Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups

[Peter Oskolkov <posk@google.com>]

On Wed, Dec 15, 2021 at 10:19 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Dec 15, 2021 at 09:56:06AM -0800, Peter Oskolkov wrote:
>
> > > Right, so the problem I'm having is that a single idle server ptr like
> > > before can trivially miss waking annother idle server.
> >
> > I believe the approach I used in my patchset, suggested by Thierry
> > Delisle, works.
> >
> > In short, there is a single idle server ptr for the kernel to work
> > with. The userspace maintains a list of idle servers. If the ptr is
> > NULL, the list is empty. When the kernel wakes the idle server it
> > sees, the server reaps the runnable worker list and wakes another idle
> > server from the userspace list, if available. This newly woken idle
> > server repoints the ptr to itself, checks the runnable worker list, to
> > avoid missing a woken worker, then goes to sleep.
> >
> > Why do you think this approach is not OK?
>
> Suppose at least 4 servers, 2 idle, 2 working.
>
> Now, the first of the working servers (lets call it S0) gets a wakeup
> (say Ta), it finds the idle server (say S3) and consumes it, sticking Ta
> on S3 and kicking it alive.

TL;DR: our models are different here. In your model a single server
can have a bunch of workers interacting with it; in my model only a
single RUNNING worker is assigned to a server, which it wakes when it
blocks.

More details:

"Working servers" cannot get wakeups, because a "working server" has a
single RUNNING worker attached to it. When a worker blocks, it wakes
its attached server and becomes a detached blocked worker (same is
true if the worker is "preempted": it blocks and wakes its assigned
server).

Blocked workers upon wakeup do this, in order:

- always add themselves to the runnable worker list (the list is
shared among ALL servers, it is NOT per server);
- wake a server pointed to by idle_server_ptr, if not NULL;
- sleep, waiting for a wakeup from a server;

Server S, upon becoming IDLE (no worker to run, or woken on idle
server list) does this, in order, in userspace (simplified, see
umcg_get_idle_worker() in
https://lore.kernel.org/lkml/20211122211327.5931-5-posk@google.com/):
- take a userspace (spin) lock (so the steps below are all within a
single critical section):
- compare_xchg(idle_server_ptr, NULL, S);
  - if failed, there is another server in idle_server_ptr, so S adds
itself to the userspace idle server list, releases the lock, goes to
sleep;
  - if succeeded:
    - check the runnable worker list;
        - if empty, release the lock, sleep;
        - if not empty:
           - get the list
           - xchg(idle_server_ptr, NULL) (either S removes itself, or
a worker in the kernel does it first, does not matter);
           - release the lock;
           - wake server S1 on idle server list. S1 goes through all
of these steps.

The protocol above serializes the userspace dealing with the idle
server ptr/list. Wakeups in the kernel will be caught if there are
idle servers. Yes, the protocol in the userspace is complicated (more
complicated than outlined above, as the reaped idle/runnable worker
list from the kernel is added to the userspace idle/runnable worker
list), but the kernel side is very simple. I've tested this
interaction extensively, I'm reasonably sure that no worker wakeups
are lost.

>
> Concurrently and loosing the race the other working server (S1) gets a
> wake-up from Tb, like said, it lost, no idle server, so Tb goes on the
> queue of S1.
>
> So then S3 wakes, finds Ta and they live happily ever after.
>
> While S2 and Tb fail to meet one another and both linger in sadness.
>