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    SubjectRe: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out
    On Mon, Oct 03, 2011 at 09:27:56AM -0700, Jeremy Fitzhardinge wrote:
    > On 10/03/2011 08:02 AM, Jason Baron wrote:
    > > Hi,
    > >
    > > (Sorry for the late reply - I was away for a few days).
    > >
    > > The early enable is really nice - it means there are not restrictions on
    > > when jump_label_inc()/dec() can be called which is nice.
    > >
    > > comments below.
    > >
    > >
    > > On Sat, Oct 01, 2011 at 02:55:35PM -0700, Jeremy Fitzhardinge wrote:
    > >> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    > >>
    > >> If a key has been enabled before jump_label_init() is called, don't
    > >> nop it out.
    > >>
    > >> This removes arch_jump_label_text_poke_early() (which can only nop
    > >> out a site) and uses arch_jump_label_transform() instead.
    > >>
    > >> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    > >> ---
    > >> include/linux/jump_label.h | 3 ++-
    > >> kernel/jump_label.c | 20 ++++++++------------
    > >> 2 files changed, 10 insertions(+), 13 deletions(-)
    > >>
    > >> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
    > >> index 1213e9d..c8fb1b3 100644
    > >> --- a/include/linux/jump_label.h
    > >> +++ b/include/linux/jump_label.h
    > >> @@ -45,7 +45,8 @@ extern void jump_label_lock(void);
    > >> extern void jump_label_unlock(void);
    > >> extern void arch_jump_label_transform(struct jump_entry *entry,
    > >> enum jump_label_type type);
    > >> -extern void arch_jump_label_text_poke_early(jump_label_t addr);
    > >> +extern void arch_jump_label_transform_early(struct jump_entry *entry,
    > >> + enum jump_label_type type);
    > >> extern int jump_label_text_reserved(void *start, void *end);
    > >> extern void jump_label_inc(struct jump_label_key *key);
    > >> extern void jump_label_dec(struct jump_label_key *key);
    > >> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
    > >> index a8ce450..059202d5 100644
    > >> --- a/kernel/jump_label.c
    > >> +++ b/kernel/jump_label.c
    > >> @@ -121,13 +121,6 @@ static void __jump_label_update(struct jump_label_key *key,
    > >> }
    > >> }
    > >>
    > >> -/*
    > >> - * Not all archs need this.
    > >> - */
    > >> -void __weak arch_jump_label_text_poke_early(jump_label_t addr)
    > >> -{
    > >> -}
    > >> -
    > >> static __init int jump_label_init(void)
    > >> {
    > >> struct jump_entry *iter_start = __start___jump_table;
    > >> @@ -139,12 +132,15 @@ static __init int jump_label_init(void)
    > >> jump_label_sort_entries(iter_start, iter_stop);
    > >>
    > >> for (iter = iter_start; iter < iter_stop; iter++) {
    > >> - arch_jump_label_text_poke_early(iter->code);
    > >> - if (iter->key == (jump_label_t)(unsigned long)key)
    > >> + struct jump_label_key *iterk;
    > >> +
    > >> + iterk = (struct jump_label_key *)(unsigned long)iter->key;
    > >> + arch_jump_label_transform(iter, jump_label_enabled(iterk) ?
    > >> + JUMP_LABEL_ENABLE : JUMP_LABEL_DISABLE);
    > > The only reason I called this at boot-time was that the 'ideal' x86
    > > no-op isn't known until boot time. Thus, in the enabled case we could
    > > skip the the arch_jump_label_transform() call. ie:
    > >
    > > if (!enabled)
    > > arch_jump_label_transform(iter, JUMP_LABEL_DISABLE);
    >
    >
    > Yep, fair enough.
    >
    > >
    > >
    > >> + if (iterk == key)
    > >> continue;
    > >>
    > >> - key = (struct jump_label_key *)(unsigned long)iter->key;
    > >> - atomic_set(&key->enabled, 0);
    > >> + key = iterk;
    > >> key->entries = iter;
    > >> #ifdef CONFIG_MODULES
    > >> key->next = NULL;
    > >> @@ -212,7 +208,7 @@ void jump_label_apply_nops(struct module *mod)
    > >> return;
    > >>
    > >> for (iter = iter_start; iter < iter_stop; iter++)
    > >> - arch_jump_label_text_poke_early(iter->code);
    > >> + arch_jump_label_transform(iter, JUMP_LABEL_DISABLE);
    > >> }
    > >>
    > >> static int jump_label_add_module(struct module *mod)
    > >> --
    > >> 1.7.6.2
    > >>
    > > hmmm...this is used on module load in smp - so this would introduce a number of
    > > calls to stop_machine() where we didn't have them before. Yes, module
    > > load is a very slow path to begin with, but I think its at least worth
    > > pointing out...
    >
    > Ah, that explains it - the module stuff certainly isn't "early" except -
    > I guess - in the module's lifetime.
    >
    > Well, I suppose I could introduce either second variant of the function,
    > or add a "live" flag (ie, may be updating code that a processor is
    > executing), which requires a stop_machine, or direct update if it doesn't.
    >
    > But is there any reason why we couldn't just generate a reasonably
    > efficient 5-byte atomic nop in the first place, and get rid of all that
    > fooling around? It looks like x86 is the only arch where it makes any
    > difference at all, and how much difference does it really make? Or is
    > there no one 5-byte atomic nop that works on all x86 variants, aside
    > from jmp +0?
    >
    > J

    Yes, there are really two reasons for the initial no-op patching pass:

    1) The jmp +0, is a 'safe' no-op that I know is going to initially
    boot for all x86. I'm not sure if there is a 5-byte nop that works on
    all x86 variants - but by using jmp +0, we make it much easier to debug
    cases where we may be using broken no-ops.

    2) This optimization is about as close to a 0 cost off case as possible.
    I know there have been various no-op benchmarks posted on lkml in the
    past, so the choice of no-op does seem to make a difference. see:
    http://lkml.indiana.edu/hypermail/linux/kernel/0808.1/2416.html, for
    example. So at least to me, if we are not using the lowest cost no-op,
    we are at least in-part defeating the point of this optimization.

    I like the "live" flag suggestion mentioned above. Less functions is
    better, and non-x86 arches can simply ignore the flag.

    Thanks,

    -Jason







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