Re: [RFC PATCH 4/4 v0.3] sched/umcg: RFC: implement UMCG syscalls

On Fri, Jul 23, 2021 at 12:06 PM Thierry Delisle [off-list ref] wrote:

 > In my tests reclaimed nodes have their next pointers immediately set
 > to point to the list head. If the kernel gets a node with its @next
 > pointing to something else, then yes, things break down (the kernel
 > kills the process); this has happened occasionally when I had a bug in
 > the userspace code.

I believe that approach is fine for production, but for testing it may
not detect some bugs. For example, it may not detect the race I detail
below.

While I think I have the idle servers list working, I now believe that
what peterz@ was suggesting is not much slower in the common case
(many idle workers; few, if any, idle servers) than having a list of
idle servers exposed to the kernel: I think having a single idle
server at head, not a list, is enough: when a worker is added to idle
workers list, a single idle server at head, if present, can be
"popped" and woken; the userspace can maintain the list of idle
servers itself; having the kernel wake only one is enough - it will
pop all idle workers and decide whether any other servers are needed
to process the newly available work.

[...]

 > Workers are trickier, as they can be woken by signals and then block
 > again, but stray signals are so bad here that I'm thinking of actually
 > not letting sleeping workers wake on signals. Other than signals
 > waking queued/unqueued idle workers, are there any other potential
 > races here?

Timeouts on blocked threads is virtually the same as a signal I think. I
can see that both could lead to attempts at waking workers that are not
blocked.

I've got preemption working well enough to warrant a new RFC patchset
(also have timeouts done, but these were easy). I'll clean things up,
change the idle servers logic to only one idle server exposed to the
kernel, not a list, add some additional documentation (state
transitions, userspace code snippets, etc.) and will post v0.4 RFC
patchset to LKML later this week.

[...]