Messages in this thread | | | Subject | Re: [git pull] scheduler/misc fixes | From | Peter Zijlstra <> | Date | Fri, 25 Apr 2008 12:19:38 +0200 |
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On Fri, 2008-04-25 at 09:48 +0200, Peter Zijlstra wrote:
> a) 15934a37324f32e0fda633dc7984a671ea81cd75 does indeed fix a bug in > rq->clock; without that patch it compresses nohz time to a single > jiffie, so cpu_clock() which (without the above hack) is based on > rq->clock will be short on nohz time. This can 'hide' the clock jump > and thus hide false positives. > > > b) there is commit: > > --- > commit d3938204468dccae16be0099a2abf53db4ed0505 > Author: Thomas Gleixner <tglx@linutronix.de> > Date: Wed Nov 28 15:52:56 2007 +0100 > softlockup: fix false positives on CONFIG_NOHZ > > David Miller reported soft lockup false-positives that trigger > on NOHZ due to CPUs idling for more than 10 seconds. > > The solution is touch the softlockup watchdog when we return from > idle. (by definition we are not 'locked up' when we were idle) > > http://bugzilla.kernel.org/show_bug.cgi?id=9409 > > Reported-by: David Miller <davem@davemloft.net> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > Signed-off-by: Ingo Molnar <mingo@elte.hu> > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 27a2338..cb89fa8 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -133,6 +133,8 @@ void tick_nohz_update_jiffies(void) > if (!ts->tick_stopped) > return; > > + touch_softlockup_watchdog(); > + > cpu_clear(cpu, nohz_cpu_mask); > now = ktime_get(); > > --- > > which should 'fix' this problem. > > > c) there are 'IPI' handlers on SPARC64 that look like they can wake > the CPU from idle sleep but do not appear to call irq_enter() which > has the above patch's touch_softlock_watchdog() in its callchain. > > tl0_irq1: TRAP_IRQ(smp_call_function_client, 1) > tl0_irq2: TRAP_IRQ(smp_receive_signal_client, 2) > tl0_irq3: TRAP_IRQ(smp_penguin_jailcell, 3) > tl0_irq4: TRAP_IRQ(smp_new_mmu_context_version_client, 4)
OK, so David came up with the idea that it might be the reschedule IPI (smp_receive_signal_client) that did the wakeup resulting in the following scenario:
<idle > 60s > <resched-IPI> -> nohz_restart() -> restart timer -> schedule()
<run stuff> <timer> -> softlockup_tick() -> BUG! doing irq_enter/exit() for smp_receive_signal_client() did indeed fix the whole issue. (x86 also has this bug - its just darn hard to generate 60s+ nohz periods due to the shitty clocks/timers)
So per b) any nohz wake needs to be done with an interrupt _and_ all such interrupts must pass through irq_enter().
As far as I can tell this is not nessecarily true (or desired from a performance POV) for all platforms, imagine the core2 monitor/mwait idle that wakes up because of a memory write. This doesn't require an interrupt at all to wake up.
So, are we going to require all waking interrupts (IPIs and regular) to do the irq_enter/exit() dance and add the perhaps unneeded overhead to these paths and require the non-interrupt driven wake-ups like monitor/mwait to do the touch_softlockup_watchdog() themselves?
Or,
Is Ingo's initial patch to make nohz_restart() also touch the softlockup watchdog the best fix (now that we understand what happens)?
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