Messages in this thread | | | Date | Tue, 8 Mar 2022 15:44:03 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock |
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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?
Thanx, Paul
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