synchronise_rcu() takes 18ms on an idle system

[David Laight <David.Laight@ACULAB.COM>]

I'm looking into some scheduling delays in application that does
audio processing.

The code calls sys_perf_event_open() (to get a cycle counter).
After static_key_enable() has done some text_poke()s
the code calls synchronise_rcu().
This is all not unreasonable.

__wait_rcu_gp() then calls wait_for_completion().
Which sleeps in schedule_timeout().

At this point I'd expect the 'rcu' to complete pretty quickly.

However my process doesn't get scheduled again for 18ms.
The actual wakeup seems to be from smp_apic_timer_interrupt().
Probably because rcu_core_si() called wakeup_after_rcu() an
then complete().

I think I understand 'rcu', I thought any code relying on it
had to disable pre-emption - so that a process switch on
every cpu was more then enough to release the waiter.

In this case my 4 cpu are largely idle.

Waiting 18ms seems odd, I'm pretty sure I've seen softint
callbacks running for well over 1ms it isn't long enough
for a slow system - but is too long for a fast one.

This is a 5.4-rc7 kernel with CONFIG_HZ=250.

Now I can mitigate this particular case by calling
sys_perf_event_open() much earlier.
But any other calls are problematic.

	David

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