Re: [RFC PATCH V2 11/22] x86/intel_rdt: Associate pseudo-locked regions with its domain

[Reinette Chatre]

Hi Thomas,

On 2/20/2018 2:00 AM, Thomas Gleixner wrote:
> On Mon, 19 Feb 2018, Reinette Chatre wrote:
>> On 2/19/2018 3:19 PM, Thomas Gleixner wrote:
>>> On Mon, 19 Feb 2018, Reinette Chatre wrote:
>>>> On 2/19/2018 1:19 PM, Thomas Gleixner wrote:
>>>>> On Tue, 13 Feb 2018, Reinette Chatre wrote:
>>>>>
>>>>>> After a pseudo-locked region is locked it needs to be associated with
>>>>>> the RDT domain representing the pseudo-locked cache so that its life
>>>>>> cycle can be managed correctly.
>>>>>>
>>>>>> Only a single pseudo-locked region can exist on any cache instance so we
>>>>>> maintain a single pointer to a pseudo-locked region from each RDT
>>>>>> domain.
>>>>>
>>>>> Why is only a single pseudo locked region possible? 
>>>>
>>>> The setup of a pseudo-locked region requires the usage of wbinvd. If a
>>>> second pseudo-locked region is thus attempted it will evict the
>>>> pseudo-locked data of the first.
>>>
>>> Why does it neeed wbinvd? wbinvd is a big hammer. What's wrong with clflush?
>>
>> wbinvd is required by this hardware supported feature but limited to the
>> creation of the pseudo-locked region. An administrator could dedicate a
>> portion of cache to pseudo-locking and applications using this region
>> can come and go. The pseudo-locked region lifetime need not be tied to
>> application lifetime. The pseudo-locked region could be set up once on
>> boot and remain for lifetime of system.
>>
>> Even so, understanding that it is a big hammer I did explore the
>> alternatives. Trying clflush, clflushopt, as well as clwb. Finding them
>> all to perform poorly(*) I went further to explore if it is possible to
>> use these other instructions with some additional work in support to
>> make them perform as well as wbinvd. The additional work included,
>> looping over the data more times than done for wbinvd, reducing the size
>> of memory locked in relationship to cache size, unused spacing between
>> pseudo-locked region and other regions, unmapped memory at end of
>> pseudo-locked region.
>>
>> In addition to the above research from my side I also followed up with
>> the CPU architects directly to question the usage of these instructions
>> instead of wbinvd.
> 
> What was their answer? This really wants a proper explanation and not just
> experimentation results as it makes absolutely no sense at all.

I always prefer to provide detailed answers but here I find myself at
the threshold where I may end up sharing information not publicly known.
This cannot be the first time you find yourself in this situation. How
do you prefer to proceed?

Reinette