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 <pet...@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 <o...@redhat.com>
> Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> Cc: Rusty Russell <ru...@rustcorp.com.au>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@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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