Re: [PATCH v4 1/4] Improve the performance of symbol searching for kernel modules

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Hi Kazu,

Sorry for the late reply, it was a public holiday for the past few days...

On Tue, Oct 5, 2021 at 4:51 PM HAGIO KAZUHITO(萩尾 一仁) <k-hagio-ab@xxxxxxx> wrote:
>
> Hi Tao,
>
> Found another issue.
>
> The first symbol of duplicated ones will not be shown by sym command
> after mod -s.
>
> # crash -s
> crash> sym cleanup_module | head -n 1
> ffffffffc02527ff (t) cleanup_module [fuse]
> crash> mod -s fuse
>      MODULE       NAME                         BASE           SIZE  OBJECT FILE
> ffffffffc025eac0  fuse                   ffffffffc023f000   151552  /usr/lib/debug/lib/modules/4.18.0-305.el8.x86_64/kernel/fs/fuse/fuse.ko.debug
> crash> sym cleanup_module | grep fuse
>
> It looks like symbol_search() has to return the lowest address symbol and
> symbol_search_next() returns the next lowest symbol, so removing/installing
> module symbols from/to the hash table breaks this.
>
> The patch will need sorted installation or other solution.
>

Agreed, I haven't considered this case. Yes, a sorting installation is
necessary.

> Also, I've not looked at this enough, but kallsyms_module_symbol() changes
> spx->cnt from lm->mod_ext_symtable.  Is there no interference with the
> patch?
>

No, this patch didn't interfere with it. Actually I didn't notice
kallsyms_module_symbol() also changed spx->cnt, so the patch
definitely needs further improvement. I will think of a better
solution for handling sp->cnt field, please wait for a while before I
send the v5 patch.

Thanks,
Tao Liu


> Thanks,
> Kazu
>
>
> -----Original Message-----
> > Currently the sequence for symbol_search to search a symbol is: 1) kernel
> > symname hash table, 2) iterate all kernel symbols, 3) iterate all kernel
> > modules and their symbols. In the worst case, if a non-exist symbol been
> > searched, all 3 stages will be went through. The time consuming status for
> > each stage is like:
> >
> >     stage 1         stage 2         stage 3
> >     0.007000(ms)    0.593000(ms)    2.421000(ms)
> >
> > stage 3 takes too much time when comparing to stage 1. This patch introduces
> > a symname hash table for kernel modules, to improve the performance of symbol
> > searching.
> >
> > Functions symbol_search and symbol_exists are fundamental and widely used by
> > other crash functions, thus the benefit of performance improvement can
> > get accumulated. For example, "ps -m" and "irq" commands, which call
> > the functions many times, will become faster with the patch.
> >
> > Signed-off-by: Tao Liu <ltao@xxxxxxxxxx>
> > Reviewed-by: Philipp Rudo <prudo@xxxxxxxxxx>
> > ---
> >  defs.h    |   1 +
> >  kernel.c  |   1 +
> >  symbols.c | 176 ++++++++++++++++++++++++++++--------------------------
> >  3 files changed, 92 insertions(+), 86 deletions(-)
> >
> > diff --git a/defs.h b/defs.h
> > index eb1c71b..c10ebff 100644
> > --- a/defs.h
> > +++ b/defs.h
> > @@ -2751,6 +2751,7 @@ struct symbol_table_data {
> >          double val_hash_searches;
> >          double val_hash_iterations;
> >          struct syment *symname_hash[SYMNAME_HASH];
> > +     struct syment *mod_symname_hash[SYMNAME_HASH];
> >       struct symbol_namespace kernel_namespace;
> >       struct syment *ext_module_symtable;
> >       struct syment *ext_module_symend;
> > diff --git a/kernel.c b/kernel.c
> > index b2c8a0c..307eeee 100644
> > --- a/kernel.c
> > +++ b/kernel.c
> > @@ -4663,6 +4663,7 @@ reinit_modules(void)
> >          kt->mods_installed = 0;
> >       clear_text_value_cache();
> >
> > +     memset(st->mod_symname_hash, 0, sizeof(st->mod_symname_hash));
> >          module_init();
> >  }
> >
> > diff --git a/symbols.c b/symbols.c
> > index bf6d94d..7ab47f2 100644
> > --- a/symbols.c
> > +++ b/symbols.c
> > @@ -64,8 +64,10 @@ static int namespace_ctl(int, struct symbol_namespace *, void *, void *);
> >  static void symval_hash_init(void);
> >  static struct syment *symval_hash_search(ulong);
> >  static void symname_hash_init(void);
> > -static void symname_hash_install(struct syment *);
> > -static struct syment *symname_hash_search(char *);
> > +static void symname_hash_install(struct syment *[], struct syment *);
> > +static void symname_hash_remove(struct syment *[], struct syment *);
> > +static struct syment *symname_hash_search(struct syment *[], char *,
> > +                                       int (*)(struct syment *, char *));
> >  static void gnu_qsort(bfd *, void *, long, unsigned int, asymbol *, asymbol *);
> >  static int check_gnu_debuglink(bfd *);
> >  static int separate_debug_file_exists(const char *, unsigned long, int *);
> > @@ -1119,7 +1121,7 @@ symname_hash_init(void)
> >          struct syment *sp;
> >
> >          for (sp = st->symtable; sp < st->symend; sp++)
> > -             symname_hash_install(sp);
> > +             symname_hash_install(st->symname_hash, sp);
> >
> >       if ((sp = symbol_search("__per_cpu_start")))
> >               st->__per_cpu_start = sp->value;
> > @@ -1127,21 +1129,42 @@ symname_hash_init(void)
> >               st->__per_cpu_end = sp->value;
> >  }
> >
> > +static void
> > +mod_symtable_hash_install_range(struct syment *from, struct syment *to)
> > +{
> > +     struct syment *sp;
> > +
> > +     for (sp = from; sp <= to; sp++)
> > +             symname_hash_install(st->mod_symname_hash, sp);
> > +}
> > +
> > +static void
> > +mod_symtable_hash_remove_range(struct syment *from, struct syment *to)
> > +{
> > +     struct syment *sp;
> > +
> > +     for (sp = from; sp <= to; sp++)
> > +             symname_hash_remove(st->mod_symname_hash, sp);
> > +}
> > +
> >  /*
> >   *  Install a single static kernel symbol into the symname_hash.
> >   */
> >  static void
> > -symname_hash_install(struct syment *spn)
> > +symname_hash_install(struct syment *table[], struct syment *spn)
> >  {
> >       struct syment *sp;
> >          int index;
> >
> > +     if (!spn)
> > +             return;
> > +
> >          index = SYMNAME_HASH_INDEX(spn->name);
> >       spn->cnt = 1;
> >
> > -        if ((sp = st->symname_hash[index]) == NULL)
> > -             st->symname_hash[index] = spn;
> > -     else {
> > +     if ((sp = table[index]) == NULL) {
> > +             table[index] = spn;
> > +     } else {
> >               while (sp) {
> >                       if (STREQ(sp->name, spn->name)) {
> >                               sp->cnt++;
> > @@ -1157,17 +1180,47 @@ symname_hash_install(struct syment *spn)
> >       }
> >  }
> >
> > +static void
> > +symname_hash_remove(struct syment *table[], struct syment *spn)
> > +{
> > +     struct syment *sp;
> > +     int index;
> > +
> > +     if (!spn)
> > +             return;
> > +
> > +     index = SYMNAME_HASH_INDEX(spn->name);
> > +
> > +     if (table[index] == spn)
> > +             table[index] = spn->name_hash_next;
> > +
> > +     for (sp = table[index]; sp; sp = sp->name_hash_next) {
> > +             if (STREQ(sp->name, spn->name))
> > +                     sp->cnt--;
> > +             if (sp->name_hash_next == spn)
> > +                     sp->name_hash_next = spn->name_hash_next;
> > +     }
> > +}
> > +
> >  /*
> >   *  Static kernel symbol value search
> >   */
> >  static struct syment *
> > -symname_hash_search(char *name)
> > +symname_hash_search(struct syment *table[], char *name,
> > +                 int (*skip_condition)(struct syment *, char *))
> >  {
> >       struct syment *sp;
> > +     int index;
> >
> > -        sp = st->symname_hash[SYMNAME_HASH_INDEX(name)];
> > +     index = SYMNAME_HASH_INDEX(name);
> > +     sp = table[index];
> >
> >       while (sp) {
> > +             if (skip_condition && skip_condition(sp, name)) {
> > +                     sp = sp->name_hash_next;
> > +                     continue;
> > +             }
> > +
> >               if (STREQ(sp->name, name))
> >                       return sp;
> >               sp = sp->name_hash_next;
> > @@ -1595,6 +1648,7 @@ store_module_symbols_v1(ulong total, int mods_installed)
> >                               lm->mod_symend = sp;
> >                       }
> >               }
> > +             mod_symtable_hash_install_range(lm->mod_symtable, lm->mod_symend);
> >       }
> >
> >       st->flags |= MODULE_SYMS;
> > @@ -2075,6 +2129,8 @@ store_module_symbols_v2(ulong total, int mods_installed)
> >                               lm->mod_init_symend = sp;
> >                       }
> >               }
> > +             mod_symtable_hash_install_range(lm->mod_symtable, lm->mod_symend);
> > +             mod_symtable_hash_install_range(lm->mod_init_symtable, lm->mod_init_symend);
> >       }
> >
> >       st->flags |= MODULE_SYMS;
> > @@ -4482,6 +4538,17 @@ symbol_query(char *s, char *print_pad, struct syment **spp)
> >       return(cnt);
> >  }
> >
> > +static int
> > +skip_symbols(struct syment *sp, char *s)
> > +{
> > +     int pseudos, skip = 0;
> > +
> > +     pseudos = (strstr(s, "_MODULE_START_") || strstr(s, "_MODULE_END_") ||
> > +             strstr(s, "_MODULE_INIT_START_") || strstr(s, "_MODULE_INIT_END_"));
> > +     if (!pseudos && MODULE_PSEUDO_SYMBOL(sp))
> > +             skip = 1;
> > +     return skip;
> > +}
> >
> >  /*
> >   *  Return the syment of a symbol.
> > @@ -4489,58 +4556,16 @@ symbol_query(char *s, char *print_pad, struct syment **spp)
> >  struct syment *
> >  symbol_search(char *s)
> >  {
> > -     int i;
> > -        struct syment *sp_hashed, *sp, *sp_end;
> > -     struct load_module *lm;
> > -     int pseudos, search_init;
> > +     struct syment *sp_hashed, *sp;
> >
> > -     sp_hashed = symname_hash_search(s);
> > +     sp_hashed = symname_hash_search(st->symname_hash, s, NULL);
> >
> >          for (sp = sp_hashed ? sp_hashed : st->symtable; sp < st->symend; sp++) {
> >                  if (STREQ(s, sp->name))
> >                          return(sp);
> >          }
> >
> > -     pseudos = (strstr(s, "_MODULE_START_") || strstr(s, "_MODULE_END_"));
> > -     search_init = FALSE;
> > -
> > -        for (i = 0; i < st->mods_installed; i++) {
> > -                lm = &st->load_modules[i];
> > -             if (lm->mod_flags & MOD_INIT)
> > -                     search_init = TRUE;
> > -             sp = lm->mod_symtable;
> > -                sp_end = lm->mod_symend;
> > -
> > -                for ( ; sp <= sp_end; sp++) {
> > -                     if (!pseudos && MODULE_PSEUDO_SYMBOL(sp))
> > -                             continue;
> > -                     if (STREQ(s, sp->name))
> > -                             return(sp);
> > -                }
> > -        }
> > -
> > -     if (!search_init)
> > -             return((struct syment *)NULL);
> > -
> > -     pseudos = (strstr(s, "_MODULE_INIT_START_") || strstr(s, "_MODULE_INIT_END_"));
> > -
> > -     for (i = 0; i < st->mods_installed; i++) {
> > -             lm = &st->load_modules[i];
> > -             if (!lm->mod_init_symtable)
> > -                     continue;
> > -             sp = lm->mod_init_symtable;
> > -             sp_end = lm->mod_init_symend;
> > -
> > -             for ( ; sp < sp_end; sp++) {
> > -                     if (!pseudos && MODULE_PSEUDO_SYMBOL(sp))
> > -                             continue;
> > -
> > -                     if (STREQ(s, sp->name))
> > -                             return(sp);
> > -             }
> > -     }
> > -
> > -        return((struct syment *)NULL);
> > +     return symname_hash_search(st->mod_symname_hash, s, skip_symbols);
> >  }
> >
> >  /*
> > @@ -5432,33 +5457,11 @@ value_symbol(ulong value)
> >  int
> >  symbol_exists(char *symbol)
> >  {
> > -     int i;
> > -        struct syment *sp, *sp_end;
> > -     struct load_module *lm;
> > -
> > -     if ((sp = symname_hash_search(symbol)))
> > +     if (symname_hash_search(st->symname_hash, symbol, NULL))
> >               return TRUE;
> >
> > -        for (i = 0; i < st->mods_installed; i++) {
> > -                lm = &st->load_modules[i];
> > -             sp = lm->mod_symtable;
> > -                sp_end = lm->mod_symend;
> > -
> > -                for ( ; sp < sp_end; sp++) {
> > -                     if (STREQ(symbol, sp->name))
> > -                             return(TRUE);
> > -                }
> > -
> > -             if (lm->mod_init_symtable) {
> > -                     sp = lm->mod_init_symtable;
> > -                     sp_end = lm->mod_init_symend;
> > -
> > -                     for ( ; sp < sp_end; sp++) {
> > -                             if (STREQ(symbol, sp->name))
> > -                                     return(TRUE);
> > -                     }
> > -             }
> > -     }
> > +     if (symname_hash_search(st->mod_symname_hash, symbol, NULL))
> > +             return TRUE;
> >
> >          return(FALSE);
> >  }
> > @@ -5513,12 +5516,7 @@ per_cpu_symbol_search(char *symbol)
> >  int
> >  kernel_symbol_exists(char *symbol)
> >  {
> > -     struct syment *sp;
> > -
> > -        if ((sp = symname_hash_search(symbol)))
> > -                return TRUE;
> > -     else
> > -             return FALSE;
> > +     return !!(symname_hash_search(st->symname_hash, symbol, NULL));
> >  }
> >
> >  /*
> > @@ -5527,7 +5525,7 @@ kernel_symbol_exists(char *symbol)
> >  struct syment *
> >  kernel_symbol_search(char *symbol)
> >  {
> > -     return symname_hash_search(symbol);
> > +     return symname_hash_search(st->symname_hash, symbol, NULL);
> >  }
> >
> >  /*
> > @@ -12464,8 +12462,10 @@ store_load_module_symbols(bfd *bfd, int dynamic, void *minisyms,
> >               error(INFO, "%s: last symbol: %s is not _MODULE_END_%s?\n",
> >                       lm->mod_name, lm->mod_load_symend->name, lm->mod_name);
> >
> > +     mod_symtable_hash_remove_range(lm->mod_symtable, lm->mod_symend);
> >          lm->mod_symtable = lm->mod_load_symtable;
> >          lm->mod_symend = lm->mod_load_symend;
> > +     mod_symtable_hash_install_range(lm->mod_symtable, lm->mod_symend);
> >
> >       lm->mod_flags &= ~MOD_EXT_SYMS;
> >       lm->mod_flags |= MOD_LOAD_SYMS;
> > @@ -12495,6 +12495,7 @@ delete_load_module(ulong base_addr)
> >                               req->name = lm->mod_namelist;
> >                               gdb_interface(req);
> >                       }
> > +                     mod_symtable_hash_remove_range(lm->mod_symtable, lm->mod_symend);
> >                       if (lm->mod_load_symtable) {
> >                               free(lm->mod_load_symtable);
> >                                  namespace_ctl(NAMESPACE_FREE,
> > @@ -12504,6 +12505,7 @@ delete_load_module(ulong base_addr)
> >                               unlink_module(lm);
> >                       lm->mod_symtable = lm->mod_ext_symtable;
> >                       lm->mod_symend = lm->mod_ext_symend;
> > +                     mod_symtable_hash_install_range(lm->mod_symtable, lm->mod_symend);
> >                       lm->mod_flags &= ~(MOD_LOAD_SYMS|MOD_REMOTE|MOD_NOPATCH);
> >                       lm->mod_flags |= MOD_EXT_SYMS;
> >                       lm->mod_load_symtable = NULL;
> > @@ -12532,6 +12534,7 @@ delete_load_module(ulong base_addr)
> >                               req->name = lm->mod_namelist;
> >                               gdb_interface(req);
> >                       }
> > +                     mod_symtable_hash_remove_range(lm->mod_symtable, lm->mod_symend);
> >                       if (lm->mod_load_symtable) {
> >                               free(lm->mod_load_symtable);
> >                               namespace_ctl(NAMESPACE_FREE,
> > @@ -12541,6 +12544,7 @@ delete_load_module(ulong base_addr)
> >                               unlink_module(lm);
> >                       lm->mod_symtable = lm->mod_ext_symtable;
> >                       lm->mod_symend = lm->mod_ext_symend;
> > +                     mod_symtable_hash_install_range(lm->mod_symtable, lm->mod_symend);
> >                          lm->mod_flags &= ~(MOD_LOAD_SYMS|MOD_REMOTE|MOD_NOPATCH);
> >                          lm->mod_flags |= MOD_EXT_SYMS;
> >                          lm->mod_load_symtable = NULL;
> > --
> > 2.29.2
>


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