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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