Re: [PATCH 5/5] x86/ioapic: Drop function pointers from __ioapic_{read,write}_entry()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.11.2021 10:06, Jan Beulich wrote:
> On 18.11.2021 01:32, Andrew Cooper wrote:
>> On 12/11/2021 10:43, Jan Beulich wrote:
>>> On 11.11.2021 18:57, Andrew Cooper wrote:
>>>> Function pointers are expensive, and the raw parameter is a constant from 
>>>> all
>>>> callers, meaning that it predicts very well with local branch history.
>>> The code change is fine, but I'm having trouble with "all" here: Both
>>> functions aren't even static, so while callers in io_apic.c may
>>> benefit (perhaps with the exception of ioapic_{read,write}_entry(),
>>> depending on whether the compiler views inlining them as warranted),
>>> I'm in no way convinced this extends to the callers in VT-d code.
>>>
>>> Further ISTR clang being quite a bit less aggressive about inlining,
>>> so the effects might not be quite as good there even for the call
>>> sites in io_apic.c.
>>>
>>> Can you clarify this for me please?
>>
>> The way the compiler lays out the code is unrelated to why this form is 
>> an improvement.
>>
>> Branch history is a function of "the $N most recently taken branches".  
>> This is because "how you got here" is typically relevant to "where you 
>> should go next".
>>
>> Trivial schemes maintain a shift register of taken / not-taken results.  
>> Less trivial schemes maintain a rolling hash of (src addr, dst addr) 
>> tuples of all taken branches (direct and indirect).  In both cases, the 
>> instantaneous branch history is an input into the final prediction, and 
>> is commonly used to select which saturating counter (or bank of 
>> counters) is used.
>>
>> Consider something like
>>
>> while ( cond )
>> {
>>      memcpy(dst1, src1, 64);
>>      memcpy(dst2, src2, 7);
>> }
>>
>> Here, the conditional jump inside memcpy() coping with the tail of the 
>> copy flips result 50% of the time, which is fiendish to predict for.
>>
>> However, because the branch history differs (by memcpy()'s return 
>> address which was accumulated by the call instruction), the predictor 
>> can actually use two different taken/not-taken counters for the two 
>> different "instances" if the tail jump.  After a few iterations to warm 
>> up, the predictor will get every jump perfect despite the fact that 
>> memcpy() is a library call and the branches would otherwise alias.
>>
>>
>> Bringing it back to the code in question.  The "raw" parameter is an 
>> explicit true or false at the top of all call paths leading into these 
>> functions.  Therefore, an individual branch history has a high 
>> correlation with said true or false, irrespective of the absolute code 
>> layout.  As a consequence, the correct result of the prediction is 
>> highly correlated with the branch history, and it will predict 
>> perfectly[1] after a few times the path has been used.
> 
> Thanks a lot for the explanation. May I suggest to make this less
> ambiguous in the description, e.g. by saying "the raw parameter is a
> constant at the root of all call trees"?

Oh, forgot to say that then:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan