Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

[Paul E. McKenney]

On Sun, Jun 12, 2022 at 09:23:14PM +0200, Paolo Bonzini wrote:
> On 6/12/22 20:49, Paul E. McKenney wrote:
> > > 
> > > 1) kvm->irq_srcu is hardly relying on the "sleepable" part; it has readers
> > > that are very very small, but it needs extremely fast detection of grace
> > > periods; see commit 719d93cd5f5c ("kvm/irqchip: Speed up
> > > KVM_SET_GSI_ROUTING", 2014-05-05) which split it off kvm->srcu.  Readers are
> > > not so frequent.
> > > 
> > > 2) kvm->srcu is nastier because there are readers all the time.  The
> > > read-side critical section are still short-ish, but they need the sleepable
> > > part because they access user memory.
> > 
> > Which one of these two is in play in this case?
> 
> The latter, kvm->srcu; though at boot time both are hammered on quite a bit
> (and then essentially not at all).
> 
> For the one involved it's still pretty rare for readers to sleep, but it
> cannot be excluded.  Most critical sections are short, I'd guess in the
> thousands of clock cycles but I can add some instrumentation tomorrow (or
> anyway before Tuesday).

And in any case, readers can be preempted.

> > The problem was not internal to SRCU, but rather due to the fact
> > that kernel live patching (KLP) had problems with the CPU-bound tasks
> > resulting from repeated synchronize_rcu_expedited() invocations.
> 
> I see.  Perhaps only add to the back-to-back counter if the
> synchronize_srcu_expedited() takes longer than a jiffy? This would
> indirectly check if syncronize_srcu_expedited() readers are actually
> blocking.  KVM uses syncronize_srcu_expedited() because it expects it to
> take very little (again I'll get hard numbers asap).

This is in effect what the patch in my previous email does.  In current
mainline, it waits for up to a jiffy before switching to sleep mode,
but with the patch it waits for between one and two jiffies before making
that switch.

Using call_srcu() with the occasional srcu_barrier() would of course be
faster still, but perhaps more complex.

							Thanx, Paul