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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
