Re: [PATCH -next] cgroup/cpuset: Reduce the lock protecting CS_SCHED_LOAD_BALANCE

[Tejun Heo <tj@kernel.org>]

On Fri, May 31, 2024 at 04:07:32PM -0400, Waiman Long wrote:
> 
> On 5/26/24 14:35, Tejun Heo wrote:
> > On Sat, May 25, 2024 at 09:46:48AM +0000, Xiu Jianfeng wrote:
> > > In the cpuset_css_online(), clearing the CS_SCHED_LOAD_BALANCE bit
> > > of cs->flags is guarded by callback_lock and cpuset_mutex. There is
> > > no problem with itself, because it is consistent with the description
> > > of there two global lock at the beginning of this file. However, since
> > > the operation of checking, setting and clearing the flag bit is atomic,
> > > protection of callback_lock is unnecessary here, see CS_SPREAD_*. so
> > > to make it more consistent with the other code, move the operation
> > > outside the critical section of callback_lock.
> > > 
> > > No functional changes intended.
> > > 
> > > Signed-off-by: Xiu Jianfeng<xiujianfeng@huawei.com>
> > > ---
> > >   kernel/cgroup/cpuset.c | 14 ++++++--------
> > >   1 file changed, 6 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index f9d2a3487645..315f8cbd6d35 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -4038,6 +4038,12 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
> > >   		set_bit(CS_SPREAD_PAGE, &cs->flags);
> > >   	if (is_spread_slab(parent))
> > >   		set_bit(CS_SPREAD_SLAB, &cs->flags);
> > > +	/*
> > > +	 * For v2, clear CS_SCHED_LOAD_BALANCE if parent is isolated
> > > +	 */
> > > +	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys) &&
> > > +	    !is_sched_load_balance(parent))
> > > +		clear_bit(CS_SCHED_LOAD_BALANCE, &cs->flags);
> > The code looks weird to me. It's doing the same thing under the
> > is_in_v2_mode() block and the cgroup_subsys_on_dfl() block and the former is
> > also run when the latter condition is true. Looks like we can get rid of the
> > latter block? Waiman?
> 
> Sorry for the late reply.
> 
> The handling of the CS_SCHED_LOAD_BALANCE flag is different between v1 and
> v2. For v1, CS_SCHED_LOAD_BALANCE is default to ON unless it is explicitly
> turned off by writing to cpuset.sched_load_balance. For v2, the state will
> follow its parent when a new cpuset is brought online. Since is_in_v2_mode()
> can be on with cgroup v1, we can't group the two together.
> 
> I agree that we don't need to protect the clearing of CS_SCHED_LOAD_BALANCE
> with the callback lock. So I don't have objection to this patch.
> 
> Acked-by: Waiman Long <longman@redhat.com>

Applied to cgroup/for-6.11.

Thanks.

-- 
tejun