On Fri, Apr 16, 2021 at 10:14:39AM +0530, Bharata B Rao wrote: > On Wed, Apr 07, 2021 at 08:28:07AM +1000, Dave Chinner wrote: > > On Mon, Apr 05, 2021 at 11:18:48AM +0530, Bharata B Rao wrote: > > > > > As an alternative approach, I have this below hack that does lazy > > > list_lru creation. The memcg-specific list is created and initialized > > > only when there is a request to add an element to that particular > > > list. Though I am not sure about the full impact of this change > > > on the owners of the lists and also the performance impact of this, > > > the overall savings look good. > > > > Avoiding memory allocation in list_lru_add() was one of the main > > reasons for up-front static allocation of memcg lists. We cannot do > > memory allocation while callers are holding multiple spinlocks in > > core system algorithms (e.g. dentry_kill -> retain_dentry -> > > d_lru_add -> list_lru_add), let alone while holding an internal > > spinlock. > > > > Putting a GFP_ATOMIC allocation inside 3-4 nested spinlocks in a > > path we know might have memory demand in the *hundreds of GB* range > > gets an NACK from me. It's a great idea, but it's just not a > > I do understand that GFP_ATOMIC allocations are really not preferrable > but want to point out that the allocations in the range of hundreds of > GBs get reduced to tens of MBs when we do lazy list_lru head allocations > under GFP_ATOMIC.
That does not make GFP_ATOMIC allocations safe or desirable. In general, using GFP_ATOMIC outside of interrupt context indicates something is being done incorrectly. Especially if it can be triggered from userspace, which is likely in this particular case... > As shown earlier, this is what I see in my experimental setup with > 10k containers: > > Number of kmalloc-32 allocations > Before During After > W/o patch 178176 3442409472 388933632 > W/ patch 190464 468992 468992 SO now we have an additional half million GFP_ATOMIC allocations when we currently have none. That's not an improvement, that rings loud alarm bells. > This does really depend and vary on the type of the container and > the number of mounts it does, but I suspect we are looking > at GFP_ATOMIC allocations in the MB range. Also the number of > GFP_ATOMIC slab allocation requests matter I suppose. They are slab allocations, which mean every single one of them could require a new slab backing page (pages!) to be allocated. Hence the likely memory demand might be a lot higher than the optimal case you are considering here... > There are other users of list_lru, but I was just looking at > dentry and inode list_lru usecase. It appears to me that for both > dentry and inode, we can tolerate the failure from list_lru_add > due to GFP_ATOMIC allocation failure. The failure to add dentry > or inode to the lru list means that they won't be retained in > the lru list, but would be freed immediately. Is this understanding > correct? No. Both retain_dentry() and iput_final() would currently leak objects that fail insertion into the LRU. They don't check for insertion success at all. But, really, this is irrelevant - GFP_ATOMIC usage is the problem, and allowing it to fail doesn't avoid the problems that unbound GFP_ATOMIC allocation can have on the stability of the rest of the system when low on memory. Being able to handle a GFP_ATOMIC memory allocation failure doesn't change the fact that you should not be doing GFP_ATOMIC allocation in the first place... Cheers, Dave. -- Dave Chinner da...@fromorbit.com
