On Mon, Nov 01, 2021 at 04:14:29PM -0700, Andrii Nakryiko wrote: > On Thu, Oct 28, 2021 at 12:12 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > On Wed, Oct 27, 2021 at 08:18:11PM +0200, Jiri Olsa wrote: > > > On Wed, Oct 27, 2021 at 10:53:55AM -0700, Andrii Nakryiko wrote: > > > > On Wed, Oct 27, 2021 at 1:53 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > > > > > > > On Tue, Oct 26, 2021 at 09:12:31PM -0700, Andrii Nakryiko wrote: > > > > > > On Tue, Oct 26, 2021 at 5:03 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > On Mon, Oct 25, 2021 at 09:54:48PM -0700, Andrii Nakryiko wrote: > > > > > > > > On Sat, Oct 23, 2021 at 5:05 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > hi, > > > > > > > > > I'm trying to enable BTF for kernel module in fedora, > > > > > > > > > and I'm getting big increase on modules sizes on s390x arch. > > > > > > > > > > > > > > > > > > Size of modules in total - kernel dir under /lib/modules/VER/ > > > > > > > > > from kernel-core and kernel-module packages: > > > > > > > > > > > > > > > > > > current new > > > > > > > > > aarch64 60M 76M > > > > > > > > > ppc64le 53M 66M > > > > > > > > > s390x 21M 41M > > > > > > > > > x86_64 64M 79M > > > > > > > > > > > > > > > > > > The reason for higher increase on s390x was that dedup algorithm > > > > > > > > > did not detect some of the big kernel structs like 'struct module', > > > > > > > > > so they are duplicated in the kernel module BTF data. The s390x > > > > > > > > > has many small modules that increased significantly in size because > > > > > > > > > of that even after compression. > > > > > > > > > > > > > > > > > > First issues was that the '--btf_gen_floats' option is not passed > > > > > > > > > to pahole for kernel module BTF generation. > > > > > > > > > > > > > > > > > > The other problem is more tricky and is the reason why this patchset > > > > > > > > > is RFC ;-) > > > > > > > > > > > > > > > > > > The s390x compiler generates multiple definitions of the same struct > > > > > > > > > and dedup algorithm does not seem to handle this at the moment. > > > > > > > > > > > > > > > > > > I put the debuginfo and btf dump of the s390x pnet.ko module in here: > > > > > > > > > http://people.redhat.com/~jolsa/kmodbtf/ > > > > > > > > > > > > > > > > > > Please let me know if you'd like to see other info/files. > > > > > > > > > > > > > > > > > > > > > > > > > Hard to tell what's going on without vmlinux itself. Can you upload a > > > > > > > > corresponding kernel image with BTF in it? > > > > > > > > > > > > > > sure, uploaded > > > > > > > > > > > > > > > > > > > vmlinux.btfdump: > > > > > > > > > > > > [174] FLOAT 'float' size=4 > > > > > > [175] FLOAT 'double' size=8 > > > > > > > > > > > > VS > > > > > > > > > > > > pnet.btfdump: > > > > > > > > > > > > [89318] INT 'float' size=4 bits_offset=0 nr_bits=32 encoding=(none) > > > > > > [89319] INT 'double' size=8 bits_offset=0 nr_bits=64 encoding=(none) > > > > > > > > > > ugh, that's with no fix applied, sry > > > > > > > > > > I applied the first patch and uploaded new files > > > > > > > > > > now when I compare the 'module' struct from vmlinux: > > > > > > > > > > [885] STRUCT 'module' size=1280 vlen=70 > > > > > > > > > > and same one from pnet.ko: > > > > > > > > > > [89323] STRUCT 'module' size=1280 vlen=70 > > > > > > > > > > they seem to completely match, all the fields > > > > > and yet it still appears in the kmod's BTF > > > > > > > > > > > > > Ok, now struct module is identical down to the types referenced from > > > > the fields, which means it should have been deduplicated completely. > > > > This will require a more time-consuming debugging, though, so I'll put > > > > it on my TODO list for now. If you get to this earlier, see where the > > > > equivalence check fails in btf_dedup (sprinkle debug outputs around to > > > > see what's going on). > > > > > > it failed for me on that hypot_type_id check where I did fix, > > > I thought it's the issue of multiple same struct in the kmod, > > > but now I see I might have confused cannon_id with cand_id ;-) > > > I'll check more on this > > > > with more checking I got to the same conclusion as before, > > now maybe with little more details ;-) > > > > the problem seems to be that in some cases the module BTF > > data stores same structs under new/different IDs, while the > > kernel BTF data is already dedup-ed > > > > the dedup algo keeps hypot_map of kernel IDs to kmod IDs, > > and in my case it will get to the point that the kernel ID > > is already 'known' and points to certain kmod ID 'A', but it > > is also equiv to another kmod ID 'B' (so kmod ID 'A' and 'B' > > are equiv structs) but the dedup will claim as not equiv > > > > > > This is where the dedup fails for me on that s390 data: > > > > The pt_regs is defined as: > > > > struct pt_regs > > { > > union { > > user_pt_regs user_regs; > > struct { > > unsigned long args[1]; > > psw_t psw; > > unsigned long gprs[NUM_GPRS]; > > }; > > }; > > ... > > }; > > > > considering just the first union: > > > > [186] UNION '(anon)' size=152 vlen=2 > > 'user_regs' type_id=183 bits_offset=0 > > '(anon)' type_id=181 bits_offset=0 > > > > [91251] UNION '(anon)' size=152 vlen=2 > > 'user_regs' type_id=91247 bits_offset=0 > > '(anon)' type_id=91250 bits_offset=0 > > > > > > --------------------------------------------------------------- > > > > Comparing the first member 'user_regs': > > > > struct pt_regs > > { > > union { > > ---> user_pt_regs user_regs; > > struct { > > unsigned long args[1]; > > psw_t psw; > > unsigned long gprs[NUM_GPRS]; > > }; > > }; > > > > Which looks like: > > > > typedef struct { > > unsigned long args[1]; > > psw_t psw; > > unsigned long gprs[NUM_GPRS]; > > } user_pt_regs; > > > > > > and is also equiv to the next union member struct.. and that's what > > kernel knows but not kmod... anyway, > > > > > > the dedup will compare 'user_pt_regs': > > > > [183] TYPEDEF 'user_pt_regs' type_id=181 > > > > [91247] TYPEDEF 'user_pt_regs' type_id=91245 > > > > > > [181] STRUCT '(anon)' size=152 vlen=3 > > 'args' type_id=182 bits_offset=0 > > 'psw' type_id=179 bits_offset=64 > > 'gprs' type_id=48 bits_offset=192 > > > > [91245] STRUCT '(anon)' size=152 vlen=3 > > 'args' type_id=91246 bits_offset=0 > > 'psw' type_id=91243 bits_offset=64 > > 'gprs' type_id=91132 bits_offset=192 > > > > and make them equiv by setting hypot_type_id for 181 to be 91245 > > > > > > --------------------------------------------------------------- > > > > Now comparing the second member: > > > > struct pt_regs > > { > > union { > > user_pt_regs user_regs; > > ---> struct { > > unsigned long args[1]; > > psw_t psw; > > unsigned long gprs[NUM_GPRS]; > > }; > > }; > > > > > > kernel knows it's same struct as user_pt_regs and uses ID 181 > > > > [186] UNION '(anon)' size=152 vlen=2 > > 'user_regs' type_id=183 bits_offset=0 > > '(anon)' type_id=181 bits_offset=0 > > > > but kmod has new ID 91250 (not 91245): > > > > [91251] UNION '(anon)' size=152 vlen=2 > > 'user_regs' type_id=91247 bits_offset=0 > > '(anon)' type_id=91250 bits_offset=0 > > > > > > and 181 and 91250 are equiv structs: > > > > [181] STRUCT '(anon)' size=152 vlen=3 > > 'args' type_id=182 bits_offset=0 > > 'psw' type_id=179 bits_offset=64 > > 'gprs' type_id=48 bits_offset=192 > > > > [91250] STRUCT '(anon)' size=152 vlen=3 > > 'args' type_id=91246 bits_offset=0 > > 'psw' type_id=91243 bits_offset=64 > > 'gprs' type_id=91132 bits_offset=192 > > > > > > now hypot_type_id for 181 is 91245, but we have brand new struct > > ID 91250, so we fail > > > > what the patch tries to do is at this point to compare ID 91250 > > with 91245 and if it passes then we are equal and we throw away > > ID 91250 because the hypot_type_id for 181 stays 91245 > > > > > > ufff.. thoughts? ;-) > > Oh, this is a really great analysis, thanks a lot! It makes everything > clear. Basically, BTF dedup algo does too good job deduping vmlinux > BTF. :) > > What's not clear is what to do about that, because a (current) > fundamental assumption of is_equiv() check is that any type within CU > (or in this case deduped vmlinux BTF) has exactly one unique mapping. > Clearly that's not the case now. That array fix you mentioned worked > around GCC bug where this assumption broke. In this case it's not a > bug of a compiler (neither of algo, really), we just need to make algo > smarter. > > Let me think about this a bit, we'll need to make the equivalence > check be aware that there could be multiple equivalent mappings and be > ok with that as long as all candidates are equivalent between > themselves. Lots of equivalence and recursion to think about. > > It would be great to have a simplified test case to play with that. Do > you mind distilling the chain of types above into a selftests and > posting it to the mailing list so that I can play with it? It > shouldn't be hard to write given BTF writing APIs. And we'll need a > selftests anyway once we improve the algo, so it's definitely not a > wasted work. > > And thanks again for analysis and writing this down, it would take me > ages to get to this otherwise. > > P.S. If the improved BTF dedup algo will be able to handle this, we > should also remove the array workaround, because that one should work > automatically. I don't know if we have a test for duplicate array > scenario, but it's probably good to have that as well. right, I'll try to add test for both thanks for checking on this jirka
