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

Hi Tao Liu, Philipp Thanks for the patch and review.

> > > > > +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; > > > > > > > > What do you need tmp for? The way I see it you only assign to it but > > > > never really use it. > > > > > > > > > > Since the elements arranged by the hash table are singly linked list. > > > If we want to remove a specific element out of the list, we need to update > > > the field which the previous element used to point to the next element. To > > > do that, I keep the address of the previous-element-pointing-to-the-next field > > > into tmp variable, > > > > well yes, but in the only case where you use tmp you have sp == spn. So > > you already have two pointers to the same element and don't need a third > > one to keep the same value. > > > > > You can see it is used in code: *tmp = sp->name_hash_next; > > > > But in that line you only store the value in tmp. Where do read it from > > tmp? I only found this > > > > tmp = &(*tmp)->name_hash_next; > > > > but that again stores the new value in tmp. For the scenario you > > described above I'd expect to have some lines like this > > > > tmp = sp->name_hash_next->name_hash_next; > > sp->name_hash_next = NULL; > > sp = tmp; > > > > I tried to elimitate the "tmp" variable but failed, I will be appreciated if > you can do it for me? > > My thought was, since struct syment is a singly linked list, sp and spn are used > to judge if the element which sp pointing to > should be removed from the list or not. To remove sp from the list, the element > which prior to sp should point to the element which follows sp. So I need a > varible which can always track the element which is prior to sp. The variable > "tmp" achieves that, actually it is the field where the prior element should > be updated when removing sp from the list. As you can see, tmp is not equivalent > to sp and spn. Is that loop replaced with this? maybe missing something. 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; }

(spn will be freed right after this, no need to update?)

> > > > > > + } > > > > > +} > > > > > + > > > > > /* > > > > > * 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 *)) > > > > > > > > this line should be indented to match the open parentheses after the > > > > function name. > > > > > > OK. > > > > > > > > > > > > { > > > > > 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; > > > > > +} > > > > > > > > It took really long for me to wrap my head around what is happening > > > > here but in the end I'm pretty sure that the extra filtering is > > > > unnecessary and can simply be dropped without problem. To be fair > > > > what you are doing seems correct it's just by cleaning up the code the > > > > problem became more obvious... > > > > > > > > > > Me too, hard for me to figure out what's going on here. My thought was don't > > > go too far at one step, for now I just tried to keep it as it was. When > > > the code is stable enough, then get this part optimized... > > > > you are right. Better to make small steps and your change already is a > > big improvement. > > > > > > Let's see what is happening here: > > > > > > > > 1) strstr returns a pointer to the start of the second string if is is > > > > contained in the first one and NULL otherwise. This means 'pseudos' > > > > is true if 's' contains any of the _MODULE_* strings, i.e. if s is a > > > > pseudo symbol. > > > > > > > > 2) MODULE_PSEUDO_SYMBOL does basically the same only that it checks > > > > 'sp->name' instead of 's' and enforces that the "_MODULE_*" strings > > > > are at the beginning of the symbol name not just contained in it. > > > > > > > > Let's look at the different cases > > > > > > > > 3.1) both 's' and 'sp' are proper, i.e. no pseudo, symbols > > > > This means skip_symbols returns false so symname_hash_search > > > > doesn't skip the symbol but compares 's' to 'sp->name' to check if > > > > 'sp' is the symbol you are searching for. This is basically the > > > > case you want. > > > > > > > > 3.2) both 's' and 'sp' are pseudo symbols > > > > Again skip_symbols returns false and symname_hash_search compares > > > > 's' with 'sp->name' to check if 'sp' is the symbol you are > > > > searching for. I'm not entirely sure if this way > > > > symname_hash_search is intended to be used but I also don't see a > > > > reason why it shouldn't be done. > > > > > > > > 3.3) 's' is a pseudo and 'sp' a proper symbol > > > > same like 3.2). > > > > > > > > 3.4) 's' is a proper symbol and 'sp' a psudo symbol > > > > here skip_symbols returns true and symname_hash_search skips this > > > > case. > > > > > > > > So the only case that is filtered out is 3.4 in which 's' must not > > > > contain any '_MODULES_*' while 'sp->name' has to start with one. But > > > > that's something a simple STREQ can handle like in case 3.3. So what's > > > > the point in having this extra filtering? > > > > > > As you pointed out, the only case to skip is 3.4): A) s is not pseudo, and B) sp is psedudo. > > > But the "pseudo" of s is different from the "psedudo" of sp. > > > > > > Let's say "_MODULE_START_", "_MODULE_END_", "_MODULE_INIT_START_", "_MODULE_INIT_END_" > > > are true pseudo symbols. > > > > > > For s is not pseudo, s can be one of "proper symbol" and "_MODULE_SECTION_" symbol. > > > For sp is pseudo, sp can be one of "true pseudo symbol" and "_MODULE_SECTION_" symbol. > > > > > > Since "proper symbol" and "true pseudo symbol" can never be the same, so skip it or not doesn't > > > matter, it cannot pass the STREQ check later. The only case left is _MODULE_SECTION_ symbol. > > > If s and sp are both _MODULE_SECTION_ symbol, even they are equal string, it will be skipped. > > > From my view it is the only use case for the skip. I agree the code should be optimized. > > > > true, I missed the _MODULE_SECTION_ case... although I'm not sure why > > this case should be treated differently to the other _MODULE_* cases... > > > > Me neither, just keep it as it was... > > > > > > /* > > > > > * 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); > > > > > } > > > > > > > > Isn't this function basically identical to symbol_search and thus can > > > > be abbreviated to > > > > > > > > return !!(symbol_search(symbol)); > > > > > > In the original symbol_search, there are 3 stages to find a symbol: > > > 1) search in kernel symbols hash table. > > > 2) iterate over all kernel symbols. > > > 3) iterate over all kernel mods and their symbols. > > > > > > As for symbol_exists, it only do 1) 3) stages. Personally I think stage 2) is > > > unnecessary, but I didn't have a strong reason to remove it. Thus I didn't > > > replace symbol_exists with symbol_search directly. If stage 2) can be removed, > > > then I'm OK with your modification. > > > > you are right. Better wait till case 2) got removed properly. Otherwise > > we might introduce a bug now... > > > > > > > @@ -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; > > > > > > > > same like above. This could be abbreviated to > > > > > > > > return !!(symname_hash_search(st->symname_hash, symbol, NULL)); > > > > > > > > > > Agreed, this one can be replaced this way. > > > > > > > > @@ -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; > > > > > > > > I must admit I don't understand how the last two functions work, so I'm > > > > relying on Kazu to comment on those. > > > > > > The difference of mod symbols and kernel symbols, is that kernel symbols will not change after loaded > > > into hash table, mod symbols can get modified by "mod" cmd. Whenever mod symbols changed, it should > > > be synced to mod symbols hash table. The above changed lines are trying to do that. > > > > Thanks for the explanation. However, my main problem is less what it > > does but more how it does it. > > > > For example in delete_load_module first all symbols from lm->mod_symtab > > are removed. Then lm->mod_symtab is changed to lm->mod_ext_symtab and > > then all symbols are installed again. Why? What's the difference > > between the mod_symtab and mod_ext_symtab? At least when looking at > > store_module_symbols_v{1,2} both should be the same... > > No, lm->mod_symtable and lm->mod_ext_symtable are not always the same. > lm->mod_symtable will be assigned to lm->mod_load_symtable in > symbols.c:store_load_module_symbols. When invoke 'mod -S/-s' in crash, > the modules(.ko) will be read into, the symbols will get refreshed. If 'mod -d' > remove the modules, the symbols will be restored to mod_ext_symtable. > > My understanding is, lm->mod_ext_symtable is read from vmcore, and > lm->mod_load_symtable is read from (.ko) file. Though mostly the symbols > are the same, but we cannot guarantee that... Right, probably if no CONFIG_KALLSYMS, the number of symbols in lm->mod_ext_symtable will be less than lm->mod_load_symbable. Thanks, Kazu