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Subject: Re: [RFC PATCH 00/16] Core scheduling v6
To:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        <torvalds@linux-foundation.org>
CC:     <linux-kernel@vger.kernel.org>, <subhra.mazumdar@oracle.com>,
        <fweisbec@gmail.com>, <keescook@chromium.org>,
        <kerrnel@google.com>, "Phil Auld" <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Joel Fernandes <joelaf@google.com>, <joel@joelfernandes.org>,
        <vineethrp@gmail.com>, Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>
References: <cover.1593530334.git.vpillai@digitalocean.com>
From:   Alexander Graf <graf@amazon.com>
Message-ID: <c4556033-4d78-0419-0114-a17f68456ec8@amazon.com>
Date:   Thu, 27 Aug 2020 02:30:39 +0200
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Hi Vineeth,

On 30.06.20 23:32, 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.
> =

> v6 is rebased on 5.7.6(a06eb423367e)
> https://github.com/digitalocean/linux-coresched/tree/coresched/v6-v5.7.y

As discussed during Linux Plumbers, here is a small repo with test =

scripts and applications that I've used to look at core scheduling =

unfairness:

   https://github.com/agraf/schedgaps

Please let me know if it's unclear how to use it or if you see issues in =

your environment.

Please also make sure to only run this on idle server class hardware. =

Notebooks will most definitely have too many uncontrollable sources of =

timing entropy to give sensible results.


Alex


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