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. > How is it decided who gets to run before the other? Is it based on > order of attachment (which can be non-deterministic)? Yeah, now it's the order of attachment. > There was a lot of discussion on something similar for tc progs, and > we went with specific flags that capture partial ordering constraints > (instead of priorities that may collide). > https://lore.kernel.org/all/20230719140858.13224-2-daniel@xxxxxxxxxxxxx > It would be nice if we can find a way of making this consistent. I'll take a look, thanks! I hope that my naive approach might be good enough for the start and we can implement something more sophisticated later, but maybe I'm wrong here.
