[Xen-devel] Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 10/04/2011 07:10 AM, Jason Baron wrote:
> 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@xxxxxxxxxx>
>>>>
>>>> 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@xxxxxxxxxx>
>>>> ---
>>>>  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.

I went the other way and added a second function,
arch_jump_label_transform_static(), which has a weak default
implementation which calls arch_jump_label_transform().  That way only
the architectures which really care about it need implement a second
variant. I did x86 and s390 by adapting the patches I had from the other
series; it didn't look like mips/sparc/power were very heavyweight at all.

    J

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