envelope-from pho) Date: Mon, 13 Mar 2017 09:21:20 +0100 From: Peter Holm To: Mark Johnston Cc: freebsd-hackers@FreeBSD.org Subject: Re: draining high-frequency callouts Message-ID: <20170313082120.GA44651@x2.osted.lan> References: <20170110205711.GA86449@wkstn-mjohnston.west.isilon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170110205711.GA86449@wkstn-mjohnston.west.isilon.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 13 Mar 2017 08:21:24 -0000 On Tue, Jan 10, 2017 at 12:57:12PM -0800, Mark Johnston wrote: > I'm occasionally seeing an assertion failure in softclock_call_cc() when > running DTrace tests on a system with hz=10000. The assertion > (c->c_flags & CALLOUT_ACTIVE) != 0 is failing while a thread is > concurrently draining the callout, which runs at a high frequency. At > the time of the panic, that thread is spinning on the per-CPU callout > lock after having been awoken from "codrain", and CALLOUT_PENDING is > set on the callout. The callout is direct, i.e., it is executed in hard > interrupt context. > > I think this is what's happening: > - callout_drain() is called while the callout is executing but after the > callout has rescheduled itself, and goes to sleep after having cleared > CALLOUT_ACTIVE. > - softclock_call_cc() wakes up the callout_drain() caller, but the > callout fires again before the caller is scheduled. > - the second softclock_call_cc() call sees that CALLOUT_ACTIVE is > cleared and panics. > > Is there anything that prevents this scenario? Is it really correct to > leave CALLOUT_ACTIVE cleared when the per-CPU callout lock must be > dropped in order to acquire a sleepqueue lock? > Is this the same problem? panic: softclock_call_cc: act 0xfffff8000de64800 0 cpuid = 10 time = 1489389893 KDB: stack backtrace: db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0f984c9660 vpanic() at vpanic+0x19c/frame 0xfffffe0f984c96e0 kassert_panic() at kassert_panic+0x126/frame 0xfffffe0f984c9750 softclock_call_cc() at softclock_call_cc+0xae/frame 0xfffffe0f984c98f0 softclock() at softclock+0x12c/frame 0xfffffe0f984c9930 intr_event_execute_handlers() at intr_event_execute_handlers+0x248/frame 0xfffffe0f984c9980 ithread_execute_handlers() at ithread_execute_handlers+0x47/frame 0xfffffe0f984c99b0 ithread_loop() at ithread_loop+0xfc/frame 0xfffffe0f984c9a30 fork_exit() at fork_exit+0x13b/frame 0xfffffe0f984c9ab0 https://people.freebsd.org/~pho/stress/log/kevent7-2.txt I first spotted this @ 12.0-CURRENT #2 r305652M: Fri Sep 9 13:09:03 CEST 2016 It's quite easy to reproduce. - Peter