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    SubjectRe: [PATCH v10 7/7] x86/tsc: use tsc early
    On Wed, Jun 20, 2018 at 12:42:40PM +0200, Thomas Gleixner wrote:
    > On Wed, 20 Jun 2018, Peter Zijlstra wrote:

    > > I'm still puzzled by the entire need for tsc_early_enabled and all that.
    > > Esp. since both branches do the exact same thing:
    > >
    > > return cycles_2_ns(rdtsc());
    >
    > Right. But up to the point where the real sched_clock initialization can be
    > done and the static keys can be flipped, there must be a way to
    > conditinally use TSC depending on availablility and early initialization.

    Ah, so we want to flip keys early, can be done, see below.

    > You might argue, that we shouldn't care becasue the jiffies case is just
    > the worst case fallback anyway. I wouldn't even disagree as those old
    > machines which have TSC varying with the CPU frequency really should not
    > matter anymore. Pavel might disagree of course.

    You forgot (rightfully) that we even use TSC on those !constant
    machines, we adjust the cycles_2_ns thing from the cpufreq notifiers.

    The only case we should _ever_ use that jiffies callback is when TSC
    really isn't there. Basically, if we kill notsc, we could make
    native_sched_clock() := cycles_2_ns(rdtsc()) (for CONFIG_X86_TSC), the
    end.

    No static keys, nothing.

    That said; flipping static keys early isn't hard. We should call
    jump_label_init() early, because we want the entries sorted and the
    key->entries link set. It will also replace the GENERIC_NOP5_ATOMIC
    thing, which means we need to also do arch_init_ideal_nop() early, but
    since that is pure CPUID based that should be doable.

    And then something like the below could be used.

    ---
    diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
    index e56c95be2808..2dd8c5bdd87b 100644
    --- a/arch/x86/kernel/jump_label.c
    +++ b/arch/x86/kernel/jump_label.c
    @@ -140,4 +140,38 @@ __init_or_module void arch_jump_label_transform_static(struct jump_entry *entry,
    __jump_label_transform(entry, type, text_poke_early, 1);
    }

    +void jump_label_update_early(struct static_key *key, bool enable)
    +{
    + struct jump_entry *entry, *stop = __stop___jump_table;
    +
    + /*
    + * We need the table sorted and key->entries set up.
    + */
    + WARN_ON_ONCE(!static_key_initialized);
    +
    + entry = static_key_entries(key);
    +
    + /*
    + * Sanity check for early users, there had beter be a core kernel user.
    + */
    + if (!entry || !entry->code || !core_kernel_text(entry->code)) {
    + WARN_ON(1);
    + return;
    + }
    +
    + for ( ; (entry < stop) && (jump_entry_key(entry) == key); entry++) {
    + enum jump_label_type type = enable ^ jump_entry_branch(entry);
    + __jump_label_transform(entry, type, text_poke_early, 0);
    + }
    +
    + atomic_set_release(&key->enabled, !!enabled);
    +}
    +
    +#else
    +
    +void jump_label_update_early(struct static_key *key, bool enable)
    +{
    + atomic_set(&key->enabled, !!enabled);
    +}
    +
    #endif
    diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
    index b46b541c67c4..cac61beca25f 100644
    --- a/include/linux/jump_label.h
    +++ b/include/linux/jump_label.h
    @@ -79,6 +79,7 @@

    #include <linux/types.h>
    #include <linux/compiler.h>
    +#include <linux/bug.h>

    extern bool static_key_initialized;

    @@ -110,6 +111,17 @@ struct static_key {
    };
    };

    +#define JUMP_TYPE_FALSE 0UL
    +#define JUMP_TYPE_TRUE 1UL
    +#define JUMP_TYPE_LINKED 2UL
    +#define JUMP_TYPE_MASK 3UL
    +
    +static inline struct jump_entry *static_key_entries(struct static_key *key)
    +{
    + WARN_ON_ONCE(key->type & JUMP_TYPE_LINKED);
    + return (struct jump_entry *)(key->type & ~JUMP_TYPE_MASK);
    +}
    +
    #else
    struct static_key {
    atomic_t enabled;
    @@ -119,6 +131,17 @@ struct static_key {

    #ifdef HAVE_JUMP_LABEL
    #include <asm/jump_label.h>
    +
    +static inline struct static_key *jump_entry_key(struct jump_entry *entry)
    +{
    + return (struct static_key *)((unsigned long)entry->key & ~1UL);
    +}
    +
    +static inline bool jump_entry_branch(struct jump_entry *entry)
    +{
    + return (unsigned long)entry->key & 1UL;
    +}
    +
    #endif

    #ifndef __ASSEMBLY__
    @@ -132,11 +155,6 @@ struct module;

    #ifdef HAVE_JUMP_LABEL

    -#define JUMP_TYPE_FALSE 0UL
    -#define JUMP_TYPE_TRUE 1UL
    -#define JUMP_TYPE_LINKED 2UL
    -#define JUMP_TYPE_MASK 3UL
    -
    static __always_inline bool static_key_false(struct static_key *key)
    {
    return arch_static_branch(key, false);
    diff --git a/kernel/jump_label.c b/kernel/jump_label.c
    index 01ebdf1f9f40..9710fa7582aa 100644
    --- a/kernel/jump_label.c
    +++ b/kernel/jump_label.c
    @@ -295,12 +295,6 @@ void __weak __init_or_module arch_jump_label_transform_static(struct jump_entry
    arch_jump_label_transform(entry, type);
    }

    -static inline struct jump_entry *static_key_entries(struct static_key *key)
    -{
    - WARN_ON_ONCE(key->type & JUMP_TYPE_LINKED);
    - return (struct jump_entry *)(key->type & ~JUMP_TYPE_MASK);
    -}
    -
    static inline bool static_key_type(struct static_key *key)
    {
    return key->type & JUMP_TYPE_TRUE;
    @@ -321,16 +315,6 @@ static inline void static_key_set_linked(struct static_key *key)
    key->type |= JUMP_TYPE_LINKED;
    }

    -static inline struct static_key *jump_entry_key(struct jump_entry *entry)
    -{
    - return (struct static_key *)((unsigned long)entry->key & ~1UL);
    -}
    -
    -static bool jump_entry_branch(struct jump_entry *entry)
    -{
    - return (unsigned long)entry->key & 1UL;
    -}
    -
    /***
    * A 'struct static_key' uses a union such that it either points directly
    * to a table of 'struct jump_entry' or to a linked list of modules which in
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