On Fri, Aug 14, 2020 at 04:06:04PM +0200, Michal Hocko wrote: > On Fri 14-08-20 06:34:50, Paul E. McKenney wrote: > > On Fri, Aug 14, 2020 at 02:48:32PM +0200, Michal Hocko wrote: > > > On Fri 14-08-20 14:15:44, Uladzislau Rezki wrote: > > > > > On Thu 13-08-20 19:09:29, Thomas Gleixner wrote: > > > > > > Michal Hocko <mhocko@xxxxxxxx> writes: > > > > > [...] > > > > > > > Why should we limit the functionality of the allocator for something > > > > > > > that is not a real problem? > > > > > > > > > > > > We'd limit the allocator for exactly ONE new user which was aware of > > > > > > this problem _before_ the code hit mainline. And that ONE user is > > > > > > prepared to handle the fail. > > > > > > > > > > If we are to limit the functionality to this one particular user then > > > > > I would consider a dedicated gfp flag a huge overkill. It would be much > > > > > more easier to have a preallocated pool of pages and use those and > > > > > completely avoid the core allocator. That would certainly only shift the > > > > > complexity to the caller but if it is expected there would be only that > > > > > single user then it would be probably better than opening a can of worms > > > > > like allocator usable from raw spin locks. > > > > > > > > > Vlastimil raised same question earlier, i answered, but let me answer again: > > > > > > > > It is hard to achieve because the logic does not stick to certain static test > > > > case, i.e. it depends on how heavily kfree_rcu(single/double) are used. Based > > > > on that, "how heavily" - number of pages are formed, until the drain/reclaimer > > > > thread frees them. > > > > > > How many pages are talking about - ball park? 100s, 1000s? > > > > Under normal operation, a couple of pages per CPU, which would make > > preallocation entirely reasonable. Except that if someone does something > > that floods RCU callbacks (close(open) in a tight userspace loop, for but > > one example), then 2000 per CPU might not be enough, which on a 64-CPU > > system comes to about 500MB. This is beyond excessive for preallocation > > on the systems I am familiar with. > > > > And the flooding case is where you most want the reclamation to be > > efficient, and thus where you want the pages. > > I am not sure the page allocator would help you with this scenario > unless you are on very large machines. Pagesets scale with the available > memory and percpu_pagelist_fraction sysctl (have a look at > pageset_set_high_and_batch). It is roughly 1000th of the zone size for > each zone. You can check that in /proc/vmstat (my 8G machine) Small systems might have ~64G. The medium-sized systems might have ~250G. There are a few big ones that might have 1.5T. None of the /proc/vmstat files from those machines contain anything resembling the list below, though. > Node 0, zone DMA > Not interesting at all > Node 0, zone DMA32 > pagesets > cpu: 0 > count: 242 > high: 378 > batch: 63 > cpu: 1 > count: 355 > high: 378 > batch: 63 > cpu: 2 > count: 359 > high: 378 > batch: 63 > cpu: 3 > count: 366 > high: 378 > batch: 63 > Node 0, zone Normal > pagesets > cpu: 0 > count: 359 > high: 378 > batch: 63 > cpu: 1 > count: 241 > high: 378 > batch: 63 > cpu: 2 > count: 297 > high: 378 > batch: 63 > cpu: 3 > count: 227 > high: 378 > batch: 63 > > Besides that do you need to be per-cpu? Having 1000 pages available and > managed under your raw spinlock should be good enough already no? It needs to be almost entirely per-CPU for performance reasons. Plus a user could do a tight close(open()) loop on each CPU. > > This of course raises the question of how much memory the lockless caches > > contain, but fortunately these RCU callback flooding scenarios also > > involve process-context allocation of the memory that is being passed > > to kfree_rcu(). That allocation should keep the lockless caches from > > going empty in the common case, correct? > > Yes, those are refilled both on the allocation/free paths. But you > cannot really rely on that to happen early enough. So the really ugly scenarios with the tight loops normally allocate something and immediately either call_rcu() or kfree_rcu() it. But you are right, someone doing "rm -rf" on a large file tree with lots of small files might not be doing that many allocations. > Do you happen to have any numbers that would show the typical usage > and how often the slow path has to be taken becase pcp lists are > depleted? In other words even if we provide a functionality to give > completely lockless way to allocate memory how useful that is? Not yet, but let's see what we can do. Thanx, Paul
