Re: [PATCHv2] KVM: optimize apic interrupt delivery

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Nov 26, 2013 at 06:24:13PM +0200, Michael S. Tsirkin wrote:
> On Wed, Sep 12, 2012 at 08:13:54AM -0700, Paul E. McKenney wrote:
> > On Wed, Sep 12, 2012 at 03:44:26PM +0300, Gleb Natapov wrote:
> > > On Wed, Sep 12, 2012 at 03:36:57PM +0300, Avi Kivity wrote:
> > > > On 09/12/2012 03:34 PM, Gleb Natapov wrote:
> > > > > On Wed, Sep 12, 2012 at 10:45:22AM +0300, Avi Kivity wrote:
> > > > >> On 09/12/2012 04:03 AM, Paul E. McKenney wrote:
> > > > >> >> > > Paul, I'd like to check something with you here:
> > > > >> >> > > this function can be triggered by userspace,
> > > > >> >> > > any number of times; we allocate
> > > > >> >> > > a 2K chunk of memory that is later freed by
> > > > >> >> > > kfree_rcu.
> > > > >> >> > > 
> > > > >> >> > > Is there a risk of DOS if RCU is delayed while
> > > > >> >> > > lots of memory is queued up in this way?
> > > > >> >> > > If yes is this a generic problem with kfree_rcu
> > > > >> >> > > that should be addressed in core kernel?
> > > > >> >> > 
> > > > >> >> > There is indeed a risk.
> > > > >> >> 
> > > > >> >> In our case it's a 2K object. Is it a practical risk?
> > > > >> > 
> > > > >> > How many kfree_rcu()s per second can a given user cause to happen?
> > > > >> 
> > > > >> Not much more than a few hundred thousand per second per process (normal
> > > > >> operation is zero).
> > > > >> 
> > > > > I managed to do 21466 per second.
> > > > 
> > > > Strange, why so slow?
> > > > 
> > > Because ftrace buffer overflows :) With bigger buffer I get 169940.
> > 
> > Ah, good, should not be a problem.  In contrast, if you ran kfree_rcu() in
> > a tight loop, you could probably do in excess of 100M per CPU per second.
> > Now -that- might be a problem.
> > 
> > Well, it -might- be a problem if you somehow figured out how to allocate
> > memory that quickly in a steady-state manner.  ;-)
> > 
> > > > >> Good idea.  Michael, is should be easy to modify kvm-unit-tests to write
> > > > >> to the APIC ID register in a loop.
> > > > >> 
> > > > > I did. Memory consumption does not grow on otherwise idle host.
> > 
> > Very good -- the checks in __call_rcu(), which is common code invoked by
> > kfree_rcu(), seem to be doing their job, then.  These do keep a per-CPU
> > counter, which can be adjusted via rcutree.blimit, which defaults
> > to taking evasive action if more than 10K callbacks are waiting on a
> > given CPU.
> > 
> > My concern was that you might be overrunning that limit in way less
> > than a grace period (as in about a hundred microseconds.  My concern
> > was of course unfounded -- you take several grace periods in push 10K
> > callbacks through.
> > 
> > 							Thanx, Paul
> 
> Gleb noted that Documentation/RCU/checklist.txt has this text:
> 
>         An especially important property of the synchronize_rcu()
>         primitive is that it automatically self-limits: if grace periods
>         are delayed for whatever reason, then the synchronize_rcu()
>         primitive will correspondingly delay updates.  In contrast,
>         code using call_rcu() should explicitly limit update rate in
>         cases where grace periods are delayed, as failing to do so can
>         result in excessive realtime latencies or even OOM conditions.
> 
> If call_rcu is self-limiting maybe this should be documented ...

It would be more accurate to say that takes has some measures to limit
the damage -- you can overwhelm these measures if you try hard enough.

And I guess I could say something to that effect.  ;-)

							Thanx, Paul