Paolo!

On Mon, May 08 2023 at 08:17, Paolo Abeni wrote:

Due to the mentioned commit, when the ksoftirqd processes take charge
of softirq processing, the system can experience high latencies.

In the past a few workarounds have been implemented for specific
side-effects of the above:

commit 1ff688209e2e ("watchdog: core: make sure the watchdog_worker is not deferred")
commit 8d5755b3f77b ("watchdog: softdog: fire watchdog even if softirqs do not get to run")
commit 217f69743681 ("net: busy-poll: allow preemption in sk_busy_loop()")
commit 3c53776e29f8 ("Mark HI and TASKLET softirq synchronous")

but the latency problem still exists in real-life workloads, see the
link below.

The reverted commit intended to solve a live-lock scenario that can now
be addressed with the NAPI threaded mode, introduced with commit
29863d41bb6e ("net: implement threaded-able napi poll loop support"),
and nowadays in a pretty stable status.

While a complete solution to put softirq processing under nice resource
control would be preferable, that has proven to be a very hard task. In
the short term, remove the main pain point, and also simplify a bit the
current softirq implementation.

Note that this change also reverts commit 3c53776e29f8 ("Mark HI and
TASKLET softirq synchronous") and commit 1342d8080f61 ("softirq: Don't
skip softirq execution when softirq thread is parking"), which are
direct follow-ups of the feature commit. A single change is preferred to
avoid known bad intermediate states introduced by a patch series
reverting them individually.

I'm fine with this change, but I definitely want that to be
acked/reviewed by the other stakeholders in the networking arena.

Thanks,

        tglx