On Wed, Mar 19, 2025 at 06:35:02PM +0000, Yosry Ahmed wrote:
> On Wed, Mar 19, 2025 at 02:06:43PM -0400, Johannes Weiner wrote:
> > (btw, why do we not have any locking around the root stats in
> > cgroup_base_stat_cputime_show()? There isn't anything preventing a
> > reader from seeing all zeroes if another reader runs the memset() on
> > cgrp->bstat, is there? Or double times...)
>
> (I think root_cgroup_cputime() operates on a stack allocated bstat, not
> cgrp->bstat)
That was the case until:
commit b824766504e49f3fdcbb8c722e70996a78c3636e
Author: Chen Ridong <chenridong@xxxxxxxxxx>
Date: Thu Jul 4 14:01:19 2024 +0000
cgroup/rstat: add force idle show helper
Now it's doing this:
void cgroup_base_stat_cputime_show(struct seq_file *seq)
{
struct cgroup *cgrp = seq_css(seq)->cgroup;
if (cgroup_parent(cgrp)) {
...
} else {
/* cgrp->bstat of root is not actually used, reuse it */
root_cgroup_cputime(&cgrp->bstat);
usage = cgrp->bstat.cputime.sum_exec_runtime;
utime = cgrp->bstat.cputime.utime;
stime = cgrp->bstat.cputime.stime;
ntime = cgrp->bstat.ntime;
}
}
and then:
static void root_cgroup_cputime(struct cgroup_base_stat *bstat)
{
struct task_cputime *cputime = &bstat->cputime;
int i;
memset(bstat, 0, sizeof(*bstat));
/* various += on bstat and cputime members */
}
So all readers are mucking with the root cgroup's bstat.
