On Wed, Apr 22, 2020 at 08:35:03AM -0700, Paul E. McKenney wrote: > On Wed, Apr 22, 2020 at 10:57:52AM -0400, Johannes Weiner wrote: > > On Thu, Apr 16, 2020 at 11:01:00AM -0700, Paul E. McKenney wrote: > > > On Thu, Apr 16, 2020 at 09:17:45AM -0400, Joel Fernandes wrote: > > > > On Thu, Apr 16, 2020 at 12:30:07PM +0200, Uladzislau Rezki wrote: > > > > > I have a question about dynamic attaching of the rcu_head. Do you think > > > > > that we should drop it? We have it because of it requires 8 + syzeof(struct rcu_head) > > > > > bytes and is used when we can not allocate 1 page what is much more for array purpose. > > > > > Therefore, dynamic attaching can succeed because of using SLAB and requesting much > > > > > less memory then one page. There will be higher chance of bypassing synchronize_rcu() > > > > > and inlining freeing on a stack. > > > > > > > > > > I agree that we should not use GFP_* flags instead we could go with GFP_NOWAIT | > > > > > __GFP_NOWARN when head attaching only. Also dropping GFP_ATOMIC to keep > > > > > atomic reserved memory for others. > > > > > > I must defer to people who understand the GFP flags better than I do. > > > The suggestion of __GFP_RETRY_MAYFAIL for no memory pressure (or maybe > > > when the CPU's reserve is not yet full) and __GFP_NORETRY otherwise came > > > from one of these people. ;-) > > > > The exact flags we want here depends somewhat on the rate and size of > > kfree_rcu() bursts we can expect. We may want to start with one set > > and instrument allocation success rates. > > > > Memory tends to be fully consumed by the filesystem cache, so some > > form of light reclaim is necessary for almost all allocations. > > > > GFP_NOWAIT won't do any reclaim by itself, but it'll wake kswapd. > > Kswapd maintains a small pool of free pages so that even allocations > > that are allowed to enter reclaim usually don't have to. It would be > > safe for RCU to dip into that. > > > > However, there are some cons to using it: > > > > - Depending on kfree_rcu() burst size, this pool could exhaust (it's > > usually about half a percent of memory, but is affected by sysctls), > > and then it would fail NOWAIT allocations until kswapd has caught up. > > > > - This pool is shared by all GFP_NOWAIT users, and many (most? all?) > > of them cannot actually sleep. Often they would have to drop locks, > > restart list iterations, or suffer some other form of deterioration to > > work around failing allocations. > > > > Since rcu wouldn't have anything better to do than sleep at this > > juncture, it may as well join the reclaim effort. > > > > Using __GFP_NORETRY or __GFP_RETRY_MAYFAIL would allow them that > > without exerting too much pressure on the VM. > > Thank you for looking this over and for the feedback! > > Good point on the sleeping. My assumption has been that sleeping waiting > for a grace period was highly likely to allow memory to eventually be > freed, and that there is a point of diminishing returns beyond which > adding additional tasks to the reclaim effort does not help much. There is when the VM is struggling, but not necessarily when there is simply a high, concurrent rate of short-lived file cache allocations. Kswapd can easily reclaim gigabytes of clean page cache each second, but there might be enough allocation concurrency from other threads to starve a kfree_rcu() that only makes a very cursory attempt at getting memory out of being able to snap up some of those returns. In that scenario it makes sense to be a bit more persistent, or even help scale out the concurrency of reclaim to that of allocations. > Here are some strategies right offhand when sleeping is required: > > 1. Always sleep in synchronize_rcu() in order to (with high > probability) free the memory. This might mean that the reclaim > effort goes slower than would be good. > > 2. Always sleep in the memory allocator in order to help reclaim > along. (This is a strawman version of what I expect your > proposal really is, but putting it here for completeness, please > see below.) > > 3. Always sleep in the memory allocator in order to help reclaim > along, but return failure at some point. Then the caller > invokes synchronize_rcu(). When to return failure? > > o After some substantial but limited amount of effort has > been spent on reclaim. > > o When it becomes likely that further reclaim effort > is not going to free up additional memory. > > I am guessing that you are thinking in terms of specifying GFP flags to > result in some variant of #3. Yes, although I would add o After making more than one attempt at the freelist to prevent merely losing races when the allocator/reclaim subsystem is mobbed by a high concurrency of requests. __GFP_NORETRY (despite its name) accomplishes this. __GFP_RETRY_MAYFAIL is yet more persistent, but may retry for quite a while if the allocation keeps losing the race for a page. This increases the chance of the allocation eventually suceeding, but also the risk of 1) trying to get memory for longer than a synchronize_rcu() might have taken and 2) exerting more temporary memory pressure on the workload* than might be productive. So I'm inclined to suggest __GFP_NORETRY as a starting point, and make further decisions based on instrumentation of the success rates of these opportunistic allocations. * Reclaim and OOM handling will be fine since no reserves are tapped
