On Tue, Nov 26, 2013 at 11:35:06AM -0800, Paul E. McKenney wrote:
> 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.
>
The question is: Is it safe to have a call_rcu() without any additional rate
limiting
on user triggerable code path?
> And I guess I could say something to that effect. ;-)
>
> Thanx, Paul
--
Gleb.
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