Re: [PATCH 3/4 v2] cgroup: separate rstat locks for subsystems

Shakeel Butt
2025-03-03T19:39:11Z

On Mon, Mar 03, 2025 at 11:23:49AM -0800, JP Kobryn wrote:
>
>
> On 3/3/25 10:40 AM, Shakeel Butt wrote:
> > On Mon, Mar 03, 2025 at 06:29:53PM +0000, Yosry Ahmed wrote:
> > > On Mon, Mar 03, 2025 at 04:22:42PM +0100, Michal Koutný wrote:
> > > > On Thu, Feb 27, 2025 at 01:55:42PM -0800, inwardvessel <inwardvessel@gmail.com> wrote:
> > > > > From: JP Kobryn <inwardvessel@gmail.com>
> > > > ...
> > > > > +static inline bool is_base_css(struct cgroup_subsys_state *css)
> > > > > +{
> > > > > + return css->ss == NULL;
> > > > > +}
> > > >
> > > > Similar predicate is also used in cgroup.c (various cgroup vs subsys
> > > > lifecycle functions, e.g. css_free_rwork_fn()). I think it'd better
> > > > unified, i.e. open code the predicate here or use the helper in both
> > > > cases (css_is_cgroup() or similar).
> > > >
> > > > > void __init cgroup_rstat_boot(void)
> > > > > {
> > > > > - int cpu;
> > > > > + struct cgroup_subsys *ss;
> > > > > + int cpu, ssid;
> > > > > - for_each_possible_cpu(cpu)
> > > > > - raw_spin_lock_init(per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu));
> > > > > + for_each_subsys(ss, ssid) {
> > > > > + spin_lock_init(&cgroup_rstat_subsys_lock[ssid]);
> > > > > + }
> > > >
> > > > Hm, with this loop I realize it may be worth putting this lock into
> > > > struct cgroup_subsys_state and initializing them in
> > > > cgroup_init_subsys() to keep all per-subsys data in one pack.
> > >
> > > I thought about this, but this would have unnecessary memory overhead as
> > > we only need one lock per-subsystem. So having a lock in every single
> > > css is wasteful.
> > >
> > > Maybe we can put the lock in struct cgroup_subsys? Then we can still
> > > initialize them in cgroup_init_subsys().
> > >
> >
> > Actually one of things I was thinking about if we can just not have
> > per-subsystem lock at all. At the moment, it is protecting
> > rstat_flush_next field (today in cgroup and JP's series it is in css).
> > What if we make it a per-cpu then we don't need the per-subsystem lock
> > all? Let me know if I missed something which is being protected by this
> > lock.
> >
> > This is help the case where there are multiple same subsystem stat
> > flushers, possibly of differnt part of cgroup tree. Though they will
> > still compete on per-cpu lock but still would be better than a
> > sub-system level lock.
>
> Right, the trade-off would mean one subsystem flushing could contend for
> a cpu where a different subsystem is updating and vice versa.
>

I meant we keep the per-subsystem per-cpu locks but remove the
per-subsystem lock (i.e. cgroup_rstat_subsys_lock) if we make
rstat_flush_next per-cpu. In that case two memory stats readers will not
compete on memory subsystem lock but they will compete on per-cpu memory
locks. Though we will have to experiment to see if this really helps or
not.