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

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Message-ID: <YifniVyoJ9NNU+pv@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Mar 2022 00:32:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Chengming Zhou <zhouchengming@...edance.com>, mingo@...hat.com,
        vincent.guittot@...aro.org, bristot@...hat.com,
        zhaolei@...fujitsu.com, tj@...nel.org, lizefan.x@...edance.com,
        hannes@...xchg.org, linux-kernel@...r.kernel.org,
        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock

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@...radead.org>
> > Signed-off-by: Chengming Zhou <zhouchengming@...edance.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?