Re: [PATCH] sched/fair: Introduce priority load balance for CFS

[Song Zhang]

On 2022/11/15 15:18, Vincent Guittot wrote:
> On Mon, 14 Nov 2022 at 17:42, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> > On Sat, 12 Nov 2022 at 03:51, Song Zhang <zhangsong34@xxxxxxxxxx> wrote:
> > > Hi, Vincent
> > >
> > > On 2022/11/3 17:22, Vincent Guittot wrote:
> > > > On Thu, 3 Nov 2022 at 10:20, Song Zhang <zhangsong34@xxxxxxxxxx> wrote:
> > > > > On 2022/11/3 16:33, Vincent Guittot wrote:
> > > > > > On Thu, 3 Nov 2022 at 04:01, Song Zhang <zhangsong34@xxxxxxxxxx> wrote:
> > > > > > > Thanks for your reply!
> > > > > > >
> > > > > > > On 2022/11/3 2:01, Vincent Guittot wrote:
> > > > > > > > On Wed, 2 Nov 2022 at 04:54, Song Zhang <zhangsong34@xxxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > >
> > > > > > > > This really looks like a v3 of
> > > > > > > > https://lore.kernel.org/all/20220810015636.3865248-1-zhangsong34@xxxxxxxxxx/
> > > > > > > >
> > > > > > > > Please keep versioning.
> > > > > > > >
> > > > > > > > > Add a new sysctl interface:
> > > > > > > > > /proc/sys/kernel/sched_prio_load_balance_enabled
> > > > > > > >
> > > > > > > > We don't want to add more sysctl knobs for the scheduler, we even
> > > > > > > > removed some. Knob usually means that you want to fix your use case
> > > > > > > > but the solution doesn't make sense for all cases.
> > > > > > >
> > > > > > > OK, I will remove this knobs later.
> > > > > > >
> > > > > > > > > 0: default behavior
> > > > > > > > > 1: enable priority load balance for CFS
> > > > > > > > >
> > > > > > > > > For co-location with idle and non-idle tasks, when CFS do load balance,
> > > > > > > > > it is reasonable to prefer migrating non-idle tasks and migrating idle
> > > > > > > > > tasks lastly. This will reduce the interference by SCHED_IDLE tasks
> > > > > > > > > as much as possible.
> > > > > > > >
> > > > > > > > I don't agree that it's always the best choice to migrate a non-idle task 1st.
> > > > > > > >
> > > > > > > > CPU0 has 1 non idle task and CPU1 has 1 non idle task and hundreds of
> > > > > > > > idle task and there is an imbalance between the 2 CPUS: migrating the
> > > > > > > > non idle task from CPU1 to CPU0 is not the best choice
> > > > > > >
> > > > > > > If the non idle task on CPU1 is running or cache hot, it cannot be
> > > > > > > migrated and idle tasks can also be migrated from CPU1 to CPU0. So I
> > > > > > > think it does not matter.
> > > > > >
> > > > > > What I mean is that migrating non idle tasks first is not a universal
> > > > > > win and not always what we want.
> > > > >
> > > > > But migrating online tasks first is mostly a trade-off that
> > > > > non-idle(Latency Sensitive) tasks can obtain more CPU time and minimize
> > > > > the interference caused by IDLE tasks. I think this makes sense in most
> > > > > cases, or you can point out what else I need to think about it ?
> > > > >
> > > > > Best regards.
> > > > >
> > > > > > > > > Testcase:
> > > > > > > > > - Spawn large number of idle(SCHED_IDLE) tasks occupy CPUs
> > > > > > > >
> > > > > > > > What do you mean by a large number ?
> > > > > > > >
> > > > > > > > > - Let non-idle tasks compete with idle tasks for CPU time.
> > > > > > > > >
> > > > > > > > > Using schbench to test non-idle tasks latency:
> > > > > > > > > $ ./schbench -m 1 -t 10 -r 30 -R 200
> > > > > > > >
> > > > > > > > How many CPUs do you have ?
> > > > > > >
> > > > > > > OK, some details may not be mentioned.
> > > > > > > My virtual machine has 8 CPUs running with a schbench process and 5000
> > > > > > > idle tasks. The idle task is a while dead loop process below:
> > > > > >
> > > > > > How can you care about latency when you start 10 workers on 8 vCPUs
> > > > > > with 5000 non idle threads ?
> > > > >
> > > > > No no no... spawn 5000 idle(SCHED_IDLE) processes not 5000 non-idle
> > > > > threads, and with 10 non-idle schbench workers on 8 vCPUs.
> > > >
> > > > yes spawn 5000 idle tasks but my point remains the same
> > >
> > > I am so sorry that I have not received your reply for a long time, and I
> > > am still waiting for it anxiously. In fact, migrating non-idle tasks 1st
> > > works well in most scenarios, so it maybe possible to add a
> > > sched_feat(LB_PRIO) to enable or disable that. Finally, I really hope
> > > you can give me some better advice.
> >
> > I have seen that you posted a v4 5 days ago which is on my list to be reviewed.
> >
> > My concern here remains that selecting non idle task 1st is not always
> > the best choices as for example when you have 1 non idle task per cpu
> > and thousands of idle tasks moving around. Then regarding your use
> > case, the weight of the 5000 idle threads is around twice more than
> > the weight of your non idle bench: sum weight of idle threads is 15k
> > whereas the weight of your bench is around 6k IIUC how RPS run. This
> > also means that the idle threads will take a significant times of the
> > system: 5000 / 7000 ticks. I don't understand how you can care about
> > latency in such extreme case and I'm interested to get the real use
> > case where you can have such situation.
> >
> > All that to say that idle task remains cfs task with a small but not
> > null weight and we should not make them special other than by not
> > preempting at wakeup.
>
> Also, as mentioned for a previous version, a task with nice prio 19
> has a weight of 15 so if you replace the 5k idle threads with 1k cfs
> w/ nice prio 19 threads, you will face a similar problem. So you can't
> really care only on the idle property of a task

Well, my original idea was to consider interference between tasks of 
different priorities when doing CFS load balancing to ensure that 
non-idle tasks get more CPU scheduler time without changing the native 
CFS load balancing policy.

Consider a simple scenario. Assume that CPU 0 has two non-idle tasks 
whose weight is 1024 * 2 = 2048, also CPU 0 has 1000 idle tasks whose 
weight is 1K x 15 = 15K. CPU 1 is idle. Therefore, IDLE load balance is 
triggered. CPU 1 needs to pull a certain number of tasks from CPU 0. If 
we do not considerate task priorities and interference between tasks, 
more than 600 idle tasks on CPU 0 may be migrated to CPU 1. As a result, 
two non-idle tasks still compete on CPU 0. However, CPU 1 is running 
with all idle but not non-idle tasks.

Let's calculate the percentage of CPU time gained by non-idle tasks in a 
scheduling period:

CPU 1: time_percent(non-idle tasks) = 0
CPU 0: time_percent(non-idle tasks) = 2048 * 2 / (2048 + 15000) = 24%

On the other hand, if we consider the interference between different 
task priorities, we change the migration policy to firstly migrate an 
non-idle task on CPU 0 to CPU 1. Migrating idle tasks on CPU 0 maybe 
interfered with the non-idle task on CPU 1. So we decide to migrate idle 
tasks on CPU 0 after non-idle tasks on CPU 1 are completed or exited.

Now the percentage of the CPU time obtained by the non-idle tasks in a 
scheduling period is as follows:

CPU 1: time_percent(non-idle tasks) = 1024 / 1024 = 100%
CPU 0: time_percent(non-idle tasks) = 1024 / (1024 + 15000) = 6.4%

Obviously, if load balance migration tasks prefer migrate non-idle tasks 
and suppress the interference of idle tasks migration on non-idle tasks, 
the latency of non-idle tasks can be significantly reduced. Although 
this will cause some idle tasks imbalance between different CPUs and 
reduce throughput of idle tasks., I think this strategy is feasible in 
some real-time business scenarios for latency tasks.

> > > Best regards.
> > >
> > > Song Zhang
> .