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

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Message-ID: <20220308234403.GC4285@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 8 Mar 2022 15:44:03 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        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
Subject: Re: [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock

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@...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?

That would reduce the number of warnings, but it also would hide bugs.

So, are you sure you really want this?

							Thanx, Paul