Re: [External] Re: [PATCH 0/2] Fix nohz_full vs rt bandwidth

[Hao Jia]

On 2023/9/11 Phil Auld wrote:
> Hi Hao,
>
> On Mon, Sep 11, 2023 at 11:39:02AM +0800 Hao Jia wrote:
> > On 2023/9/8 Phil Auld wrote:
> > > On Fri, Sep 08, 2023 at 10:57:26AM +0800 Hao Jia wrote:
> > > > On 2023/9/7 Phil Auld wrote:
> > > > > Hi Hao,
>
> ...
>
> > > > > Are you actually hitting this in the real world?
> > > > >
> > > > > We, for example, no longer enable RT_GROUP_SCHED so this is a non-issue
> > > > > for our use cases.  I'd recommend considering that. (Does it even
> > > > > work with cgroup2?)
> > > >
> > > > Yes, it has always been there. Regardless of whether RT_GROUP_SCHED is
> > > > enabled or not, rt bandwidth is always enabled. If RT_GROUP_SCHED is not
> > > > enabled, all rt tasks in the system are a group, and rt_runtime is 950000,
> > > > and rt_period is 1000000.So rt bandwidth is always enabled by default.
> > >
> > > Sure, there is that. But I think Daniel is actively trying to remove it.
> >
> > Thank you for your reply. Maybe I'm missing something. Can you give me some
> > links to discussions about it?
>
> Sure, try this one:
>        https://lore.kernel.org/lkml/cover.1693510979.git.bristot@xxxxxxxxxx/

Thanks for the information you shared.

> > > Also I'm not sure you answered my question. Are you actually hitting this
> > > in the real world?  I'd be tempted to think this is a mis-configuration or
> > > mis-use of RT.  Plus you can disable that throttling and use stalld to catch
> > > cases where the rt task goes out of control.
> >
> > > Are you actually hitting this in the real world?
> >
> > I tested on my machine using default settings (rt_runtime is 950000, and
> > rt_period is 1000000.). The rt task is supposed to be throttled after
> > running for 0.95 seconds, but due to the influence of NO_HZ_FULL, it may be
> > throttled after running for about 1.4 seconds. This will only cause the
> > rt_bandwidth throttle to be delayed, but no warning will be triggered.
>
> Yes, you can hit this in testing.  I'm asking if it's causing your real-world
> applicaton issues or is this just a theoretical problem you can contrive a
> test for?  Are you actually hitting this when running your workload?
>  From what you are showing (a test setup) I'm guessing no.

Yes, I don't see this issue in our production environment. The number of 
rt tasks is very small in our production environment, and their running 
time is very short, so the rt_bandwidth throttle will not be triggered 
unless the rt task goes out of control.

Thanks,
Hao

> > > Plus you can disable that throttling and use stalld to catch cases where
> > the rt task goes out of control.
> >
> > IIRC, if we disable rt_bandwidth. The rt task is always running, which may
> > cause cfs task starvation and hung_task warnning. This may be the reason why
> > rt_bandwidth is enabled by default (rt_runtime is 950000, and rt_period is
> > 1000000).
>
> That's what stalld is for.  Some rt applications don't like giving up 5% of
> the cpu time when they don't really need to.
>
>
> Cheers,
> Phil