On Fri 05-02-21 23:30:36, Muchun Song wrote: > On Fri, Feb 5, 2021 at 8:20 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Fri 05-02-21 19:04:19, Muchun Song wrote: > > > On Fri, Feb 5, 2021 at 6:21 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > On Fri 05-02-21 17:55:10, Muchun Song wrote: > > > > > On Fri, Feb 5, 2021 at 4:24 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > On Fri 05-02-21 14:23:10, Muchun Song wrote: > > > > > > > We call memcg_oom_recover() in the uncharge_batch() to wakeup OOM task > > > > > > > when page uncharged, but for the slab pages, we do not do this when page > > > > > > > uncharged. > > > > > > > > > > > > How does the patch deal with this? > > > > > > > > > > When we uncharge a slab page via __memcg_kmem_uncharge, > > > > > actually, this path forgets to do this for us compared to > > > > > uncharge_batch(). Right? > > > > > > > > Yes this was more more or less clear (still would have been nicer to be > > > > explicit). But you still haven't replied to my question I believe. I > > > > assume you rely on refill_stock doing draining but how does this address > > > > the problem? Is it sufficient to do wakeups in the batched way? > > > > > > Sorry, the subject title may not be suitable. IIUC, memcg_oom_recover > > > aims to wake up the OOM task when we uncharge the page. > > > > Yes, your understanding is correct. This is a way to pro-actively wake > > up oom victims when the memcg oom handling is outsourced to the > > userspace. Please note that I haven't objected to the problem statement. > > > > I was questioning the fix for the problem. > > > > > I see uncharge_batch always do this. I am confused why > > > __memcg_kmem_uncharge does not. > > > > Very likely an omission. I haven't checked closely but I suspect this > > has been introduced by the recent kmem accounting changes. > > > > Why didn't you simply do the same thing and call memcg_oom_recover > > unconditionally and instead depend on the draining? I suspect this was > > because you wanted to recover also when draining which is not necessary > > as pointed out in other email. > > Thanks for your explanations. You are right. It is my fault to depend > on the draining. I should call memcg_oom_recover directly in the > __memcg_kmem_uncharge. Right? Yes. > > [...] > > > > > > Does this lead to any code generation improvements? I would expect > > > > > > compiler to be clever enough to inline static functions if that pays > > > > > > off. If yes make this a patch on its own. > > > > > > > > > > I have disassembled the code, I see memcg_oom_recover is not > > > > > inline. Maybe because memcg_oom_recover has a lot of callers. > > > > > Just guess. > > > > > > > > > > (gdb) disassemble uncharge_batch > > > > > [...] > > > > > 0xffffffff81341c73 <+227>: callq 0xffffffff8133c420 <page_counter_uncharge> > > > > > 0xffffffff81341c78 <+232>: jmpq 0xffffffff81341bc0 <uncharge_batch+48> > > > > > 0xffffffff81341c7d <+237>: callq 0xffffffff8133e2c0 <memcg_oom_recover> > > > > > > > > So does it really help to do the inlining? > > > > > > I just think memcg_oom_recover is very small, inline maybe > > > a good choice. Maybe I am wrong. > > > > In general I am not overly keen on changes without a proper > > justification. In this particular case I would understand that a > > function call that will almost never do anything but the test (because > > oom_disabled is a rarely used) is just waste of cycles in some hot > > paths (e.g. kmem uncharge). Maybe this even has some visible performance > > benefit. If this is really the case then would it make sense to guard > > this test by the existing cgroup_subsys_on_dfl(memory_cgrp_subsys)? > > Agree. I think it can improve performance when this > function is inline. Guarding the test should be also > an improvement on cgroup v2. I would be surprised if this was measurable but you can give it a try. A static key would be a reasonable argument for inlining on its own. -- Michal Hocko SUSE Labs
