Re: [RFC PATCH 00/16] Core scheduling v6

[Li, Aubrey]

On 2020/7/1 5:32, Vineeth Remanan Pillai wrote:
> Sixth iteration of the Core-Scheduling feature.
> 
> Core scheduling is a feature that allows only trusted tasks to run
> concurrently on cpus sharing compute resources (eg: hyperthreads on a
> core). The goal is to mitigate the core-level side-channel attacks
> without requiring to disable SMT (which has a significant impact on
> performance in some situations). Core scheduling (as of v6) mitigates
> user-space to user-space attacks and user to kernel attack when one of
> the siblings enters the kernel via interrupts. It is still possible to
> have a task attack the sibling thread when it enters the kernel via
> syscalls.
> 
> By default, the feature doesn't change any of the current scheduler
> behavior. The user decides which tasks can run simultaneously on the
> same core (for now by having them in the same tagged cgroup). When a
> tag is enabled in a cgroup and a task from that cgroup is running on a
> hardware thread, the scheduler ensures that only idle or trusted tasks
> run on the other sibling(s). Besides security concerns, this feature
> can also be beneficial for RT and performance applications where we
> want to control how tasks make use of SMT dynamically.
> 
> This iteration is mostly a cleanup of v5 except for a major feature of
> pausing sibling when a cpu enters kernel via nmi/irq/softirq. Also
> introducing documentation and includes minor crash fixes.
> 
> One major cleanup was removing the hotplug support and related code.
> The hotplug related crashes were not documented and the fixes piled up
> over time leading to complex code. We were not able to reproduce the
> crashes in the limited testing done. But if they are reroducable, we
> don't want to hide them. We should document them and design better
> fixes if any.
> 
> In terms of performance, the results in this release are similar to
> v5. On a x86 system with N hardware threads:
> - if only N/2 hardware threads are busy, the performance is similar
>   between baseline, corescheduling and nosmt
> - if N hardware threads are busy with N different corescheduling
>   groups, the impact of corescheduling is similar to nosmt
> - if N hardware threads are busy and multiple active threads share the
>   same corescheduling cookie, they gain a performance improvement over
>   nosmt.
>   The specific performance impact depends on the workload, but for a
>   really busy database 12-vcpu VM (1 coresched tag) running on a 36
>   hardware threads NUMA node with 96 mostly idle neighbor VMs (each in
>   their own coresched tag), the performance drops by 54% with
>   corescheduling and drops by 90% with nosmt.
> 

We found uperf(in cgroup) throughput drops by ~50% with corescheduling.

The problem is, uperf triggered a lot of softirq and offloaded softirq
service to *ksoftirqd* thread. 

- default, ksoftirqd thread can run with uperf on the same core, we saw
  100% CPU utilization.
- coresched enabled, ksoftirqd's core cookie is different from uperf, so
  they can't run concurrently on the same core, we saw ~15% forced idle.

I guess this kind of performance drop can be replicated by other similar
(a lot of softirq activities) workloads.

Currently core scheduler picks cookie-match tasks for all SMT siblings, does
it make sense we add a policy to allow cookie-compatible task running together?
For example, if a task is trusted(set by admin), it can work with kernel thread.
The difference from corescheduling disabled is that we still have user to user
isolation.

Thanks,
-Aubrey