lkml.org 
[lkml]   [2022]   [Mar]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@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

\
 
 \ /
  Last update: 2022-03-09 02:40    [W:0.193 / U:0.384 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site