Re: [RFC PATCH v4 00/13] ktask: multithread CPU-intensive kernel work

[Zi Yan]

Hi Daniel,

On 5 Nov 2018, at 11:55, Daniel Jordan wrote:

> Hi,
>
> This version addresses some of the feedback from Andrew and Michal last year
> and describes the plan for tackling the rest.  I'm posting now since I'll be
> presenting ktask at Plumbers next week.
>
> Andrew, you asked about parallelizing in more places[0].  This version adds
> multithreading for VFIO page pinning, and there are more planned users listed
> below.
>
> Michal, you mentioned that ktask should be sensitive to CPU utilization[1].
> ktask threads now run at the lowest priority on the system to avoid disturbing
> busy CPUs (more details in patches 4 and 5).  Does this address your concern?
> The plan to address your other comments is explained below.
>
> Alex, any thoughts about the VFIO changes in patches 6-9?
>
> Tejun and Lai, what do you think of patch 5?
>
> And for everyone, questions and comments welcome.  Any suggestions for more
> users?
>
>          Thanks,
>             Daniel
>
> P.S.  This series is big to address the above feedback, but I can send patches
> 7 and 8 separately.
>
>
> TODO
> ----
>
>  - Implement cgroup-aware unbound workqueues in a separate series, picking up
>    Bandan Das's effort from two years ago[2].  This should hopefully address
>    Michal's comment about running ktask threads within the limits of the calling
>    context[1].
>
>  - Make ktask aware of power management.  A starting point is to disable the
>    framework when energy-conscious cpufreq settings are enabled (e.g.
>    powersave, conservative scaling governors).  This should address another
>    comment from Michal about keeping CPUs under power constraints idle[1].
>
>  - Add more users.  On my list:
>     - __ib_umem_release in IB core, which Jason Gunthorpe mentioned[3]
>     - XFS quotacheck and online repair, as suggested by Darrick Wong
>     - vfs object teardown at umount time, as Andrew mentioned[0]
>     - page freeing in munmap/exit, as Aaron Lu posted[4]
>     - page freeing in shmem
>    The last three will benefit from scaling zone->lock and lru_lock.
>
>  - CPU hotplug support for ktask to adjust its per-CPU data and resource
>    limits.
>
>  - Check with IOMMU folks that iommu_map is safe for all IOMMU backend
>    implementations (it is for x86).
>
>
> Summary
> -------
>
> A single CPU can spend an excessive amount of time in the kernel operating
> on large amounts of data.  Often these situations arise during initialization-
> and destruction-related tasks, where the data involved scales with system size.
> These long-running jobs can slow startup and shutdown of applications and the
> system itself while extra CPUs sit idle.
>
> To ensure that applications and the kernel continue to perform well as core
> counts and memory sizes increase, harness these idle CPUs to complete such jobs
> more quickly.
>
> ktask is a generic framework for parallelizing CPU-intensive work in the
> kernel.  The API is generic enough to add concurrency to many different kinds
> of tasks--for example, zeroing a range of pages or evicting a list of
> inodes--and aims to save its clients the trouble of splitting up the work,
> choosing the number of threads to use, maintaining an efficient concurrency
> level, starting these threads, and load balancing the work between them.
>
> The first patch has more documentation, and the second patch has the interface.
>
> Current users:
>  1) VFIO page pinning before kvm guest startup (others hitting slowness too[5])
>  2) deferred struct page initialization at boot time
>  3) clearing gigantic pages
>  4) fallocate for HugeTLB pages

Do you think if it makes sense to use ktask for huge page migration (the data
copy part)?

I did some experiments back in 2016[1], which showed that migrating one 2MB page
with 8 threads could achieve 2.8x throughput of the existing single-threaded method.
The problem with my parallel page migration patchset at that time was that it
has no CPU-utilization awareness, which is solved by your patches now.

Thanks.

[1]https://lkml.org/lkml/2016/11/22/457

--
Best Regards
Yan Zi

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