CPU affinity behavior of liburcu call-rcu per-cpu worker threads

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

Hi Paul,

I have a question related to liburcu per-cpu call-rcu worker threads.

So far in the current liburcu releases no affinity is set when the
worker threads are started. We have to wait for 
SET_AFFINITY_CHECK_PERIOD_MASK grace periods before the affinity is set
(it takes about 2.5s in my benchmarks).

This means that short-lived test programs will get poor CPU affinity
for call-rcu worker threads at the beginning of their lifetime. So for
short lived programs, this means poor performance.

OTOH, if we have short-lived applications on a large machine, setting
the affinity immediately when the worker thread is starting means
we use CPU time on CPUs which may never be actually used by the
application (no call-rcu activity), which can be detrimental to other
use-cases as well.

So I was wondering: is the choice of skipping setting the affinity
on call-rcu worker thread startup done on purpose ? And if so,
should we perhaps consider setting the affinity as soon as the
worker thread is woken up for the first time rather than after
SET_AFFINITY_CHECK_PERIOD_MASK grace periods ?

Note that there is a commit in liburcu changing this behavior right
now, but it's not part of any release yet. I want to figure out the
wanted behavior before releasing this change:

commit 28f282af1905a1cf50ff9b5835b4cd9de4416ddf
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date:   Wed Jul 1 12:49:43 2026 -0400

     call_rcu: pin per-CPU worker at thread startup

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com