On Wed, 9 Sep 2020, Alex Shi wrote: > 在 2020/9/9 上午7:41, Hugh Dickins 写道: > > > > [PATCH v18 05/32] mm/thp: remove code path which never got into > > This is a good simplification, but I see no sign that you understand > > why it's valid: it relies on lru_add_page_tail() being called while > > head refcount is frozen to 0: we would not get this far if someone > > else holds a reference to the THP - which they must hold if they have > > isolated the page from its lru (and that's true before or after your > > per-memcg changes - but even truer after those changes, since PageLRU > > can then be flipped without lru_lock at any instant): please explain > > something of this in the commit message. > > Is the following commit log better? > > split_huge_page() will never call on a page which isn't on lru list, so > this code never got a chance to run, and should not be run, to add tail > pages on a lru list which head page isn't there. > > Hugh Dickins' mentioned: > The path should never be called since lru_add_page_tail() being called > while head refcount is frozen to 0: we would not get this far if someone > else holds a reference to the THP - which they must hold if they have > isolated the page from its lru. > > Although the bug was never triggered, it'better be removed for code > correctness, and add a warn for unexpected calling. Not much better, no. split_huge_page() can easily be called for a page which is not on the lru list at the time, and I don't know what was the bug which was never triggered. Stick with whatever text you end up with for the combination of 05/32 and 18/32, and I'll rewrite it after. > > [PATCH v18 06/32] mm/thp: narrow lru locking > > Why? What part does this play in the series? "narrow lru locking" can > > also be described as "widen page cache locking": > > Uh, the page cache locking isn't widen, it's still on the old place. I'm not sure if you're joking there. Perhaps just a misunderstanding. Yes, patch 06/32 does not touch the xa_lock(&mapping->i_pages) and xa_lock(&swap_cache->i_pages) lines (odd how we've arrived at two of those, but please do not get into cleaning it up now); but it removes the spin_lock_irqsave(&pgdata->lru_lock, flags) which used to come before them, and inserts a spin_lock(&pgdat->lru_lock) after them. You call that narrowing the lru locking, okay, but I see it as also pushing the page cache locking outwards: before this patch, page cache lock was taken inside lru_lock; after this patch, page cache lock is taken outside lru_lock. If you cannot see that, then I think you should not have touched this code at all; but it's what we have been testing, and I think we should go forward with it. > > But I wish you could give some reason for it in the commit message! > > It's a head scratch task. Would you like to tell me what's detailed info > should be there? Thanks! So, you don't know why you did it either: then it will be hard to justify. I guess I'll have to write something for it later. I'm strongly tempted just to drop the patch, but expect it will become useful later, for using lock_page_memcg() before getting lru_lock. > > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx> > > Is that correct? Or Wei Yang suggested some part of it perhaps? > > Yes, we talked a lot to confirm the locking change is safe. Okay, but the patch was written by you, and sent by you to Andrew: that is not a case for "Signed-off-by: Someone Else". > > [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock > > Could we please drop this one for the moment? And come back to it later > > when the basic series is safely in. It's a good idea to try sorting > > together those pages which come under the same lock (though my guess is > > that they naturally gather themselves together quite well already); but > > I'm not happy adding 360 bytes to the kernel stack here (and that in > > addition to 192 bytes of horrid pseudo-vma in the shmem swapin case), > > though that could be avoided by making it per-cpu. But I hope there's > > a simpler way of doing it, as efficient, but also useful for the other > > pagevec operations here: perhaps scanning the pagevec for same page-> > > mem_cgroup (and flags node bits), NULLing entries as they are done. > > Another, easily fixed, minor defect in this patch: if I'm reading it > > right, it reverses the order in which the pages are put on the lru? > > this patch could give about 10+% performance gain on my multiple memcg > readtwice testing. fairness locking cost the performance much. Good to know, should have been mentioned. s/fairness/Repeated/ But what was the gain or loss on your multiple memcg readtwice testing without this patch, compared against node-only lru_lock? The 80% gain mentioned before, I presume. So this further optimization can wait until the rest is solid. > > I also tried per cpu solution but that cause much trouble of per cpu func > things, and looks no benefit except a bit struct size of stack, so if > stack size still fine. May we could use the solution and improve it better. > like, functionlize, fix the reverse issue etc. I don't know how important the stack depth consideration is nowadays: I still care, maybe others don't, since VMAP_STACK became an option. Yes, please fix the reversal (if I was right on that); and I expect you could use a singly linked list instead of the double. But I'll look for an alternative - later, once the urgent stuff is completed - and leave the acks on this patch to others. Hugh
