Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Joel Fernandes <joel@joelfernandes.org>]

Hi Vincent,

On Mon, Jan 25, 2021 at 03:42:41PM +0100, Vincent Guittot wrote:
> On Fri, 22 Jan 2021 at 20:10, Joel Fernandes <joel@joelfernandes.org> wrote:
> > On Fri, Jan 22, 2021 at 05:56:22PM +0100, Vincent Guittot wrote:
> > > On Fri, 22 Jan 2021 at 16:46, Joel Fernandes (Google)
> > > <joel@joelfernandes.org> wrote:
> > > >
> > > > On an octacore ARM64 device running ChromeOS Linux kernel v5.4, I found
> > > > that there are a lot of calls to update_blocked_averages(). This causes
> > > > the schedule loop to slow down to taking upto 500 micro seconds at
> > > > times (due to newidle load balance). I have also seen this manifest in
> > > > the periodic balancer.
> > > >
> > > > Closer look shows that the problem is caused by the following
> > > > ingredients:
> > > > 1. If the system has a lot of inactive CGroups (thanks Dietmar for
> > > > suggesting to inspect /proc/sched_debug for this), this can make
> > > > __update_blocked_fair() take a long time.
> > >
> > > Inactive cgroups are removed from the list so they should not impact
> > > the duration
> >
> > I meant blocked CGroups. According to this code, a cfs_rq can be partially
> > decayed and not have any tasks running on it but its load needs to be
> > decayed, correct? That's what I meant by 'inactive'. I can reword it to
> > 'blocked'.
> 
> How many blocked cgroups have you got ?

I put a counter in for_each_leaf_cfs_rq_safe() { } to count how many times
this loop runs per new idle balance. When the problem happens I see this loop
run 35-40 times (for one single instance of newidle balance). So in total
there are at least these many cfs_rq load updates.

I also see that new idle balance can be called 200-500 times per second.

> >
> >                   * There can be a lot of idle CPU cgroups.  Don't let fully
> >                   * decayed cfs_rqs linger on the list.
> >                   */
> >                  if (cfs_rq_is_decayed(cfs_rq))
> >                          list_del_leaf_cfs_rq(cfs_rq);
> >
> > > > 2. The device has a lot of CPUs in a cluster which causes schedutil in a
> > > > shared frequency domain configuration to be slower than usual. (the load
> > >
> > > What do you mean exactly by it causes schedutil to be slower than usual ?
> >
> > sugov_next_freq_shared() is order number of CPUs in the a cluster. This
> > system is a 6+2 system with 6 CPUs in a cluster. schedutil shared policy
> > frequency update needs to go through utilization of other CPUs in the
> > cluster. I believe this could be adding to the problem but is not really
> > needed to optimize if we can rate limit the calls to update_blocked_averages
> > to begin with.
> 
> Qais mentioned half of the time being used by
> sugov_next_freq_shared(). Are there any frequency changes resulting in
> this call ?

I do not see a frequency update happening at the time of the problem. However
note that sugov_iowait_boost() does run even if frequency is not being
updated. IIRC, this function is also not that light weight and I am not sure
if it is a good idea to call this that often.

> > > > average updates also try to update the frequency in schedutil).
> > > >
> > > > 3. The CPU is running at a low frequency causing the scheduler/schedutil
> > > > code paths to take longer than when running at a high CPU frequency.
> > >
> > > Low frequency usually means low utilization so it should happen that much.
> >
> > It happens a lot as can be seen with schbench. It is super easy to reproduce.
> 
> Happening a lot in itself is not a problem if there is nothing else to
> do so it's not a argument in itself

It is a problem - it shows up in the preempt off critical section latency
tracer. Are you saying its Ok for preemption to be disabled on system for 500
micro seconds?  That hurts real-time applications (audio etc).

> So why is it a problem for you ? You are mentioning newly idle load
> balance so I assume that your root problem is the scheduling delay
> generated by the newly idle load balance which then calls
> update_blocked_averages

Yes, the new idle balance is when I see it happen quite often. I do see it
happen with other load balance as well, but it not that often as those LB
don't run as often as new idle balance.

> 
> rate limiting the call to update_blocked_averages() will only reduce
> the number of time it happens but it will not prevent it to happen.

Sure, but soft real-time issue can tolerate if the issue does not happen very
often. In this case though, it is frequent.

> IIUC, your real problem is that newidle_balance is running whereas a
> task is about to wake up on the cpu and we don't abort quickly during
> this load_balance

Yes.

> so we could also try to abort earlier in case of newly idle load balance

I think interrupts are disabled when the load balance runs, so there's no way
for say an audio interrupt to even run in order to wake up a task. How would
you know to abort the new idle load balance?

Could you elaborate more also on the drawback of the rate limiting patch we
posted? Do you see a side effect?

> > > > sometimes, which seems overkill.
> > > >
> > > > schbench shows a clear improvement with the change:
> > >
> > > Have you got more details about your test setup ?
> > > which platform ?
> > > which kernel ?
> >
> > I mentioned in the commit message it is a v5.4 kernel.
> 
> I was not sure if the tests results done with this kernel because we
> usually ask for results against mainline to make sure you are not
> facing a problem that has solved since v5.4 has been released

Ok, yes I have a userspace up and running only on 5.4 kernel unfortunately. I
was hoping that is recent enough for this debug.

thanks,

 - Joel