Messages in this thread | | | Date | Wed, 04 Feb 2015 14:42:51 -0800 | From | Stephen Boyd <> | Subject | Re: [PATCH] ARM: Don't use complete() during __cpu_die |
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On 02/04/15 08:53, Krzysztof Kozlowski wrote: > The complete() should not be used on offlined CPU. Rewrite the > wait-complete mechanism with wait_on_bit_timeout(). > > The CPU triggering hot unplug (e.g. CPU0) will loop until some bit is > cleared. In each iteration schedule_timeout() is used with initial sleep > time of 1 ms. Later it is increased to 10 ms. > > The dying CPU will clear the bit which is safe in that context. > > This fixes following RCU warning on ARMv8 (Exynos 4412, Trats2) during > suspend to RAM: > > [ 31.113925] =============================== > [ 31.113928] [ INFO: suspicious RCU usage. ] > [ 31.113935] 3.19.0-rc7-next-20150203 #1914 Not tainted > [ 31.113938] ------------------------------- > [ 31.113943] kernel/sched/fair.c:4740 suspicious rcu_dereference_check() usage! > [ 31.113946] > [ 31.113946] other info that might help us debug this: > [ 31.113946] > [ 31.113952] > [ 31.113952] RCU used illegally from offline CPU! > [ 31.113952] rcu_scheduler_active = 1, debug_locks = 0 > [ 31.113957] 3 locks held by swapper/1/0: > [ 31.113988] #0: ((cpu_died).wait.lock){......}, at: [<c005a114>] complete+0x14/0x44 > [ 31.114012] #1: (&p->pi_lock){-.-.-.}, at: [<c004a790>] try_to_wake_up+0x28/0x300 > [ 31.114035] #2: (rcu_read_lock){......}, at: [<c004f1b8>] select_task_rq_fair+0x5c/0xa04 > [ 31.114038] > [ 31.114038] stack backtrace: > [ 31.114046] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.19.0-rc7-next-20150203 #1914 > [ 31.114050] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree) > [ 31.114076] [<c0014ce4>] (unwind_backtrace) from [<c0011c30>] (show_stack+0x10/0x14) > [ 31.114091] [<c0011c30>] (show_stack) from [<c04dc048>] (dump_stack+0x70/0xbc) > [ 31.114105] [<c04dc048>] (dump_stack) from [<c004f83c>] (select_task_rq_fair+0x6e0/0xa04) > [ 31.114118] [<c004f83c>] (select_task_rq_fair) from [<c004a83c>] (try_to_wake_up+0xd4/0x300) > [ 31.114129] [<c004a83c>] (try_to_wake_up) from [<c00598a0>] (__wake_up_common+0x4c/0x80) > [ 31.114140] [<c00598a0>] (__wake_up_common) from [<c00598e8>] (__wake_up_locked+0x14/0x1c) > [ 31.114150] [<c00598e8>] (__wake_up_locked) from [<c005a134>] (complete+0x34/0x44) > [ 31.114167] [<c005a134>] (complete) from [<c04d6ca4>] (cpu_die+0x24/0x84) > [ 31.114179] [<c04d6ca4>] (cpu_die) from [<c005a508>] (cpu_startup_entry+0x328/0x358) > [ 31.114189] [<c005a508>] (cpu_startup_entry) from [<40008784>] (0x40008784) > [ 31.114226] CPU1: shutdown > > Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com> > ---
Would it be better to use IPIs instead? The IPI handler could even call complete() as long as we IPI off the dying CPU to the killing CPU. I suppose this could be an extension of the current IPI_COMPLETION that we already have.
> arch/arm/kernel/smp.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 49 insertions(+), 3 deletions(-) > > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c > index 86ef244c5a24..bb8ff465975f 100644 > --- a/arch/arm/kernel/smp.c > +++ b/arch/arm/kernel/smp.c > @@ -213,7 +219,40 @@ int __cpu_disable(void) > return 0; > } > > -static DECLARE_COMPLETION(cpu_died); > +/* > + * Wait for 5000*1 ms for 'wait_cpu_die' bit to be cleared. > + * Actually the real wait time will be longer because of schedule() > + * called bit_wait_timeout. > + * > + * Returns 0 if bit was cleared (CPU died) or non-zero > + * otherwise (1 or negative ERRNO). > + */ > +static int wait_for_cpu_die(void) > +{ > + int retries = 5000, sleep_ms = 1, ret = 0; > + > + might_sleep(); > + > + smp_mb__before_atomic(); > + while (test_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die)) { > + ret = out_of_line_wait_on_bit_timeout(&wait_cpu_die, > + CPU_DIE_WAIT_BIT, bit_wait_timeout, > + TASK_UNINTERRUPTIBLE, > + msecs_to_jiffies(sleep_ms)); > + if (!ret || (--retries <= 0)) > + break; > + > + if (retries < 4000) { > + /* After ~1000 ms increase sleeping time to 10 ms */ > + retries = 400; > + sleep_ms = 10; > + } > + > + smp_mb__before_atomic(); /* For next test_bit() in loop */ > + } > + > + return ret; > +} >
Is there any reason we test the bit before testing it again in out_of_line_wait_on_bit_timeout()? Why can't we just call that function in a loop and let it handle checking the bit?
> @@ -267,7 +312,8 @@ void __ref cpu_die(void) > * this returns, power and/or clocks can be removed at any point > * from this CPU and its cache by platform_cpu_kill(). > */ > - complete(&cpu_died); > + clear_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die); > + smp_mb__after_atomic(); > > /* > * Ensure that the cache lines associated with that completion are
This comment here should be updated because the completion is gone.
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