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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
