Re: task isolation discussion at Linux Plumbers

From: Thomas Gleixner
Date: Mon Nov 07 2016 - 11:59:20 EST

On Sat, 5 Nov 2016, Chris Metcalf wrote:
> == Remote statistics ==
> We discussed the possibility of remote statistics gathering, i.e. load
> average etc. The idea would be that we could have housekeeping
> core(s) periodically iterate over the nohz cores to load their rq
> remotely and do update_current etc. Presumably it should be possible
> for a single housekeeping core to handle doing this for all the
> nohz_full cores, as we only need to do it quite infrequently.
> Thomas suggested that this might be the last remaining thing that
> needed to be done to allow disabling the current behavior of falling
> back to a 1 Hz clock in nohz_full.
> I believe Thomas said he had a patch to do this already.

No, Riek was working on that.

> == Remote LRU cache drain ==
> One of the issues with task isolation currently is that the LRU cache
> drain must be done prior to entering userspace, but it requires
> interrupts enabled and thus can't be done atomically. My previous
> patch series have handled this by checking with interrupts disabled,
> but then looping around with interrupts enabled to try to drain the
> LRU pagevecs. Experimentally this works, but it's not provable that
> it terminates, which is worrisome. Andy suggested adding a percpu
> flag to disable creation of deferred work like LRU cache pages.
> Thomas suggested using an RT "local lock" to guard the LRU cache
> flush; he is planning on bringing the concept to mainline in any case.
> However, after some discussion we converged on simply using a spinlock
> to guard the appropriate resources. As a result, the
> lru_add_drain_all() code that currently queues work on each remote cpu
> to drain it, can instead simply acquire the lock and drain it remotely.
> This means that a task isolation task no longer needs to worry about
> being interrupted by SMP function call IPIs, so we don't have to deal
> with this in the task isolation framework any more.
> I don't recall anyone else volunteering to tackle this, so I will plan
> to look at it. The patch to do that should be orthogonal to the
> revised task isolation patch series.

I offered to clean up the patch from RT. I'll do that in the next days.

> == Missing oneshot_stopped callbacks ==
> I raised the issue that various clock_event_device sources don't
> always support oneshot_stopped, which can cause an additional
> final interrupt to occur after the timer infrastructure believes the
> interrupt has been stopped. I have patches to fix this for tile and
> arm64 in my patch series; Thomas volunteered to look at adding
> equivalent support for x86.