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
> > Ok, thanks.
> >
> >
> > --
> > error compiling committee.c: too many arguments to function
>
> --
> Gleb.
>
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
