Messages in this thread Patch in this message | | | Date | Wed, 9 Mar 2022 00:20:33 +0100 | Subject | Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock | From | Marek Szyprowski <> |
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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
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