Messages in this thread | | | Date | Wed, 17 Jun 2015 17:20:43 -0700 | From | "Paul E. McKenney" <> | Subject | Re: call_rcu from trace_preempt |
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On Wed, Jun 17, 2015 at 04:58:48PM -0700, Alexei Starovoitov wrote: > On 6/17/15 2:36 PM, Paul E. McKenney wrote: > >Well, you do need to have something in each element to allow them to be > >tracked. You could indeed use llist_add() to maintain the per-CPU list, > >and then use llist_del_all() bulk-remove all the elements from the per-CPU > >list. You can then pass each element in turn to kfree_rcu(). And yes, > >I am suggesting that you open-code this, as it is going to be easier to > >handle your special case then to provide a fully general solution. For > >one thing, the general solution would require a full rcu_head to track > >offset and next. In contrast, you can special-case the offset. And > >ignore the overload special cases. > > yes. all makes sense. > > > Locklessly enqueue onto a per-CPU list, but yes. The freeing is up to > > yes. per-cpu llist indeed. > > > you -- you get called just before exit from __call_rcu(), and get to > > figure out what to do. > > > > My guess would be if not in interrupt and not recursively invoked, > > atomically remove all the elements from the list, then pass each to > > kfree_rcu(), and finally let things take their course from there. > > The llist APIs look like they would work. > > Above and 'just before the exit from __call_rcu()' part of suggestion > I still don't understand. > To avoid reentry into call_rcu I can either create 1 or N new kthreads > or work_queue and do manual wakeups, but that's very specialized and I > don't want to permanently waste them, so I'm thinking to llist_add into > per-cpu llists and do llist_del_all in rcu_process_callbacks() to take > them from these llists and call kfree_rcu on them.
Another option is to drain the lists the next time you do an allocation. That would avoid hooking both __call_rcu() and rcu_process_callbacks().
Thanx, Paul
> The llist_add part will also do: > if (!rcu_is_watching()) invoke_rcu_core(); > to raise softirq when necessary. > So at the end it will look like two phase kfree_rcu. > I'll try to code it up and see it explodes :)
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