Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes: > On Thu, 21 Aug 2025 at 02:25, Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: >> >> Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes: >> >> > On Mon, 18 Aug 2025 at 19:01, Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: >> >> >> >> Introduce a bpf struct ops for implementing custom OOM handling policies. >> >> >> >> The struct ops provides the bpf_handle_out_of_memory() callback, >> >> which expected to return 1 if it was able to free some memory and 0 >> >> otherwise. >> >> >> >> In the latter case it's guaranteed that the in-kernel OOM killer will >> >> be invoked. Otherwise the kernel also checks the bpf_memory_freed >> >> field of the oom_control structure, which is expected to be set by >> >> kfuncs suitable for releasing memory. It's a safety mechanism which >> >> prevents a bpf program to claim forward progress without actually >> >> releasing memory. The callback program is sleepable to enable using >> >> iterators, e.g. cgroup iterators. >> >> >> >> The callback receives struct oom_control as an argument, so it can >> >> easily filter out OOM's it doesn't want to handle, e.g. global vs >> >> memcg OOM's. >> >> >> >> The callback is executed just before the kernel victim task selection >> >> algorithm, so all heuristics and sysctls like panic on oom, >> >> sysctl_oom_kill_allocating_task and sysctl_oom_kill_allocating_task >> >> are respected. >> >> >> >> The struct ops also has the name field, which allows to define a >> >> custom name for the implemented policy. It's printed in the OOM report >> >> in the oom_policy=<policy> format. "default" is printed if bpf is not >> >> used or policy name is not specified. >> >> >> >> [ 112.696676] test_progs invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0 >> >> oom_policy=bpf_test_policy >> >> [ 112.698160] CPU: 1 UID: 0 PID: 660 Comm: test_progs Not tainted 6.16.0-00015-gf09eb0d6badc #102 PREEMPT(full) >> >> [ 112.698165] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-5.fc42 04/01/2014 >> >> [ 112.698167] Call Trace: >> >> [ 112.698177] <TASK> >> >> [ 112.698182] dump_stack_lvl+0x4d/0x70 >> >> [ 112.698192] dump_header+0x59/0x1c6 >> >> [ 112.698199] oom_kill_process.cold+0x8/0xef >> >> [ 112.698206] bpf_oom_kill_process+0x59/0xb0 >> >> [ 112.698216] bpf_prog_7ecad0f36a167fd7_test_out_of_memory+0x2be/0x313 >> >> [ 112.698229] bpf__bpf_oom_ops_handle_out_of_memory+0x47/0xaf >> >> [ 112.698236] ? srso_alias_return_thunk+0x5/0xfbef5 >> >> [ 112.698240] bpf_handle_oom+0x11a/0x1e0 >> >> [ 112.698250] out_of_memory+0xab/0x5c0 >> >> [ 112.698258] mem_cgroup_out_of_memory+0xbc/0x110 >> >> [ 112.698274] try_charge_memcg+0x4b5/0x7e0 >> >> [ 112.698288] charge_memcg+0x2f/0xc0 >> >> [ 112.698293] __mem_cgroup_charge+0x30/0xc0 >> >> [ 112.698299] do_anonymous_page+0x40f/0xa50 >> >> [ 112.698311] __handle_mm_fault+0xbba/0x1140 >> >> [ 112.698317] ? srso_alias_return_thunk+0x5/0xfbef5 >> >> [ 112.698335] handle_mm_fault+0xe6/0x370 >> >> [ 112.698343] do_user_addr_fault+0x211/0x6a0 >> >> [ 112.698354] exc_page_fault+0x75/0x1d0 >> >> [ 112.698363] asm_exc_page_fault+0x26/0x30 >> >> [ 112.698366] RIP: 0033:0x7fa97236db00 >> >> >> >> It's possible to load multiple bpf struct programs. In the case of >> >> oom, they will be executed one by one in the same order they been >> >> loaded until one of them returns 1 and bpf_memory_freed is set to 1 >> >> - an indication that the memory was freed. This allows to have >> >> multiple bpf programs to focus on different types of OOM's - e.g. >> >> one program can only handle memcg OOM's in one memory cgroup. >> >> But the filtering is done in bpf - so it's fully flexible. >> > >> > I think a natural question here is ordering. Is this ability to have >> > multiple OOM programs critical right now? >> >> Good question. Initially I had only supported a single bpf policy. >> But then I realized that likely people would want to have different >> policies handling different parts of the cgroup tree. >> E.g. a global policy and several policies handling OOMs only >> in some memory cgroups. >> So having just a single policy is likely a no go. > > If the ordering is more to facilitate scoping, would it then be better > to support attaching the policy to specific memcg/cgroup? Well, it has some advantages and disadvantages. First, it will require way more infrastructure on the memcg side. Second, the interface is not super clear: we don't want to have a struct ops per cgroup, I guess. And in many case a single policy for all memcgs is just fine, so asking the user to attach it to all memcgs is just adding a toil and creating all kinds of races. So I see your point, but I'm not yet convinced, to be honest. Thanks!
