Re: [External] Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/3/9 7:20 上午, Marek Szyprowski wrote:
> On 20.02.2022 06:14, Chengming Zhou wrote:
>> Since cpuacct_charge() is called from the scheduler update_curr(),
>> we must already have rq lock held, then the RCU read lock can
>> be optimized away.
>>
>> And do the same thing in it's wrapper cgroup_account_cputime(),
>> but we can't use lockdep_assert_rq_held() there, which defined
>> in kernel/sched/sched.h.
>>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> 
> This patch landed recently in linux-next as commit dc6e0818bc9a 
> ("sched/cpuacct: Optimize away RCU read lock"). On my test systems I 
> found that it triggers a following warning in the early boot stage:

Hi, thanks for the report. I've send a fix patch[1] for review.

[1] https://lore.kernel.org/lkml/20220305034103.57123-1-zhouchengming@bytedance.com/

> 
> Calibrating delay loop (skipped), value calculated using timer 
> frequency.. 48.00 BogoMIPS (lpj=240000)
> pid_max: default: 32768 minimum: 301
> Mount-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
> Mountpoint-cache hash table entries: 2048 (order: 1, 8192 bytes, linear)
> CPU: Testing write buffer coherency: ok
> CPU0: Spectre v2: using BPIALL workaround
> 
> =============================
> WARNING: suspicious RCU usage
> 5.17.0-rc5-00050-gdc6e0818bc9a #11458 Not tainted
> -----------------------------
> ./include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 2 locks held by kthreadd/2:
>   #0: c1d7972c (&p->pi_lock){....}-{2:2}, at: task_rq_lock+0x30/0x118
>   #1: ef7b52d0 (&rq->__lock){-...}-{2:2}, at: 
> raw_spin_rq_lock_nested+0x24/0x34
> 
> stack backtrace:
> CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.17.0-rc5-00050-gdc6e0818bc9a 
> #11458
> Hardware name: Samsung Exynos (Flattened Device Tree)
>   unwind_backtrace from show_stack+0x10/0x14
>   show_stack from dump_stack_lvl+0x58/0x70
>   dump_stack_lvl from update_curr+0x1bc/0x35c
>   update_curr from dequeue_task_fair+0xb0/0x8e8
>   dequeue_task_fair from __do_set_cpus_allowed+0x19c/0x258
>   __do_set_cpus_allowed from __set_cpus_allowed_ptr_locked+0x130/0x1d8
>   __set_cpus_allowed_ptr_locked from __set_cpus_allowed_ptr+0x48/0x64
>   __set_cpus_allowed_ptr from kthreadd+0x44/0x16c
>   kthreadd from ret_from_fork+0x14/0x2c
> Exception stack(0xc1cb9fb0 to 0xc1cb9ff8)
> 9fa0:                                     00000000 00000000 00000000 
> 00000000
> 9fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000
> 9fe0: 00000000 00000000 00000000 00000000 00000013 00000000
> CPU0: thread -1, cpu 0, socket 9, mpidr 80000900
> cblist_init_generic: Setting adjustable number of callback queues.
> cblist_init_generic: Setting shift to 1 and lim to 1.
> Running RCU-tasks wait API self tests
> Setting up static identity map for 0x40100000 - 0x40100060
> rcu: Hierarchical SRCU implementation.
> 
> =============================
> WARNING: suspicious RCU usage
> 5.17.0-rc5-00050-gdc6e0818bc9a #11458 Not tainted
> -----------------------------
> ./include/linux/cgroup.h:481 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 1 lock held by migration/0/13:
>   #0: ef7b52d0 (&rq->__lock){-...}-{2:2}, at: 
> raw_spin_rq_lock_nested+0x24/0x34
> 
> stack backtrace:
> CPU: 0 PID: 13 Comm: migration/0 Not tainted 
> 5.17.0-rc5-00050-gdc6e0818bc9a #11458
> Hardware name: Samsung Exynos (Flattened Device Tree)
> Stopper: 0x0 <- 0x0
>   unwind_backtrace from show_stack+0x10/0x14
>   show_stack from dump_stack_lvl+0x58/0x70
>   dump_stack_lvl from put_prev_task_stop+0x16c/0x25c
>   put_prev_task_stop from __schedule+0x698/0x964
>   __schedule from schedule+0x54/0xe0
>   schedule from smpboot_thread_fn+0x218/0x288
>   smpboot_thread_fn from kthread+0xf0/0x134
>   kthread from ret_from_fork+0x14/0x2c
> Exception stack(0xc1ccffb0 to 0xc1ccfff8)
> ffa0:                                     00000000 00000000 00000000 
> 00000000
> ffc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
> 00000000
> ffe0: 00000000 00000000 00000000 00000000 00000013 00000000
> smp: Bringing up secondary CPUs ...
> CPU1: thread -1, cpu 1, socket 9, mpidr 80000901
> CPU1: Spectre v2: using BPIALL workaround
> smp: Brought up 1 node, 2 CPUs
> SMP: Total of 2 processors activated (96.00 BogoMIPS).
> 
> The above log comes from ARM 32bit Samsung Exnyos4210 based Trats board.
> 
>> ---
>>   include/linux/cgroup.h | 2 --
>>   kernel/sched/cpuacct.c | 4 +---
>>   2 files changed, 1 insertion(+), 5 deletions(-)
>>
>> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
>> index 75c151413fda..9a109c6ac0e0 100644
>> --- a/include/linux/cgroup.h
>> +++ b/include/linux/cgroup.h
>> @@ -791,11 +791,9 @@ static inline void cgroup_account_cputime(struct task_struct *task,
>>   
>>   	cpuacct_charge(task, delta_exec);
>>   
>> -	rcu_read_lock();
>>   	cgrp = task_dfl_cgroup(task);
>>   	if (cgroup_parent(cgrp))
>>   		__cgroup_account_cputime(cgrp, delta_exec);
>> -	rcu_read_unlock();
>>   }
>>   
>>   static inline void cgroup_account_cputime_field(struct task_struct *task,
>> diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
>> index 307800586ac8..f79f88456d72 100644
>> --- a/kernel/sched/cpuacct.c
>> +++ b/kernel/sched/cpuacct.c
>> @@ -337,12 +337,10 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>>   	unsigned int cpu = task_cpu(tsk);
>>   	struct cpuacct *ca;
>>   
>> -	rcu_read_lock();
>> +	lockdep_assert_rq_held(cpu_rq(cpu));
>>   
>>   	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))
>>   		*per_cpu_ptr(ca->cpuusage, cpu) += cputime;
>> -
>> -	rcu_read_unlock();
>>   }
>>   
>>   /*
> 
> Best regards