On 08/13/2017 03:44 PM, Tejun Heo wrote:

Hello, Waiman.

On Fri, Aug 11, 2017 at 04:19:07PM -0400, Waiman Long wrote:

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* CPU usage stats are collected and shown in "cgroup.stat" with "cpu."
  prefix.  Total usage is collected from scheduling events.  User/sys
  breakdown is sourced from tick sampling and adjusted to the usage
  using cputime_adjuts().

Typo: cputime_adjust()

Oops, will fix.

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+static DEFINE_MUTEX(cgroup_stat_mutex);
+static DEFINE_PER_CPU(raw_spinlock_t, cgroup_cpu_stat_lock);

If the hierarchy is large enough and the stat data hasn't been read
recently, it may take a while to accumulate all the stat data even for
one cpu in cgroup_stat_flush_locked(). So I think it will make more
sense to use regular spinlock_t instead of raw_spinlock_t.

They need to be raw spinlocks because the accounting side may grab
them while scheduling.  If the accumulating latency becomes
problematic, we can test for need_resched and spin_needbreak and break
out as necessary.  The iteration logic is built to allow that and an
earlier revision actually did that but I wasn't sure whether it's
actually necessary and removed it for simplicity.

If the latency ever becomes a problem, resurrecting that isn't
difficult at all.

Right, I forgot they will be used by the scheduler.

I think it is prudent to limit the latency, but it can be another patch
when the need arises.

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+static struct cgroup *cgroup_cpu_stat_next_updated(struct cgroup *pos,
+						   struct cgroup *root, int cpu)

This function is trying to unwind one cgrp from the updated_children and
updated_next linkage. It is somewhat like the opposite of
cgroup_cpu_stat_updated(). I just feel like its name isn't intuitive
enough to convey what it is doing. Maybe use name like
cgroup_cpu_stat_unlink_one() to match cgroup_cpu_stat_flush_one().

Hmm... the name comes from it being an iterator - most interators are
named _next.  But, yeah, the name doesn't signifiy that it unlinks as
it goes along.  I'll rename it to cgroup_cpu_stat_pop_updated().

I am fine with that.

Thanks,
Longman