Re: [RFC PATCH 0/5] softirq: Per vector threading v2

[Linus Torvalds]

On Wed, Jan 17, 2018 at 10:07 AM, Frederic Weisbecker
<frederic@xxxxxxxxxx> wrote:
>
> I see, so you may want to test (possibly much) higher values of MAX_SOFTIRQ_RESTART,
> such as 50 or 100.

I suspect the "number of softiqs per jiffy" is hardly interesting at all.

We used to allow up to 2mS or ten iterations per _invocation_, never
mind per timer tick.

I thought you were going to actally account for time, but I don't
think you ended up doing that.

Maybe time isn't necessarily the thing to do, but just pure "count per
jiffy" seems very bad.

What I might suggest using instead:

 - do it by time. This may be too expensive, though. Keeping track of
ns-level timing per invocation can be nasty.

 - do it by "we got a new softirq event while handling another softirq
event". That was our old count per invocation, except you could do it
per softirq, and just allow *one* (ie keep a bitmask of "I've already
handled this softirq", and if the restart results in it being
triggered *again* you say "ok, I'll just move this to a workqueue"

 - .. something else?

I'd suggest trying the "if we get a new softirq event that we've
already seen while we were already handling softirq events" thing.
That should really take care of the networking case of "90% time spend
in softirq handling during packet storms" thing. If we spend that much
time on softirqs, we *will* get a new softirq while handling an old
one occasionally.

            Linus