Re: [RFC PATCH v3 1/2] cgroup: cgroup.freeze.stat.local time accounting

Hello.

On Mon, Aug 04, 2025 at 08:29:41PM -0700, Tiffany Yang ynaffit@google.com wrote:

• cgroup.freeze.stat.local
• A read-only flat-keyed file which exists in non-root cgroups.
• The following entry is defined:

•  freeze_time_total
  

• Cumulative time that this cgroup has spent between freezing and
  

• thawing, regardless of whether by self or ancestor groups.
  

• NB: (not) reaching "frozen" state is not accounted here.
  

• Using the following ASCII representation of a cgroup's freezer
  

• state, ::
  

• 	       1    _____
  

• 	frozen 0 __/     \__
  

• 	          ab    cd
  

• .. Originally contributed by Michal Koutný <mkoutny@suse.com>
  

• the duration being measured is the span between a and c.
  

This is so little "artwork" that a mere mention in commit message is OK ;-)

+static int cgroup_freeze_local_stat_show(struct seq_file *seq, void *v) +{

• struct cgroup *cgrp = seq_css(seq)->cgroup;
• unsigned int sequence;
• u64 freeze_time;
• do {

• sequence = read_seqcount_begin(&cgrp->freezer.freeze_seq);
  

• freeze_time = cgrp->freezer.freeze_time_total_ns;
  

• /* Add in current freezer interval if the task is now frozen */
  

Nit: cgrp is frozen, not a task here

@@ -179,10 +179,16 @@ static void cgroup_do_freeze(struct cgroup *cgrp, bool freeze) lockdep_assert_held(&cgroup_mutex); spin_lock_irq(&css_set_lock);

• if (freeze)

• write_seqcount_begin(&cgrp->freezer.freeze_seq);
• if (freeze) { set_bit(CGRP_FREEZE, &cgrp->flags);

• else

• cgrp->freezer.freeze_time_start_ns = ktime_get_ns();
  

I wonder whether it wouldn't achieve more stable results if the reference timestamp was taken only once in cgroup_freeze(). Measuring the rate of cgroup traversal is rather noise in this case.

Thanks, Michal