Messages in this thread | | | Subject | Re: [PATCH v5 07/15] locking/lockdep: Free lock classes that are no longer in use | From | Bart Van Assche <> | Date | Thu, 10 Jan 2019 13:56:41 -0800 |
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On Thu, 2019-01-10 at 20:44 +0100, Peter Zijlstra wrote: > On Thu, Jan 10, 2019 at 10:51:27AM -0800, Bart Van Assche wrote: > > On Thu, 2019-01-10 at 16:24 +0100, Peter Zijlstra wrote: > > > /* > > > * A data structure for delayed freeing of data structures that may be > > > - * accessed by RCU readers at the time these were freed. The size of the array > > > - * is a compromise between minimizing the amount of memory used by this array > > > - * and minimizing the number of wait_event() calls by get_pending_free_lock(). > > > + * accessed by RCU readers at the time these were freed. > > > */ > > > static struct pending_free { > > > - struct list_head zapped_classes; > > > struct rcu_head rcu_head; > > > + int index; > > > int pending; > > > -} pending_free[2]; > > > -static DECLARE_WAIT_QUEUE_HEAD(rcu_cb); > > > + struct list_head zapped[2]; > > > +} pending_free; > > > > Hi Peter, > > > > If the zapped[] array only has two elements there is no guarantee that an > > element will be free when zap_class() is called. I think we need at least > > num_online_cpus() elements to guarantee that at least one element is free > > when zap_class() is called. So removing the wait loop from > > get_pending_free_lock() seems wrong to me. Have you tried to run a workload > > that keeps all CPUs busy and that triggers get_pending_free_lock() > > frequently? > > I have not ran (yet); but I do not quite follow your argument. There is > only a single rcu_head, yes? Thereby only a single list can be pending > at any one time, and the other list is free to be appended to during > this time -- all is serialized by the graph lock after all. > > When the rcu callback happens, we flush the list we started the QS for, > which then becomes empty and if the open list contains entries, we > flip the lot and requeue the rcu_head for another QS. > > Therefore we only ever need 2 lists; 1 closed with entries waiting for > the callback, 1 open, to which we can append all newly freed entries.
Hi Peter,
Now that I had a closer look at your patch I think the approach of your patch is fine. Sorry for the confusion.
Bart.
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