Messages in this thread | | | Date | Thu, 10 Mar 2022 09:45:17 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock |
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On Tue, Mar 08, 2022 at 03:44:03PM -0800, Paul E. McKenney wrote: > On Wed, Mar 09, 2022 at 12:32:25AM +0100, Peter Zijlstra wrote: > > On Wed, Mar 09, 2022 at 12:20:33AM +0100, 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: > > > > > > 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! > > > > Arguably, with the flavours folded again, rcu_dereference_check() ought > > to default include rcu_read_lock_sched_held() or its equivalent I > > suppose. > > > > Paul? > > That would reduce the number of warnings, but it also would hide bugs. > > So, are you sure you really want this?
I don't understand... Since the flavours got merged regular RCU has it's quescent state held off by preempt_disable. So how can relying on that cause bugs?
And if we can rely on that, then surely rcu_dereferenced_check() ought to play by the same rules, otherwise we get silly warnings like these at hand.
Specifically, we removed the rcu_read_lock() here because this has rq->lock held, which is a raw_spinlock_t which very much implies preempt disable, on top of that, it's also an IRQ-safe lock and thus IRQs will be disabled.
There is no possible way for RCU to make progress.
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