On Fri, May 21, 2021 at 12:32 PM Andy Lutomirski [off-list ref] wrote:

On Thu, May 20, 2021 at 11:36 AM Peter Oskolkov [off-list ref] wrote:

quoted

Implement version 1 of core UMCG API (wait/wake/swap).

As has been outlined in
https://lore.kernel.org/lkml/20200722234538.166697-1-posk@posk.io/ (local),
efficient and synchronous on-CPU context switching is key
to enabling two broad use cases: in-process M:N userspace scheduling
and fast X-process RPCs for security wrappers.

High-level design considerations/approaches used:
- wait & wake can race with each other;
- offload as much work as possible to libumcg in tools/lib/umcg,
  specifically:
  - most state changes, e.g. RUNNABLE <=> RUNNING, are done in
    the userspace (libumcg);
  - retries are offloaded to the userspace.

Do you have some perf numbers as to how long a UMCG context switch
takes compared to a normal one?

I'm not sure what is a "normal context switch" in this context. Futex
wakeup on a remote idle CPU takes 5-10usec; an on-CPU UMCG context
switch takes less than 1usec; futex wake + futex wait on the same CPU
(taskset ***) takes about 1-1.5usec in my benchmarks.

--Andy