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SubjectRe: Kernel broken on processors without performance counters
On Tue, Jul 21, 2015 at 11:50 AM, Jason Baron <jasonbaron0@gmail.com> wrote:
>
>
> On 07/21/2015 02:15 PM, Borislav Petkov wrote:
>> On Tue, Jul 21, 2015 at 06:12:15PM +0200, Peter Zijlstra wrote:
>>> Yes, if you start out false, you must be unlikely. If you start out
>>> true, you must be likely.
>>>
>>> We could maybe try and untangle that if there really is a good use case,
>>> but this is the current state.
>>>
>>> The whole reason this happened is because 'false' is like:
>>>
>>>
>>> ...
>>> <nop>
>>> 1:
>>> ...
>>>
>>>
>>>
>>> label:
>>> <unlikely code>
>>> jmp 1b
>>>
>>>
>>> Where the code if out-of-line by default. The enable will rewrite the
>>> <nop> with a jmp label.
>> Btw, native_sched_clock() is kinda botched because of that, see below.
>>
>> I'd want that RDTSC to come first with a NOP preceding it which can
>> become a JMP in case some idiotic CPU can't do RDTSC and needs to use
>> jiffies. Instead, we *unconditionally* jump to RDTSC which is WTF?! We
>> can just as well do a normal unlikely() without the static_key:
>
> hmmm...so this is a case where need to the default the branch
> to the out-of-line branch at boot. That is, we can't just enable
> the out-of-line branch at boot time, b/c it might be too late at
> that point? IE native_sched_clock() gets called very early?
>

Could this be done at link time, or perhaps when compressing the
kernel image, instead of at boot time?

--Andy


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