On Tue, 16 Oct 2018, Frederic Weisbecker wrote: > On Mon, Oct 15, 2018 at 10:50:48AM +0100, Mel Gorman wrote: > > On Fri, Oct 12, 2018 at 09:21:41AM +0200, Vlastimil Babka wrote: > > > On 9/14/18 4:59 PM, Sebastian Andrzej Siewior wrote: > > > I think this evaluation is missing the other side of the story, and > > > that's the cost of using a spinlock (even uncontended) instead of > > > disabling preemption. The expectation for LRU pagevec is that the local > > > operations will be much more common than draining of other CPU's, so > > > it's optimized for the former. > > > > > > > Agreed, the drain operation should be extremely rare except under heavy > > memory pressure, particularly if mixed with THP allocations. The overall > > intent seems to be improving lockdep coverage but I don't think we > > should take a hit in the common case just to get that coverage. Bear in > > mind that the main point of the pagevec (whether it's true or not) is to > > avoid the much heavier LRU lock. > > So indeed, if the only purpose of this patch were to make lockdep wiser, > a pair of spin_lock_acquire() / spin_unlock_release() would be enough to > teach it and would avoid the overhead. > > Now another significant incentive behind this change is to improve CPU isolation. > Workloads relying on owning the entire CPU without being disturbed are interested > in this as it allows to offload some noise. It's no big deal for those who can > tolerate rare events but often CPU isolation is combined with deterministic latency > requirements. > > So, I'm not saying this per-CPU spinlock is necessarily the right answer, I > don't know that code enough to have an opinion, but I still wish we can find > a solution. One way to solve this and I had played with it already is to make the smp function call based variant and the lock based variant switchable at boot time with a static key. That way CPU isolation can select it and take the penalty while normal workloads are not affected. Thanks, tglx
