* Peter Zijlstra <pet...@infradead.org> wrote: > On Thu, Apr 02, 2015 at 12:42:27PM +0200, Ingo Molnar wrote: > > So why not use a suitable CPU_DOWN* notifier for this, instead of open > > coding it all into a random place in the hotplug machinery? > > Because notifiers are crap? ;-) [...]
No doubt - but I didn't feel this poorly named random call into the hotplug code, with no comments was any better. > [...] Its entirely impossible to figure out what's happening to core > code in hotplug. You need to go chase down and random order notifier > things. > > I'm planning on taking out many of the core hotplug notifiers and > hard coding their callbacks into the hotplug code. That's very welcome news - but please also lets put in place a proper namespace for all these callbacks, to make them easy to find and change: hotplug_cpu__*() or so, which in this case would turn into hotplug_cpu__tick_pull() or so? > That way at least its clear wtf happens when. Okay. I'll resurrect the fix with a hotplug_cpu__tick_pull() name - agreed? > > Also, I improved the changelog (attached below), but decided > > against applying it until these questions are cleared - please use > > that for future versions of this patch. > > > > Fixes: > > http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html > > You forgot to fix the Fixes line ;-) > > My copy has: > > Fixes: 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast") Hm, not sure how that got lost - my git-log of the patch ported on top of timers/core still has it: ==========================> From 413fbf5193b330c5f478ef7aaeaaee08907a993e Mon Sep 17 00:00:00 2001 From: Preeti U Murthy <pre...@linux.vnet.ibm.com> Date: Mon, 30 Mar 2015 14:59:19 +0530 Subject: [PATCH] clockevents: Fix cpu_down() race for hrtimer based broadcasting It was found when doing a hotplug stress test on POWER, that the machine either hit softlockups or rcu_sched stall warnings. The issue was traced to commit: 7cba160ad789 ("powernv/cpuidle: Redesign idle states management") which exposed the cpu_down() race with hrtimer based broadcast mode: 5d1638acb9f6 ("tick: Introduce hrtimer based broadcast") The race is the following: Assume CPU1 is the CPU which holds the hrtimer broadcasting duty before it is taken down. CPU0 CPU1 cpu_down() take_cpu_down() disable_interrupts() cpu_die() while (CPU1 != CPU_DEAD) { msleep(100); switch_to_idle(); stop_cpu_timer(); schedule_broadcast(); } tick_cleanup_cpu_dead() take_over_broadcast() So after CPU1 disabled interrupts it cannot handle the broadcast hrtimer anymore, so CPU0 will be stuck forever. Fix this by explicitly taking over broadcast duty before cpu_die(). This is a temporary workaround. What we really want is a callback in the clockevent device which allows us to do that from the dying CPU by pushing the hrtimer onto a different cpu. That might involve an IPI and is definitely more complex than this immediate fix. Changelog was picked up from: https://lkml.org/lkml/2015/2/16/213 Suggested-by: Thomas Gleixner <t...@linutronix.de> Tested-by: Nicolas Pitre <n...@linaro.org> Signed-off-by: Preeti U. Murthy <pre...@linux.vnet.ibm.com> Cc: linuxppc-dev@lists.ozlabs.org Cc: m...@ellerman.id.au Cc: nicolas.pi...@linaro.org Cc: pet...@infradead.org Cc: r...@rjwysocki.net Fixes: http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html Link: http://lkml.kernel.org/r/20150330092410.24979.59887.st...@preeti.in.ibm.com [ Merged it to the latest timer tree, tidied up the changelog. ] Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/cpu.c | 2 ++ kernel/time/tick-broadcast.c | 19 +++++++++++-------- kernel/time/tick-internal.h | 2 ++ 3 files changed, 15 insertions(+), 8 deletions(-) diff --git a/kernel/cpu.c b/kernel/cpu.c index 1972b161c61e..f9ca351a404a 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -20,6 +20,7 @@ #include <linux/gfp.h> #include <linux/suspend.h> #include <linux/lockdep.h> +#include <linux/tick.h> #include <trace/events/power.h> #include "smpboot.h" @@ -411,6 +412,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen) while (!idle_cpu(cpu)) cpu_relax(); + tick_takeover(cpu); /* This actually kills the CPU. */ __cpu_die(cpu); diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c index 19cfb381faa9..81174cd9a29c 100644 --- a/kernel/time/tick-broadcast.c +++ b/kernel/time/tick-broadcast.c @@ -680,14 +680,19 @@ static void broadcast_shutdown_local(struct clock_event_device *bc, clockevents_set_state(dev, CLOCK_EVT_STATE_SHUTDOWN); } -static void broadcast_move_bc(int deadcpu) +void tick_takeover(int deadcpu) { - struct clock_event_device *bc = tick_broadcast_device.evtdev; + struct clock_event_device *bc; + unsigned long flags; - if (!bc || !broadcast_needs_cpu(bc, deadcpu)) - return; - /* This moves the broadcast assignment to this cpu */ - clockevents_program_event(bc, bc->next_event, 1); + raw_spin_lock_irqsave(&tick_broadcast_lock, flags); + bc = tick_broadcast_device.evtdev; + + if (bc && broadcast_needs_cpu(bc, deadcpu)) { + /* This moves the broadcast assignment to this CPU: */ + clockevents_program_event(bc, bc->next_event, 1); + } + raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags); } /* @@ -924,8 +929,6 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup) cpumask_clear_cpu(cpu, tick_broadcast_pending_mask); cpumask_clear_cpu(cpu, tick_broadcast_force_mask); - broadcast_move_bc(cpu); - raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags); } diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h index b6ba0a44e740..d0794a21ab44 100644 --- a/kernel/time/tick-internal.h +++ b/kernel/time/tick-internal.h @@ -126,6 +126,7 @@ extern int tick_broadcast_oneshot_active(void); extern void tick_check_oneshot_broadcast_this_cpu(void); bool tick_broadcast_oneshot_available(void); extern struct cpumask *tick_get_broadcast_oneshot_mask(void); +extern void tick_takeover(int deadcpu); #else /* !(BROADCAST && ONESHOT): */ static inline void tick_broadcast_setup_oneshot(struct clock_event_device *bc) { BUG(); } static inline int tick_broadcast_oneshot_control(unsigned long reason) { return 0; } @@ -134,6 +135,7 @@ static inline void tick_shutdown_broadcast_oneshot(unsigned int *cpup) { } static inline int tick_broadcast_oneshot_active(void) { return 0; } static inline void tick_check_oneshot_broadcast_this_cpu(void) { } static inline bool tick_broadcast_oneshot_available(void) { return tick_oneshot_possible(); } +static inline void tick_takeover(int deadcpu) { } #endif /* !(BROADCAST && ONESHOT) */ /* NO_HZ_FULL internal */ _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
