Johannes Weiner <hannes@xxxxxxxxxxx> 于2023年2月9日周四 03:20写道：
> On Wed, Feb 08, 2023 at 06:29:56PM +0100, Michal Koutný wrote:
> > On Thu, Feb 09, 2023 at 12:16:54AM +0800, Kairui Song <ryncsn@xxxxxxxxx> wrote:
> > > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > > Signed-off-by: Kairui Song <ryncsn@xxxxxxxxx>
> >
> > Typo?
> >
> > > -static inline struct psi_group *task_psi_group(struct task_struct *task)
> > > +static inline struct psi_group *psi_iter_first(struct task_struct *task, void **iter)
> > > {
> > > #ifdef CONFIG_CGROUPS
> > > - if (static_branch_likely(&psi_cgroups_enabled))
> > > - return cgroup_psi(task_dfl_cgroup(task));
> > > + if (static_branch_likely(&psi_cgroups_enabled)) {
> > > + struct cgroup *cgroup = task_dfl_cgroup(task);
> > > +
> > > + *iter = cgroup_parent(cgroup);
> >
> > This seems to skip a cgroup level -- maybe that's the observed
> > performance gain?
>
> Hm, I don't think it does. It sets up *iter to point to the parent for
> the _next() call, but it returns task_dfl_cgroup()->psi. The next call
> does the same: cgroup = *iter, *iter = parent, return cgroup->psi.
>
> It could be a bit more readable to have *iter always point to the
> current cgroup - but no strong preference either way from me:
>
> psi_groups_first(task, iter)
> {
> #ifdef CONFIG_CGROUPS
> if (static_branch_likely(&psi_cgroups_enabled)) {
> struct cgroup *cgroup = task_dfl_cgroup(task);
>
> *iter = cgroup;
> return cgroup_psi(cgroup);
> }
> #endif
> return &psi_system;
> }
>
> psi_groups_next(iter)
> {
> #ifdef CONFIG_CGROUPS
> if (static_branch_likely(&psi_cgroups_enabled)) {
> struct cgroup *cgroup = *iter;
>
> if (cgroup) {
> *iter = cgroup_parent(cgroup);
> return cgroup_psi(cgroup);
> }
> }
> return NULL;
> #endif
> }
> psi_groups_next(iter)
> {
> #ifdef CONFIG_CGROUPS
> if (static_branch_likely(&psi_cgroups_enabled)) {
> struct cgroup *cgroup = *iter;
>
> if (cgroup) {
> *iter = cgroup_parent(cgroup);
> return cgroup_psi(cgroup);
> }
> }
> return NULL;
> #endif
> }
It should be like this, right? For psi_groups_next, retrieving cgroup
parent should be done before "if (cgroup)".
+ psi_groups_next(iter)
+ {
+ #ifdef CONFIG_CGROUPS
+ if (static_branch_likely(&psi_cgroups_enabled)) {
+ struct cgroup *cgroup = *iter;
+
+ cgroup = cgroup_parent(cgroup);
+ if (cgroup) {
+ *iter = cgroup;
+ return cgroup_psi(cgroup);
+ }
+ }
+ return NULL;
+ #endif
+ }
Thanks for the suggestion!
I think your style is better indeed.
I tried to re-benchmark the code just in case, and found it seems my
previous benchmark result is not accurate enough now, some results
changed after I did a rebase to latest master, or maybe just 100 times
of perfpipe is not enough to get a stable result.
With a few times of retest, the final conclusion of the commit message
is still valid, will post V2 later just after more test.
