Date: Mon, 13 Mar 2017 09:27:59 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Peter Holm <peter@holm.cc>, Mark Johnston <markj@FreeBSD.org> Cc: freebsd-hackers@FreeBSD.org Subject: Re: draining high-frequency callouts Message-ID: <ac7c766a-98fe-7f96-ecba-6f4d083f854a@selasky.org> In-Reply-To: <20170313082120.GA44651@x2.osted.lan> References: <20170110205711.GA86449@wkstn-mjohnston.west.isilon.com> <20170313082120.GA44651@x2.osted.lan>
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On 03/13/17 09:21, Peter Holm wrote: > 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. > Hi, It depends on the trigger. The panic above just means the callout structure was referred after being freed. Either a drain call is missing or the callout escaped drain. Can you run this test with a kernel built from projects/hps_head aswell? --HPS
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