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

[Marek Szyprowski <m.szyprowski@samsung.com>]

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:

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
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland