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SubjectRe: RFD: multithreading in padata
On Tue, Dec 03, 2019 at 10:58:41AM -0500, Daniel Jordan wrote:
> [resending in modified form since this didn't seem to reach the lists]
>
> Hi,
>
> padata has been undergoing some surgery lately[0] and now seems ready for
> another enhancement: splitting up and multithreading CPU-intensive kernel work.
>
> I'm planning to send an RFC for this, but I wanted to post some thoughts on the
> design and a work-in-progress branch first to see if the direction looks ok.
>
> Quoting from an earlier series[1], this is the problem I'm trying to solve:
>
> 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.
>
> There are several paths where this problem exists, but here are three to start:
>
> - struct page initialization (at boot-time, during memory hotplug, and for
> persistent memory)
> - VFIO page pinning (kvm guest initialization)
> - fallocating a HugeTLB page (database initialization)
>
> padata is a general mechanism for parallel work and so seems natural for this
> functionality[2], but now it can only manage a series of small, ordered jobs.
>
> The coming RFC will bring enhancements to split up a large job among a set of
> helper threads according to the user's wishes, load balance the work between
> them, set concurrency limits to control the overall number of helpers in the
> system and per NUMA node, and run extra helper threads beyond the first at a
> low priority level so as not to disturb other activity on the system for the
> sake of an optimization. (While extra helpers are run at low priority for most
> of the job, their priority is raised one by one at job end to match the
> caller's to avoid starvation on a busy system.)
>
> The existing padata interfaces and features will remain, but serialization
> becomes optional because these sorts of jobs don't need it.
>
> The advantage to enhancing padata rather than having the multithreading stand
> alone is that there would be one central place in the kernel to manage the
> number of helper threads that run at any given time. A machine's idle CPU
> resources can be harnessed yet controlled (the low priority idea) to provide
> the right amount of multithreading for the system.
>
> Here's a work-in-progress branch with some of this already done in the last
> five patches.
>
> git://oss.oracle.com/git/linux-dmjordan.git padata-mt-wip
> https://oss.oracle.com/git/gitweb.cgi?p=linux-dmjordan.git;a=shortlog;h=refs/heads/padata-mt-wip
>
> Thoughts? Questions?

I'm ok with this. Please consider to add yourself as
a Co-Maintainer, I guess you know the code in between
much better than I do :)

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