Hello, Christoph, Shaohua. On Thu, Apr 21, 2011 at 04:43:00PM +0200, Tejun Heo wrote: > > In order to make it simple I avoided an preempt enable/disable. With > > Shaohua's patches there will be a simple atomic_add within the last if > > cluase. I was able to consolidate multiple code paths into the cmpxchg > > loop with this approach. > > > > The one below avoids the #ifdef that is ugly... > > That said, combined with Shaohua's patch, maybe it's better this way. > Let's see... Unfortunately, I have a new concern for __percpu_counter_sum(), which applies to both your and Shaohua's change. Before these changes, percpu_counter->lock protects whole of batch transfer. IOW, while __percpu_counter_sum() is holding ->lock, it can be sure that batch transfer from percpu counter to the main counter isn't in progress and that the deviation it might see is limited by the number of on-going percpu inc/dec's which is much lower than batch transfers. With the proposed changes to percpu counter, this no longer holds. cl's patch de-couples local counter update from the global counter update and __percpu_counter_sum() can see batch amount of deviation per concurrent updater making the whole visit-each-counter thing more or less meaningless. This, however, can be fixed by putting the whole slow path inside spin_lock() as suggested before so that the whole batch transferring from local to global is enclosed inside spinlock. Unfortunately, Shaohua's atomic64_t update ain't that easy. The whole point of that update was avoiding spinlocks in favor of atomic64_t, which naturally collides with the ability to enclosing local and global updates into the same exclusion block, which is necessary for __percpu_counter_sum() accuracy. So, Christoph, please put the whole slow path inside spin_lock(). Shaohua, unfortunately, I think your change is caught inbetween rock and hard place. Any ideas? Thanks. -- tejun -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>