On Fri, Jun 5, 2020 at 11:01 AM Hao Luo <haoluo@xxxxxxxxxx> wrote: > > > > On Wed, Jun 3, 2020 at 9:04 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: >> >> On Wed, Jun 3, 2020 at 10:23 AM Hao Luo <haoluo@xxxxxxxxxx> wrote: >> > >> > On SMP systems, the global percpu variables are placed in a speical >> > '.data..percpu' section, which is stored in a segment whose initial >> > address is set to 0, the addresses of per-CPU variables are relative >> > positive addresses [1]. >> > >> > This patch extracts these variables from vmlinux and places them with >> > their type information in BTF. More specifically, when we encode BTF, >> > we find the shndx of the '.data..percpu' section and then traverse >> > the symbol table to find those global objects which are in this section. >> > For each of these objects, we push a BTF_KIND_VAR into the types buffer, >> > and a BTF_VAR_SECINFO into another buffer, percpu_secinfo. When all the >> > CUs have finished processing, we push a BTF_KIND_DATASEC into the >> > types buffer, followed by the percpu_secinfo's content, that is, an >> > array of BTF_VAR_SECINFOs. >> > >> > In a v5.7-rc7 linux kernel, I was able to extract 294 such variables. >> > The space overhead is small. >> > >> > Testing: >> > >> > Before: >> > $ readelf -SW vmlinux | grep BTF >> > [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d2bf8 00 A 0 0 1 >> > >> > After: >> > $ pahole -J vmlinux >> > $ readelf -SW vmlinux | grep BTF >> > [25] .BTF PROGBITS ffffffff821a905c 13a905c 2d5bca 00 A 0 0 1 >> > >> > Common percpu vars can be found in the BTF section. >> > >> > $ bpftool btf dump file vmlinux | grep runqueues >> > [14098] VAR 'runqueues' type_id=13725, linkage=global-alloc >> > >> > $ bpftool btf dump file vmlinux | grep 'cpu_stopper' >> > [17592] STRUCT 'cpu_stopper' size=72 vlen=5 >> > [17612] VAR 'cpu_stopper' type_id=17592, linkage=static >> > >> > $ bpftool btf dump file vmlinux | grep ' DATASEC ' >> > [63652] DATASEC '.data..percpu' size=0 vlen=294 >> > >> > References: >> > [1] https://lwn.net/Articles/531148/ >> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx> >> > --- >> >> It all looks good to me, very minor things below only. >> >> Few questions for you: >> 1. Have you tried building kernel with these global variables and >> checking that kernel can self-validate it correctly? > > > Do you mean running selftests? I haven't run selftests but do have built and booted, it looked good. Well yeah, anything that relies on kernel to recognize its own BTF properly (so tp_btf or fentry/fexit programs, for instance). > >> >> 2. It's not explicitly stated, but right now within struct and >> datasec, all offsets are in a sorted order by offset, can you please >> check that this is preserved for per-cpu variables withing >> .data..percpu data section? > > > No problem, I can add a qsort before copying all var_secinfos. Right now it seems it doesn't follow any particular order. Sounds good, thanks. > >> >> 3. As an experiment, can you please try to estimate how much data >> would be to encode all of the variables, not just per-cpu ones? E.g., >> you can do it very hacky by appending them to single percpu datasec, >> but for all variables across all data sections. That would give us a >> good idea if it's feasible to have all kernel global variables >> available eventually. > > > I tried encoding all the global object-type symbols (including static vars and globally allocated vars). There are a total of 27516 such symbols. The .BTF size is 0x41499e, in comparison, it was 0x2d2c10. The space increase is around 1.3Mb. The build time is roughly the same. So I think this is totally affordable. I see, that's quite a bump, but not unreasonable. Let's add it later as needed, though. > > I think 4 sections are of interest: .data..percpu, .data, .rodata and .bss. I can encode the rest of the sections the same way as .data..percpu. But, compared to .data..percpu, the other three contain a lot more unconventional symbols mixed with some useful variables. > > Maybe, add them in future patches upon request, if this change looks good? Any comments? > >> >> >> Thanks! >> >> > btf_encoder.c | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++ >> > dwarves.c | 6 +++ >> > dwarves.h | 2 + >> > libbtf.c | 86 ++++++++++++++++++++++++++++++++++ >> > libbtf.h | 7 +++ >> > pahole.c | 1 + >> > 6 files changed, 227 insertions(+) >> > >> >> [...] >> >> > + cu__for_each_variable(cu, core_id, pos) { >> > + var = tag__variable(pos); >> > + if (var->declaration) >> > + continue; >> > + /* percpu variables are allocated in global space */ >> > + if (variable__scope(var) != VSCOPE_GLOBAL) >> > + continue; >> > + has_global_var = true; >> > + struct hlist_head *head = &hash_addr[hashaddr__fn(var->ip.addr)]; >> >> variable declarations in C89 should be at the top of the block, >> separate from statements. > > > Ack. Will fix. > >> >> >> > + hlist_add_head(&var->tool_hnode, head); >> > + } >> > + if (!has_global_var) >> > + goto out; >> > + >> > + /* search within symtab for percpu variables */ >> > + symtab = elf_symtab__new(NULL, btfe->elf, &btfe->ehdr); >> > + if (!symtab) >> > + goto out; >> > + >> > + elf_symtab__for_each_symbol(symtab, core_id, sym) { >> > + uint32_t linkage, type, size, offset; >> > + int32_t btf_var_id, btf_var_secinfo_id; >> > + uint64_t addr; >> > + >> > + /* compare a symbol's shndx to determine if it's a percpu variable */ >> > + if (elf_sym__section(&sym) != percpu_ndx) >> > + continue; >> > + >> > + addr = elf_sym__value(&sym); >> > + /* >> > + * Store only those symbols that have allocated space in the percpu section. >> > + * This excludes the following three types of symbols: >> > + * >> > + * 1. __ADDRESSABLE(sym), which are forcely emitted as symbols. >> > + * 2. __UNIQUE_ID(prefix), which are introduced to generate unique ids. >> > + * 3. __exitcall(fn), functions which are labeled as exit calls. >> > + * >> > + * In addition, the variables defined using DEFINE_PERCPU_FIRST are >> > + * also not included, which currently includes: >> > + * >> > + * 1. fixed_percpu_data >> > + */ >> > + if (!addr) >> > + continue; >> >> offset == 0 is always invalid for per-cpu variables? > > > On x86_64, the first area is always for irq_stack. On other architectures, this seems not necessary as I didn't see anywhere used DEFINE_PERCPU_FIRST like x86_64. But I don't have a non-x86 64 machine to check right now. I was asking because it seemed arbitrary to ignore one symbol that happened to be at offset 0. > >> >> >> > + var = hashaddr__find_variable(hash_addr, addr); >> > + if (var == NULL) >> > + continue; >> > + >> > + /* add a BTF_KIND_VAR in btfe->types */ >> > + linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : BTF_VAR_STATIC; >> > + type = var->ip.tag.type + type_id_off; >> > + btf_var_id = btf_elf__add_var_type(btfe, type, var->name, linkage); >> > + if (btf_var_id < 0) { >> > + err = -1; >> > + printf("error: failed to encode variable '%s'\n", >> > + variable__name(var, cu)); >> > + break; >> > + } >> > + >> > + /* >> >> [...] >> >> > static int btf_elf__write(const char *filename, struct btf *btf) >> > { >> > GElf_Shdr shdr_mem, *shdr; >> > @@ -727,6 +810,9 @@ int btf_elf__encode(struct btf_elf *btfe, uint8_t flags) >> > if (gobuffer__size(&btfe->types) == 0) >> > return 0; >> > >> > + if (gobuffer__size(&btfe->percpu_secinfo) != 0) >> > + btf_elf__add_datasec_type(btfe, ".data..percpu", &btfe->percpu_secinfo); >> >> would probably be better to extract ".data..percpu" into a constant >> and use it everywhere > > > Indeed. I'll add that. > >> >> >> > + >> > btfe->size = sizeof(*hdr) + (gobuffer__size(&btfe->types) + gobuffer__size(btfe->strings)); >> > btfe->data = zalloc(btfe->size); >> > >> >> [...]
