On Tue, 14 Jul 2020 17:54:17 +0200
Peter Zijlstra <pet...@infradead.org> wrote:
> > I guess that is, why did moving the initialization early require an
> > allocation where initializing it later did not? What allocation are we
> > avoiding?
>
> The other way around. Before this patch initialization required an
> allocation, with this patch init no longer requires one.
>
Yeah, that's what I figured. Just didn't express myself that way.
> > I'm not seeing why this trick is needed when we moved the init early as
> > compared to doing the same thing later on.
>
> We use the trick to avoid the alloc, which is what enables early init.
>
> So, before:
> - init required alloc
> - alloc prohibits early use
>
> after:
> - init no longer require alloc
> - because horrible pointer reuse
> - early use possible
>
> > > @@ -192,16 +222,35 @@ static int __static_call_init(struct mod
> > > if (key != prev_key) {
> > > prev_key = key;
> > >
> > > + if (!mod) {
> > > + key->sites = site;
> > > + key->type |= 1;
> > > + goto do_transform;
> > > + }
> > > +
> >
> > We want to avoid calling kzalloc() early?
> >
> > If so, this should have a comment here stating so and why.
>
> Fair enough; and I think I even see a further optimiation.
>
> How's this:
>
> ---
> Subject: static_call: Allow early init
> From: Peter Zijlstra <pet...@infradead.org>
> Date: Fri, 4 Oct 17:21:10 CEST 2019
>
> In order to use static_call() to wire up x86_pmu, we need to
> initialize earlier; copy some of the tricks from jump_label to enable
> this.
>
> Primarily we overload key->next to store a sites pointer when there
> are no modules, this avoids having to use kmalloc() to initialize the
> sites and allows us to run much earlier.
Can we add a statement that says something like: "Because x86 now calls
static_call_init() before the setup of the memory allocator, we must
avoid using kmalloc() and friends for core kernel static calls." ?
This was the missing piece for me.
>
> (arguably, this is much much earlier than needed for perf, but it
> might allow other uses.)
>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> arch/x86/kernel/setup.c | 2 +
> arch/x86/kernel/static_call.c | 8 ++++-
> include/linux/static_call.h | 15 ++++++++-
> kernel/static_call.c | 67
> +++++++++++++++++++++++++++++++++++++++---
> 4 files changed, 85 insertions(+), 7 deletions(-)
>
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -19,6 +19,7 @@
> #include <linux/hugetlb.h>
> #include <linux/tboot.h>
> #include <linux/usb/xhci-dbgp.h>
> +#include <linux/static_call.h>
>
> #include <uapi/linux/mount.h>
>
> @@ -848,6 +849,7 @@ void __init setup_arch(char **cmdline_p)
> early_cpu_init();
> arch_init_ideal_nops();
> jump_label_init();
> + static_call_init();
> early_ioremap_init();
>
> setup_olpc_ofw_pgd();
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -11,7 +11,7 @@ enum insn_type {
> RET = 3, /* tramp / site cond-tail-call */
> };
>
> -static void __static_call_transform(void *insn, enum insn_type type, void
> *func)
> +static void __ref __static_call_transform(void *insn, enum insn_type type,
> void *func)
> {
> int size = CALL_INSN_SIZE;
> const void *code;
> @@ -33,11 +33,17 @@ static void __static_call_transform(void
> code = text_gen_insn(RET_INSN_OPCODE, insn, func);
> size = RET_INSN_SIZE;
> break;
> +
> + default: /* GCC is a moron -- it figures @code can be uninitialized
> below */
> + BUG();
> }
>
> if (memcmp(insn, code, size) == 0)
> return;
>
> + if (unlikely(system_state == SYSTEM_BOOTING))
> + return text_poke_early(insn, code, size);
> +
> text_poke_bp(insn, code, size, NULL);
> }
>
> --- a/include/linux/static_call.h
> +++ b/include/linux/static_call.h
> @@ -136,6 +136,8 @@ extern void arch_static_call_transform(v
>
> #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
>
> +extern void __init static_call_init(void);
> +
> struct static_call_mod {
> struct static_call_mod *next;
> struct module *mod; /* for vmlinux, mod == NULL */
> @@ -144,7 +146,12 @@ struct static_call_mod {
>
> struct static_call_key {
> void *func;
> - struct static_call_mod *mods;
> + union {
> + /* bit 0: 0 = mods, 1 = sites */
> + unsigned long type;
> + struct static_call_mod *mods;
> + struct static_call_site *sites;
> + };
> };
>
> extern void __static_call_update(struct static_call_key *key, void *tramp,
> void *func);
> @@ -155,7 +162,7 @@ extern int static_call_text_reserved(voi
> DECLARE_STATIC_CALL(name, _func); \
> struct static_call_key STATIC_CALL_KEY(name) = { \
> .func = _func, \
> - .mods = NULL, \
> + .type = 1, \
> }; \
> ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
>
> @@ -180,6 +187,8 @@ extern int static_call_text_reserved(voi
>
> #elif defined(CONFIG_HAVE_STATIC_CALL)
>
> +static inline void static_call_init(void) { }
> +
> struct static_call_key {
> void *func;
> };
> @@ -225,6 +234,8 @@ static inline int static_call_text_reser
>
> #else /* Generic implementation */
>
> +static inline void static_call_init(void) { }
> +
> struct static_call_key {
> void *func;
> };
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -94,10 +94,31 @@ static inline void static_call_sort_entr
> static_call_site_cmp, static_call_site_swap);
> }
>
> +static inline bool static_call_key_has_mods(struct static_call_key *key)
> +{
> + return !(key->type & 1);
> +}
> +
> +static inline struct static_call_mod *static_call_key_next(struct
> static_call_key *key)
> +{
> + if (!static_call_key_has_mods(key))
> + return NULL;
> +
> + return key->mods;
> +}
> +
> +static inline struct static_call_site *static_call_key_sites(struct
> static_call_key *key)
> +{
> + if (static_call_key_has_mods(key))
> + return NULL;
> +
> + return (struct static_call_site *)(key->type & ~1);
> +}
> +
> void __static_call_update(struct static_call_key *key, void *tramp, void
> *func)
> {
> struct static_call_site *site, *stop;
> - struct static_call_mod *site_mod;
> + struct static_call_mod *site_mod, first;
>
> cpus_read_lock();
> static_call_lock();
> @@ -116,13 +137,22 @@ void __static_call_update(struct static_
> if (WARN_ON_ONCE(!static_call_initialized))
> goto done;
>
> - for (site_mod = key->mods; site_mod; site_mod = site_mod->next) {
> + first = (struct static_call_mod){
> + .next = static_call_key_next(key),
> + .mod = NULL,
> + .sites = static_call_key_sites(key),
> + };
> +
> + for (site_mod = &first; site_mod; site_mod = site_mod->next) {
> struct module *mod = site_mod->mod;
>
> if (!site_mod->sites) {
> /*
> * This can happen if the static call key is defined in
> * a module which doesn't use it.
> + *
> + * It also happens in the has_mods case, where the
> + * 'first' entry has no sites associated with it.
> */
> continue;
> }
> @@ -192,16 +222,45 @@ static int __static_call_init(struct mod
> if (key != prev_key) {
> prev_key = key;
>
> + /*
Can we add to this comment:
* Some architectures (x86) call this before the memory
* allocator is set up.
> + * For vmlinux (!mod) avoid the allocation by storing
> + * the sites pointer in the key itself. Also see
> + * __static_call_update()'s @first.
-- Steve
> + */
> + if (!mod) {
> + key->sites = site;
> + key->type |= 1;
> + goto do_transform;
> + }
> +
> site_mod = kzalloc(sizeof(*site_mod), GFP_KERNEL);
> if (!site_mod)
> return -ENOMEM;
>
> + /*
> + * When the key has a direct sites pointer, extract
> + * that into an explicit struct static_call_mod, so we
> + * can have a list of modules.
> + */
> + if (static_call_key_sites(key)) {
> + site_mod->mod = NULL;
> + site_mod->next = NULL;
> + site_mod->sites = static_call_key_sites(key);
> +
> + key->mods = site_mod;
> +
> + site_mod = kzalloc(sizeof(*site_mod),
> GFP_KERNEL);
> + if (!site_mod)
> + return -ENOMEM;
> + }
> +
> site_mod->mod = mod;
> site_mod->sites = site;
> - site_mod->next = key->mods;
> + site_mod->next = static_call_key_next(key);
> key->mods = site_mod;
> }
>
> +do_transform:
> arch_static_call_transform(site_addr, NULL, key->func,
> static_call_is_tail(site));
> }
> @@ -348,7 +407,7 @@ int static_call_text_reserved(void *star
> return __static_call_mod_text_reserved(start, end);
> }
>
> -static void __init static_call_init(void)
> +void __init static_call_init(void)
> {
> int ret;
>
