Re: [PATCH v2] Improve the performance of symbol searching for kernel modules

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On Mon, Aug 23, 2021 at 08:24:28AM +0000, HAGIO KAZUHITO(萩尾 一仁) wrote:
> Hi Tao Liu,
> 
> -----Original Message-----
> > Currently the sequence for crash searching 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)
> 
> Just to clarify, which parts of code are meant by the stage 1 and 2?
> 
> Do you have a result with the patch?
> 
> > 
> > stage 3 takes too much time when comparing to stage 1. So let's introduce a
> > symname hash table for kernel modules to improve the performance of symbol
> > searching.
> 
> I see that 3) is relatively time-consuming, but I have not had a delay
> due to the symbol search.  Which command did you observe a delay?

Hello Kazu,

The target function is symbols.c:symbol_search. The code involved in the 3 stages are:

1) sp_hashed = symname_hash_search(s);
2) for (sp = sp_hashed ? sp_hashed : st->symtable; sp < st->symend; sp++) {
                 if (STREQ(s, sp->name)) 
                         return(sp);
3) for (i = 0; i < st->mods_installed; i++) {
                 lm = &st->load_modules[i];
                 ....
   }

I measured each stage's time by adding the following code to symbol_search:

        struct load_module *lm;
        int pseudos, search_init;
+       double T1, T2, T3;
+       clock_t start, end;
 
+       start = clock();
        sp_hashed = symname_hash_search(s);
+       end = clock();
+       T1 = (double)(end - start) * 1000.0 / CLOCKS_PER_SEC;
 
+       start = clock();
         for (sp = sp_hashed ? sp_hashed : st->symtable; sp < st->symend; sp++) {
                 if (STREQ(s, sp->name)) 
                         return(sp);
         }
+        end = clock();
+        T2 = (double)(end - start) * 1000.0 / CLOCKS_PER_SEC;
 
        pseudos = (strstr(s, "_MODULE_START_") || strstr(s, "_MODULE_END_"));
        search_init = FALSE;
 
+       start = clock();
         for (i = 0; i < st->mods_installed; i++) {
                 lm = &st->load_modules[i];
                if (lm->mod_flags & MOD_INIT)
         ....
                                return(sp);
                 }
         }
+        end = clock();
+        T3 = (double)(end - start) * 1000.0 / CLOCKS_PER_SEC;
+       printf("%lf %lf %lf\n", T1, T2, T3);
 
        if (!search_init)
                return((struct syment *)NULL);

crash> sym blah
T1:0.008000 T2:0.562000 T3:2.435000
symbol not found: blah
possible alternatives:
  (none found)

With the v2 patch applied and the time measurement code added:

crash> sym blah
T1:0.003000 T2:0.545000 T3:0.017000
symbol not found: blah
possible alternatives:
  (none found)

We can see T3 performs better.

Thanks,
Tao Liu

> 
> Thanks,
> Kazu
> 
> 
> > 
> > Signed-off-by: Tao Liu <ltao@xxxxxxxxxx>
> > ---
> > v1 -> v2:
> > - Removed unused variables within the modified functions.
> > 
> > ---
> >  defs.h    |   1 +
> >  kernel.c  |   1 +
> >  symbols.c | 189 +++++++++++++++++++++++++++++++-----------------------
> >  3 files changed, 111 insertions(+), 80 deletions(-)
> > 
> > diff --git a/defs.h b/defs.h
> > index eb1c71b..58b8546 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 36fdea2..c56cc34 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..9b64734 100644
> > --- a/symbols.c
> > +++ b/symbols.c
> > @@ -64,8 +64,9 @@ 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 +1120,7 @@ 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 +1128,48 @@ 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++) {
> > +		if (sp != NULL) {
> > +			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++) {
> > +		if (sp != NULL) {
> > +			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;
> > 
> >          index = SYMNAME_HASH_INDEX(spn->name);
> > +	index = index > 0 ? index : -index;
> > +
> >  	spn->cnt = 1;
> > 
> > -        if ((sp = st->symname_hash[index]) == NULL)
> > -        	st->symname_hash[index] = spn;
> > -	else {
> > +        if ((sp = table[index]) == NULL) {
> > +		table[index] = spn;
> > +		spn->name_hash_next = NULL;
> > +	} else {
> >  		while (sp) {
> >  	        	if (STREQ(sp->name, spn->name)) {
> >  	                	sp->cnt++;
> > @@ -1151,23 +1179,67 @@ symname_hash_install(struct syment *spn)
> >  				sp = sp->name_hash_next;
> >  			else {
> >  				sp->name_hash_next = spn;
> > +				spn->name_hash_next = NULL;
> >  				break;
> >  			}
> >  		}
> >  	}
> >  }
> > 
> > +static void
> > +symname_hash_remove(struct syment *table[], struct syment *spn)
> > +{
> > +	struct syment *sp, **tmp;
> > +        int index, first_encounter = 1;
> > +
> > +        index = SYMNAME_HASH_INDEX(spn->name);
> > +	index = index > 0 ? index : -index;
> > +
> > +        if ((sp = table[index]) == NULL)
> > +		return;
> > +
> > +	for (tmp = &table[index], sp = table[index]; sp; ) {
> > +		if (STREQ(sp->name, spn->name)) {
> > +			if (sp != spn) {
> > +				sp->cnt--;
> > +				spn->cnt--;
> > +			} else if (!first_encounter) {
> > +				sp->cnt--;
> > +			} else {
> > +				*tmp = sp->name_hash_next;
> > +				first_encounter = 0;
> > +
> > +				tmp = &(*tmp)->name_hash_next;
> > +				sp = sp->name_hash_next;
> > +				spn->name_hash_next = NULL;
> > +				continue;
> > +			}
> > +		}
> > +		tmp = &sp->name_hash_next;
> > +		sp = sp->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);
> > +	index = index > 0 ? index : -index;
> > +        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 +1667,7 @@ 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 +2148,8 @@ 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 +4557,16 @@ 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 +4574,16 @@ 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 +5475,13 @@ value_symbol(ulong value)
> >  int
> >  symbol_exists(char *symbol)
> >  {
> > -	int i;
> > -        struct syment *sp, *sp_end;
> > -	struct load_module *lm;
> > +        struct syment *sp;
> > 
> > -	if ((sp = symname_hash_search(symbol)))
> > +	if ((sp = 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 ((sp = symname_hash_search(st->mod_symname_hash, symbol, NULL)))
> > +		return TRUE;
> > 
> >          return(FALSE);
> >  }
> > @@ -5515,7 +5538,7 @@ kernel_symbol_exists(char *symbol)
> >  {
> >  	struct syment *sp;
> > 
> > -        if ((sp = symname_hash_search(symbol)))
> > +        if ((sp = symname_hash_search(st->symname_hash, symbol, NULL)))
> >                  return TRUE;
> >  	else
> >          	return FALSE;
> > @@ -5527,7 +5550,7 @@ 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 +12487,10 @@ 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 +12520,7 @@ 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 +12530,7 @@ 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 +12559,7 @@ 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 +12569,7 @@ 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
> > 
> > --
> > Crash-utility mailing list
> > Crash-utility@xxxxxxxxxx
> > https://listman.redhat.com/mailman/listinfo/crash-utility
> 
> 
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