Messages in this thread | | | Date | Thu, 26 Feb 2015 10:28:17 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC][PATCH] module: Optimize __module_address() using a latched RB-tree |
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On Thu, Feb 26, 2015 at 05:43:56PM +0100, Peter Zijlstra wrote: > On Thu, Feb 26, 2015 at 04:02:43PM +0000, Mathieu Desnoyers wrote: > > Perhaps you could use mod_value() below, and introduce a > > "mod_size()" too. This would keep the init vs core selection > > out of the traversal code. > > Indeed! > > > Is it customary to define static variables in the > > middle of a C file ? > > Dunno, it seemed like a good a place as any. > > > The rest looks good, especially for use of the latch. > > I'd be tempted to turn "0, 1, 2, 3" into an enum though, > > so we can follow in the code what each of those array > > entry really means. > > Agreed. > > --- > Subject: module: Optimize __module_address() using a latched RB-tree > From: Peter Zijlstra <peterz@infradead.org> > Date: Thu Feb 26 10:57:34 CET 2015 > > Currently __module_address() is using a linear search through all > modules in order to find the module corresponding to the provided > address. With a lot of modules this can take a lot of time. > > One of the users of this is __kernel_text_address() which is employed > in many stack unwinders; which in turn are used by perf-callchain and > ftrace (possibly from NMI context). > > So by optimizing __module_address() we optimize many stack unwinders > which are used by both perf and tracing in performance sensitive code. > > Cc: Oleg Nesterov <oleg@redhat.com> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > Cc: Rusty Russell <rusty@rustcorp.com.au> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Color me confused, both by the existing code and the modifications.
It appears that you are using seqlock to force readers to retry when a concurrent update occurs, but I don't see what is ensuring that the readers see good data when racing with an insertion or a deletion. Yes, the reader will be retried, courtesy of the seqlock, but that won't help if the reader segfaults before it gets to the read_seqcount_retry().
Questions below.
> --- > include/linux/module.h | 19 +++- > kernel/module.c | 212 +++++++++++++++++++++++++++++++++++++++++++++++-- > 2 files changed, 224 insertions(+), 7 deletions(-) > > --- a/include/linux/module.h > +++ b/include/linux/module.h > @@ -17,6 +17,7 @@ > #include <linux/moduleparam.h> > #include <linux/jump_label.h> > #include <linux/export.h> > +#include <linux/rbtree.h> > > #include <linux/percpu.h> > #include <asm/module.h> > @@ -210,6 +211,11 @@ enum module_state { > MODULE_STATE_UNFORMED, /* Still setting it up. */ > }; > > +struct module_node { > + struct module *mod; > + struct rb_node node; > +}; > + > struct module { > enum module_state state; > > @@ -269,8 +275,15 @@ struct module { > /* Startup function. */ > int (*init)(void); > > - /* If this is non-NULL, vfree after init() returns */ > - void *module_init; > + /* > + * If this is non-NULL, vfree after init() returns > + * > + * cacheline align here, such that: > + * module_init, module_core, init_size, core_size and > + * tree_node[0] > + * are on the same cacheline. > + */ > + void *module_init ____cacheline_aligned; > > /* Here is the actual code + data, vfree'd on unload. */ > void *module_core; > @@ -281,6 +294,8 @@ struct module { > /* The size of the executable code in each section. */ > unsigned int init_text_size, core_text_size; > > + struct module_node tree_node[4]; > + > /* Size of RO sections of the module (text+rodata) */ > unsigned int init_ro_size, core_ro_size; > > --- a/kernel/module.c > +++ b/kernel/module.c > @@ -102,6 +102,204 @@ > DEFINE_MUTEX(module_mutex); > EXPORT_SYMBOL_GPL(module_mutex); > static LIST_HEAD(modules); > + > + > +/* > + * Use a latched RB-tree for __module_address() > + * > + * The latch concept is a multi-value concurrency data structure which allows > + * unserialized access and guarantees at least one version is stable. > + * > + * It is employed here to optimize __module_address(), which needs to find the > + * module (if any) associated with an address. Such questions are best answered > + * using a binary search tree. > + * > + * Since modules use non-overlapping memory ranges we can use a regular RB-tree > + * (as opposed to the interval tree). However, BSTs cannot be iterated while > + * modified. > + * > + * To overcome this we use the latched RB-tree, it basically consists of two > + * RB-trees which are modified in order, ensuring one is always consistent. > + * > + * Things are somewhat more complicated because there are two ranges per > + * module, but other than that its pretty straight forward. > + * > + */ > + > +enum { > + latch0_core = 0, > + latch1_core = 1, > + latch0_init = 2, > + latch1_init = 3, > +}; > + > +enum { > + latch_bit = 0x01, > + init_bit = 0x02, > +}; > + > +struct latch_tree_root { > + seqcount_t seq; > + struct rb_root tree[2]; > +}; > + > +static unsigned long mod_value(struct module *mod, int idx) > +{ > + if (idx & init_bit) > + return (unsigned long)mod->module_init; > + else > + return (unsigned long)mod->module_core; > +} > + > +static unsigned long mod_size(struct module *mod, int idx) > +{ > + if (idx & init_bit) > + return mod->init_size; > + else > + return mod->core_size; > +} > + > +static struct module *mod_entry(struct rb_node *n) > +{ > + struct module_node *mn = container_of(n, struct module_node, node); > + return mn->mod; > +} > + > +static int mod_node_idx(struct module *m, struct rb_node *n) > +{ > + struct module_node *mn = container_of(n, struct module_node, node); > + int idx = mn - m->tree_node; > + > + BUG_ON(mn->mod != m); > + BUG_ON((unsigned)idx > 3); > + > + return idx; > +} > + > +static void __tree_insert(struct latch_tree_root *mod_tree, struct module *mod, int idx) > +{ > + struct rb_root *root = &mod_tree->tree[idx & latch_bit]; > + struct module_node *mn = &mod->tree_node[idx]; > + struct rb_node **link = &root->rb_node; > + struct rb_node *parent = NULL; > + unsigned long mod_val, m_val; > + struct module *m; > + int i; > + > + mn->mod = mod; > + mod_val = mod_value(mod, idx); > + > + while (*link) { > + parent = *link; > + m = mod_entry(parent); > + i = mod_node_idx(m, parent); > + m_val = mod_value(m, i); > + > + if (mod_val < m_val) > + link = &parent->rb_left; > + else > + link = &parent->rb_right; > + } > + > + rb_link_node(&mn->node, parent, link);
This makes the new module visible to readers, if I understand correctly. There needs to be a memory barrier between initialization and this call to rb_link_node(), otherwise, both the CPU (well, for Alpha) and the compiler (for everyone) can reorder, which could result in some hapless reader seeing pre-initialized data.
Or did I miss the memory barrier?
> + rb_insert_color(&mn->node, root);
This -might- be OK -- the rotations do not make new nodes visible, instead simply rearranging nodes that are already visible.
For both rb_link_node() and rb_insert_color(), given that we are updating pointers while readers are traversing them, READ_ONCE() and WRITE_ONCE() would be good. Yes, the compiler -probably- doesn't mess you up, but it would be completely within its rights to do so. :-/
Alpha would like either rcu_dereference() or lockless_dereference() instead of READ_ONCE(), of course!
> +} > + > +static void __tree_remove(struct latch_tree_root *mod_tree, struct module *mod, int idx) > +{ > + struct rb_root *root = &mod_tree->tree[idx & latch_bit]; > + struct module_node *mn = &mod->tree_node[idx]; > + > + rb_erase(&mn->node, root);
This is just removing and not freeing, so OK from a read-side viewpoint. WRITE_ONCE() would be good.
> +} > + > +/* > + * struct module is arranged such that: > + * > + * module_init, module_core, init_size, core_size, > + * init_text_size, core_text_size and tree_node[0] > + * > + * are on the same cacheline, therefore if the below iteration is > + * on latch0 and all module init has completed, we'll only hit > + * tree_node[0] and every intermediate level will hit only a single > + * cacheline. > + * > + * Furthermore, by ensuring init_text_size and core_text_size are > + * also in this same cacheline we've made sure is_module_text_address() > + * will also not require additional lines. > + */ > +static struct module *__tree_find(struct rb_root *r, unsigned long addr) > +{ > + struct rb_node *n = r->rb_node; > + unsigned long m_val, m_size; > + > + while (n) { > + struct module *m = mod_entry(n); > + int idx = mod_node_idx(m, n); > + > + m_val = mod_value(m, idx); > + m_size = mod_size(m, idx); > + > + if (addr < m_val) > + n = n->rb_left;
We need a rcu_dereference() or lockless_dereference() here, I think.
> + else if (addr >= m_val + m_size) > + n = n->rb_right;
And here.
> + else > + return m; > + } > + > + return NULL;
I did finally find the RCU read-side critical section, supplied by is_module_text_address()'s preempt_disable() and preempt_enable(). The matching synchronize_sched() is supplied by do_init_module() and load_module(). I expected a synchronize_sched() in free_module() as well, but I only see a synchronize_rcu(). Is the required synchronize_sched() on this code path hiding somewhere?
Or did I miss an rcu_read_lock()/rcu_read_unlock() pair somewhere?
Thanx, Paul
> +} > + > +static struct latch_tree_root mod_tree; > + > +static void mod_tree_insert(struct module *mod) > +{ > + raw_write_seqcount_latch(&mod_tree.seq); > + __tree_insert(&mod_tree, mod, latch0_core); > + if (mod->init_size) > + __tree_insert(&mod_tree, mod, latch0_init); > + raw_write_seqcount_latch(&mod_tree.seq); > + __tree_insert(&mod_tree, mod, latch1_core); > + if (mod->init_size) > + __tree_insert(&mod_tree, mod, latch1_init); > +} > + > +static void mod_tree_remove_init(struct module *mod) > +{ > + raw_write_seqcount_latch(&mod_tree.seq); > + if (mod->init_size) > + __tree_remove(&mod_tree, mod, latch0_init); > + raw_write_seqcount_latch(&mod_tree.seq); > + if (mod->init_size) > + __tree_remove(&mod_tree, mod, latch1_init); > +} > + > +static void mod_tree_remove(struct module *mod) > +{ > + raw_write_seqcount_latch(&mod_tree.seq); > + __tree_remove(&mod_tree, mod, latch0_core); > + if (mod->init_size) > + __tree_remove(&mod_tree, mod, latch0_init); > + raw_write_seqcount_latch(&mod_tree.seq); > + __tree_remove(&mod_tree, mod, latch1_core); > + if (mod->init_size) > + __tree_remove(&mod_tree, mod, latch1_init); > +} > + > +static struct module *mod_tree_find(unsigned long addr) > +{ > + struct module *m; > + unsigned int seq; > + > + do { > + seq = raw_read_seqcount(&mod_tree.seq); > + m = __tree_find(&mod_tree.tree[seq & latch_bit], addr); > + } while (read_seqcount_retry(&mod_tree.seq, seq)); > + > + return m; > +} > + > #ifdef CONFIG_KGDB_KDB > struct list_head *kdb_modules = &modules; /* kdb needs the list of modules */ > #endif /* CONFIG_KGDB_KDB */ > @@ -1854,6 +2052,7 @@ static void free_module(struct module *m > mutex_lock(&module_mutex); > /* Unlink carefully: kallsyms could be walking list. */ > list_del_rcu(&mod->list); > + mod_tree_remove(mod); > /* Remove this module from bug list, this uses list_del_rcu */ > module_bug_cleanup(mod); > /* Wait for RCU synchronizing before releasing mod->list and buglist. */ > @@ -3098,6 +3297,7 @@ static noinline int do_init_module(struc > mod->symtab = mod->core_symtab; > mod->strtab = mod->core_strtab; > #endif > + mod_tree_remove_init(mod); > unset_module_init_ro_nx(mod); > module_arch_freeing_init(mod); > mod->module_init = NULL; > @@ -3168,6 +3368,7 @@ static int add_unformed_module(struct mo > goto out; > } > list_add_rcu(&mod->list, &modules); > + mod_tree_insert(mod); > err = 0; > > out: > @@ -3367,6 +3568,7 @@ static int load_module(struct load_info > mutex_lock(&module_mutex); > /* Unlink carefully: kallsyms could be walking list. */ > list_del_rcu(&mod->list); > + mod_tree_remove(mod); > wake_up_all(&module_wq); > /* Wait for RCU synchronizing before releasing mod->list. */ > synchronize_rcu(); > @@ -3810,13 +4012,13 @@ struct module *__module_address(unsigned > if (addr < module_addr_min || addr > module_addr_max) > return NULL; > > - list_for_each_entry_rcu(mod, &modules, list) { > + mod = mod_tree_find(addr); > + if (mod) { > + BUG_ON(!within_module(addr, mod)); > if (mod->state == MODULE_STATE_UNFORMED) > - continue; > - if (within_module(addr, mod)) > - return mod; > + mod = NULL; > } > - return NULL; > + return mod; > } > EXPORT_SYMBOL_GPL(__module_address); > >
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