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Date: Sun, 15 Sep 2013 17:29:17 +0400
From: Kirill Tkhai <tkhai@...dex.ru>
To: "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH] rcu: Fix CONFIG_RCU_NOCB_CPU_ALL panic on machines with sparse CPU mask
14.09.2013, 22:48, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>:
> On Sat, Sep 14, 2013 at 05:03:20PM +0400, Kirill Tkhai wrote:
>
>> When a system has a sparse cpumask and CONFIG_RCU_NOCB_CPU_ALL is enabled,
>> rcu_spawn_nocb_kthreads() creates nocb threads for nonexistent CPUS.
>>
>> The problem is in rcu_bootup_announce_oddness(). cpumask_setall() sets all
>> bits from 0 to nr_cpu_ids existent and nonexistent CPUs both.
>>
>> The sample. My cpuinfo(sparc64):
>>
>> CPU0: online
>> CPU2: online
>>
>> I get these oopses every boot:
>>
>> Unable to handle kernel NULL pointer dereference
>> CPU: 0 PID: 23 Comm: rcuos/1 Not tainted 3.11.0+ #103
>> task: fffff800be9792c0 ti: fffff800be98c000 task.ti: fffff800be98c000
>> TSTATE: 0000004480e01604 TPC: 000000000047cffc TNPC: 000000000047d000 Y: 00000000 Not tainted
>> TPC: <prepare_to_wait+0x38/0x5c>
>> g0: 0000000000000000 g1: 0000000000000000 g2: fffff800be98fd58 g3: 0000000000acb718
>> g4: fffff800be9792c0 g5: fffff800be536000 g6: fffff800be98c000 g7: 0000000000000000
>> o0: 0000000000000000 o1: 0000000000a33d34 o2: 0000000000000001 o3: 0000000000000000
>> o4: 0000000000000000 o5: 0000000000000000 sp: fffff800be98f3d1 ret_pc: 000000000047cfd8
>> RPC: <prepare_to_wait+0x14/0x5c>
>> l0: 0000000000a33ce8 l1: 0000000000000001 l2: 0000000000a7d800 l3: fffff800bf004dc0
>> l4: fffff800be880310 l5: 0000000000a33ce8 l6: 0000000000ad0000 l7: 0000000000a33800
>> i0: 0000000000acb710 i1: fffff800be98fd40 i2: 0000000000000001 i3: fffff800be98fd40
>> i4: 0000000000000000 i5: 0000000000000000 i6: fffff800be98f491 i7: 00000000004d3f50
>> I7: <rcu_nocb_kthread+0x5c/0x224>
>> Call Trace:
>> [00000000004d3f50] rcu_nocb_kthread+0x5c/0x224
>> [000000000047c604] kthread+0x88/0x9c
>> [0000000000406084] ret_from_fork+0x1c/0x2c
>> [0000000000000000] (null)
>>
>> So, not possible CPUs have to be cleared.
>
> Good catch!
>
> I am hoping that your setup always has CPU 0 defined. If not, there are
> many other problems in addition to CONFIG_RCU_NOCB_CPU_ZERO=y!
I've never seen machines like you said, but it seem they are possible. boot_cpu_init() accounts
the fact kernel may boot from not zero CPU:
static void __init boot_cpu_init(void)
{
int cpu = smp_processor_id();
/* Mark the boot cpu "present", "online" etc for SMP and UP case */
set_cpu_online(cpu, true);
set_cpu_active(cpu, true);
set_cpu_present(cpu, true);
set_cpu_possible(cpu, true);
}
>> Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
>> CC: Dipankar Sarma <dipankar@...ibm.com>
>> CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> ---
>> kernel/rcutree_plugin.h | 1 +
>> 1 files changed, 1 insertions(+), 0 deletions(-)
>> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
>> index 130c97b..1f17e99 100644
>> --- a/kernel/rcutree_plugin.h
>> +++ b/kernel/rcutree_plugin.h
>> @@ -97,6 +97,7 @@ static void __init rcu_bootup_announce_oddness(void)
>> #ifdef CONFIG_RCU_NOCB_CPU_ALL
>> pr_info("\tOffload RCU callbacks from all CPUs\n");
>> cpumask_setall(rcu_nocb_mask);
>> + cpumask_and(rcu_nocb_mask, cpu_possible_mask, rcu_nocb_mask);
>
> Would it make sense to combine the cpumask_setall() and the cpumask_and()
> into a single cpumask_copy()?
>
>> #endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
>> #endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
>> if (have_rcu_nocb_mask) {
>
> Do we need a cpumask_and() within this "if" statement to handle the
> case where the kernel's rcu_nocbs= boot parameter specified a CPU
> that is not present? (If so, probably also giving a warning about the
> non-present CPUs specified.)
>
> I believe that the answer to both questions is "yes", and I would
> welcome an updated patch that covered these cases.
Yes and yes. New patch is below.
> All that said... Why does the system have a sparse cpu_possible_mask?
> This does cause overallocation of memory in a number of areas, including
> RCU's combining tree of rcu_node structures, to say nothing of the
> cpumasks themselves.
In case of sparc64 the cpu number is configured by boot/openprom. Kernel
doesn't change it. I have not touched inside of them, so I don't know.
[PATCH] rcu: Fix CONFIG_RCU_NOCB_CPU_ALL panic on machines with sparse CPU mask
Some architectures have sparse cpu mask. UltraSparc's cpuinfo for example:
CPU0: online
CPU2: online
So, set only possible CPUs when CONFIG_RCU_NOCB_CPU_ALL is enabled.
Also, check that user passes right 'rcu_nocbs=' option.
Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
CC: Dipankar Sarma <dipankar@...ibm.com>
CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
kernel/rcutree_plugin.h | 8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 130c97b..0b5d673 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -96,10 +96,16 @@ static void __init rcu_bootup_announce_oddness(void)
#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
#ifdef CONFIG_RCU_NOCB_CPU_ALL
pr_info("\tOffload RCU callbacks from all CPUs\n");
- cpumask_setall(rcu_nocb_mask);
+ cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
if (have_rcu_nocb_mask) {
+ if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
+ pr_info("\tNote: kernel parameter 'rcu_nocbs=' "
+ "contains nonexistent CPUs.\n");
+ cpumask_and(rcu_nocb_mask, cpu_possible_mask,
+ rcu_nocb_mask);
+ }
cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
if (rcu_nocb_poll)
--
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