On 7/28/2023 12:33 PM, YiFei Zhu wrote: > In internal testing of test_maps, we sometimes observed failures like: > test_maps: test_maps.c:173: void test_hashmap_percpu(unsigned int, void *): > Assertion `bpf_map_update_elem(fd, &key, value, BPF_ANY) == 0' failed. > where the errno is ENOMEM. After some troubleshooting and enabling > the warnings, we saw: > [ 91.304708] percpu: allocation failed, size=8 align=8 atomic=1, atomic alloc failed, no space left > [ 91.304716] CPU: 51 PID: 24145 Comm: test_maps Kdump: loaded Tainted: G N 6.1.38-smp-DEV #7 > [ 91.304719] Hardware name: Google Astoria/astoria, BIOS 0.20230627.0-0 06/27/2023 > [ 91.304721] Call Trace: > [ 91.304724] <TASK> > [ 91.304730] [<ffffffffa7ef83b9>] dump_stack_lvl+0x59/0x88 > [ 91.304737] [<ffffffffa7ef83f8>] dump_stack+0x10/0x18 > [ 91.304738] [<ffffffffa75caa0c>] pcpu_alloc+0x6fc/0x870 > [ 91.304741] [<ffffffffa75ca302>] __alloc_percpu_gfp+0x12/0x20 > [ 91.304743] [<ffffffffa756785e>] alloc_bulk+0xde/0x1e0 > [ 91.304746] [<ffffffffa7566c02>] bpf_mem_alloc_init+0xd2/0x2f0 > [ 91.304747] [<ffffffffa7547c69>] htab_map_alloc+0x479/0x650 > [ 91.304750] [<ffffffffa751d6e0>] map_create+0x140/0x2e0 > [ 91.304752] [<ffffffffa751d413>] __sys_bpf+0x5a3/0x6c0 > [ 91.304753] [<ffffffffa751c3ec>] __x64_sys_bpf+0x1c/0x30 > [ 91.304754] [<ffffffffa7ef847a>] do_syscall_64+0x5a/0x80 > [ 91.304756] [<ffffffffa800009b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd > > This makes sense, because in atomic context, percpu allocation would > not create new chunks; it would only create in non-atomic contexts. > And if during prefill all precpu chunks are full, -ENOMEM would > happen immediately upon next unit_alloc. > > Prefill phase does not actually run in atomic context, so we can > use this fact to allocate non-atomically with GFP_KERNEL instead > of GFP_NOWAIT. This avoids the immediate -ENOMEM. > > GFP_NOWAIT has to be used in unit_alloc when bpf program runs > in atomic context. Even if bpf program runs in non-atomic context, > in most cases, rcu read lock is enabled for the program so > GFP_NOWAIT is still needed. This is often also the case for > BPF_MAP_UPDATE_ELEM syscalls. > > Signed-off-by: YiFei Zhu <zhuyifei@xxxxxxxxxx> Acked-by: Hou Tao <houtao1@xxxxxxxxxx>
