Re: [PATCH] modpost: Optimize symbol search from linear to binary search

[Fangrui Song]

On 2023-09-23, Masahiro Yamada wrote:
> On Tue, Sep 19, 2023 at 6:06 AM Jack Brennen <jbrennen@xxxxxxxxxx> wrote:
> > Modify modpost to use binary search for converting addresses back
> > into symbol references.  Previously it used linear search.
> >
> > This change saves a few seconds of wall time for defconfig builds,
> > but can save several minutes on allyesconfigs.
>
> Thanks.
> Binary search is a good idea.
>
>
> > Before:
> > $ make LLVM=1 -j128 allyesconfig vmlinux -s KCFLAGS="-Wno-error"
> >         Elapsed (wall clock) time (h:mm:ss or m:ss): 13:30.31
>
> Instead of the time for the entire build,
> can you put the time for the modpost command?
>
> If you allyesconfig case,
>
> $ time scripts/mod/modpost -M -m -a -N -o vmlinux.symvers vmlinux.o
>
>
>
>
>
> > diff --git a/scripts/mod/symsearch.c b/scripts/mod/symsearch.c
> > new file mode 100644
> > index 000000000000..aab79262512b
> > --- /dev/null
> > +++ b/scripts/mod/symsearch.c
> > @@ -0,0 +1,233 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +/* Helper functions for finding the symbol in an ELF which is "nearest"
> > + * to a given address.
> > + */
>
> Can you use the following block comment style?
>
> /*
> * Helper functions for finding the symbol in an ELF which is "nearest"
> * to a given address.
> */
>
>
>
> > +#include "modpost.h"
> > +
> > +/* Struct used for binary search. */
>
> I think this obvious comment is unneeded.
>
>
>
> > +struct syminfo {
> > +       unsigned int symbol_index;
> > +       unsigned int section_index;
> > +       Elf_Addr addr;
> > +};
> > +
> > +/* Container used to hold an entire binary search table.
> > + * Entries in table are ascending, sorted first by section_index,
> > + * then by addr, and last by symbol_index.  The sorting by
> > + * symbol_index is used to duplicate the quirks of the prior
> > + * find_nearest_sym() function, where exact matches to an address
> > + * return the first symtab entry seen, but near misses return the
> > + * last symtab entry seen.
>
> Preserving this quirk makes the code complicated.
>
> I do not mind changing the behavior of the corner case.
>
>
>
>
>
> > + * The first and last entries of the table are sentinels and their
> > + * values only matter in two places:  when we sort the table, and
> > + * on lookups, the end sentinel should not have an addr field which
> > + * matches its immediate predecessor.  To meet these requirements,
> > + * we initialize them to (0,0,0) and (max,max,max), and then after
> > + * sorting, we tweak the end sentinel's addr field accordingly.
> > + */
> > +struct symsearch {
> > +       size_t table_size;
> > +       struct syminfo table[];
> > +};
>
>
>
> syminfo::symbol_index is unsigned int.
> symsearch::table_size is size_t.
>
>
> symbol_index of the last element is always larger than
> elf->symsearch->table_size.
>
> So, the code works only within 32-bit width anyway.
>
>
>
>
>
>
>
>
>
>
>
>
> > +
> > +static inline bool is_sym_searchable(struct elf_info *elf, Elf_Sym *sym)
> > +{
> > +       return is_valid_name(elf, sym) != 0;
> > +}
>
> If you call is_valid_name() directly, this function was unneeded?
>
>
>
>
>
>
> > +
> > +static int syminfo_compare(const void *s1, const void *s2)
> > +{
> > +       const struct syminfo *sym1 = s1;
> > +       const struct syminfo *sym2 = s2;
> > +
> > +       if (sym1->section_index > sym2->section_index)
> > +               return 1;
> > +       if (sym1->section_index < sym2->section_index)
> > +               return -1;
> > +       if (sym1->addr > sym2->addr)
> > +               return 1;
> > +       if (sym1->addr < sym2->addr)
> > +               return -1;
> > +       if (sym1->symbol_index > sym2->symbol_index)
> > +               return 1;
> > +       if (sym1->symbol_index < sym2->symbol_index)
> > +               return -1;
> > +       return 0;
> > +}
> > +
> > +static size_t symbol_count(struct elf_info *elf)
> > +{
> > +       size_t result = 0;
> > +
> > +       for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
> > +               if (is_sym_searchable(elf, sym))
> > +                       result++;
> > +       }
> > +       return result;
> > +}
> > +
> > +/* Populate the search array that we just allocated.
> > + * Be slightly paranoid here.  If the ELF file changes during processing,
>
> I could not understand. In which case, the ELF file changes?
>
> modpost loads the entire file to memory first..
>
> In which scenario, the memory content changes?
>
>
>
>
>
>
> > + * or if the behavior of is_sym_searchable() changes during processing,
> > + * we want to catch it; neither of those is acceptable.
> > + */
> > +static void symsearch_populate(struct elf_info *elf,
> > +                              struct syminfo *table,
> > +                              size_t table_size)
> > +{
> > +       bool is_arm = (elf->hdr->e_machine == EM_ARM);
> > +
> > +       /* Start sentinel */
> > +       if (table_size-- == 0)
> > +               fatal("%s: size mismatch\n", __func__);
> > +       table->symbol_index = 0;
> > +       table->section_index = 0;
> > +       table->addr = 0;
> > +       table++;
> > +
> > +       for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
> > +               if (is_sym_searchable(elf, sym)) {
> > +                       if (table_size-- == 0)
> > +                               fatal("%s: size mismatch\n", __func__);
> > +                       table->symbol_index = sym - elf->symtab_start;
> > +                       table->section_index = get_secindex(elf, sym);
> > +                       table->addr = sym->st_value;
> > +
> > +                       /*
> > +                        * For ARM Thumb instruction, the bit 0 of st_value is
> > +                        * set if the symbol is STT_FUNC type. Mask it to get
> > +                        * the address.
> > +                        */
> > +                       if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC)
> > +                               table->addr &= ~1;
> > +
> > +                       table++;
> > +               }
> > +       }
> > +
> > +       /* End sentinel; all values are unsigned so -1 wraps to max */
> > +       if (table_size != 1)
> > +               fatal("%s: size mismatch\n", __func__);
> > +       table->symbol_index = -1;
> > +       table->section_index = -1;
> > +       table->addr = -1;
> > +}
> > +
> > +void symsearch_init(struct elf_info *elf)
> > +{
> > +       /* +2 here to allocate space for the start and end sentinels */
> > +       size_t table_size = symbol_count(elf) + 2;
> > +
> > +       elf->symsearch = NOFAIL(malloc(
> > +                                       sizeof(struct symsearch) +
> > +                                       sizeof(struct syminfo) * table_size));
> > +       elf->symsearch->table_size = table_size;
> > +
> > +       symsearch_populate(elf, elf->symsearch->table, table_size);
> > +       qsort(elf->symsearch->table, table_size,
> > +             sizeof(struct syminfo), syminfo_compare);
> > +
> > +       /* A bit of paranoia; make sure that the end sentinel's address is
> > +        * different than its predecessor.  Not doing this could cause
> > +        * possible undefined behavior if anybody ever inserts a symbol
> > +        * with section_index and addr both at their max values.
>
> I could not understand this comment.
>
> If section_index and addr both at their max values at [table_size - 2],
> ->table[table_size - 2].addr + 1 wraps to zero.
>
> The table is not sorted any longer?
>
>
>
>
> > +        * Doing this little bit of defensive programming is more efficient
> > +        * than checking for array overruns later.
> > +        */
> > +       elf->symsearch->table[table_size - 1].addr =
> > +               elf->symsearch->table[table_size - 2].addr + 1;
> > +}
> > +
> > +void symsearch_finish(struct elf_info *elf)
> > +{
> > +       free(elf->symsearch);
> > +       elf->symsearch = NULL;
> > +}
> > +
> > +/* Find the syminfo which is in secndx and "nearest" to addr.
> > + * allow_negative: allow returning a symbol whose address is > addr.
> > + * min_distance: ignore symbols which are further away than this.
> > + *
> > + * Returns a nonzero index into the symsearch table for success.
> > + * Returns NULL if no legal symbol is found within the requested range.
> > + */
> > +static size_t symsearch_find_impl(struct elf_info *elf, Elf_Addr addr,
> > +                                 unsigned int secndx, bool allow_negative,
> > +                                 Elf_Addr min_distance)
> > +{
> > +       /* Find the target in the array; it will lie between two elements.
> > +        * Invariant here: table[lo] < target <= table[hi]
> > +        * For the purposes of search, exact hits in the search array are
> > +        * considered greater than the target.  This means that if we do
> > +        * get an exact hit, then once the search terminates, table[hi]
> > +        * will be the exact match which has the lowest symbol index.
> > +        */
> > +       struct syminfo *table = elf->symsearch->table;
> > +       size_t hi = elf->symsearch->table_size - 1;
> > +       size_t lo = 0;
>
>
>
>
> The binary search code was implemented in a too complex way
> to preserve the previous quirks.
>
>
> I want to use the same comparison function for
> qsort() and bsearch() to avoid paranoia.
>
>
>
>
> How about this implementation?
>
>
>
> static struct syminfo *symsearch_find_impl(struct elf_info *elf, Elf_Addr addr,
>                                           unsigned int secndx, bool
> allow_negative,
>                                           Elf_Addr min_distance)
> {
>        struct syminfo target = { .symbol_index = -1, .section_index =
> secndx, .addr = addr };
>        struct syminfo *table = elf->symsearch->table;
>        unsigned int hi = elf->symsearch->table_size - 1;
>        unsigned int lo = 0;
>        struct syminfo *result = NULL;
>        Elf_Addr distance;
>
>        while (lo < hi) {
>                unsigned int mid = (lo + hi + 1) / 2;
>
>                if (syminfo_compare(&table[mid], &target) > 0)
>                        hi = mid - 1;
>                else
>                        lo = mid;
>        }
>
>        /*
>         * The target resides between lo and (lo + 1).
>         * If allow_negative is true, check both of them.
>         */
>
>        if (allow_negative && lo + 1 < elf->symsearch->table_size &&
>            table[lo + 1].section_index == secndx) {
>                distance = table[lo + 1].addr - addr;
>                if (distance <= min_distance) {
>                        min_distance = distance;
>                        result = &table[lo + 1];
>                }
>        }
>
>        if (table[lo].section_index == secndx) {
>                distance = addr - table[lo].addr;
>                if (distance <= min_distance)
>                  result = &table[lo];
>        }
>
>        return result;
> }

I think this implementation (shrinking [lo,hi] to [lo,mid-1] or
[mid,hi]) is better than the original one (shrinking [lo,hi] to [lo,mid]
or [mid,hi], a bit wasteful).

The original patch uses `if (mid == lo) break;`, which I consider not so
elegant.

However, the `- 1` part in `unsigned int hi = elf->symsearch->table_size - 1;` can be improved.
I'd prefer an implementation similar to typical C++ https://en.cppreference.com/w/cpp/algorithm/upper_bound implementation.

lo = 0;
hi = n;  // or replace hi with count
while (lo < hi) {
  mid = (lo + hi) / 2;  // we don't care about (lo+hi) overflow
  if (less_or_eq(&table[mid], &target))
    lo = mid+1;
  else
    hi = mid;
}

// lo == hi: the index of the first element that is > target
// if elements equal to target are present, they are on the left of lo

> Elf_Sym *symsearch_find_nearest(struct elf_info *elf, Elf_Addr addr,
>                                unsigned int secndx, bool allow_negative,
>                                Elf_Addr min_distance)
> {
>        struct syminfo *result;
>
>        result = symsearch_find_impl(elf, addr, secndx,
>                                     allow_negative, min_distance);
>        if (!result)
>                return NULL;
>
>        return &elf->symtab_start[result->symbol_index];
> }
>
>
>
> This does not preserve the previous quirks.
>
> If there are multiple entries with the same address,
> it always returns the last element.
>
> I did not expect sentinels.
>
> I did not do thorough tests, but it seems to be working for me.
>
>
>
>
> Also, please call symsearch_find_nearest() directly
> and remove symfind_nearest_sym().
>
>
>
>
>
>
> --
> Best Regards
>
> Masahiro Yamada