Date:	Fri, 31 Jul 2015 09:41:58 -0700 (PDT)
From:	Vikas Shivappa <vikas.shivappa@...el.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
cc:	Vikas Shivappa <vikas.shivappa@...el.com>,
	"Auld, Will" <will.auld@...el.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"tj@...nel.org" <tj@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"Fleming, Matt" <matt.fleming@...el.com>,
	"Williamson, Glenn P" <glenn.p.williamson@...el.com>,
	"Juvva, Kanaka D" <kanaka.d.juvva@...el.com>
Subject: [summary] Re: [PATCH 3/9] x86/intel_rdt: Cache Allocation documentation
 and cgroup usage guide


To summarize  the ever growing thread :

1. the rdt_cgroup can be used to configure exclusive cache bitmaps for the child 
nodes which can be used for the scenarios which Marcello mentions.

simle examples which were mentioned :
max bitmask length : 16 . hence full mask is 0xffff
groupx_realtime - 0xff .
group2_systemtraffic - 0xf. : put a lot of tasks from root node to here or which 
ever is offending and thrashing.
groupy_<mytraffic> - 0x0f

Now the groupx has its own area of cache that can used by the realtime/(specific 
scenario) apps. Similarly configure any groupy.

2. Can the maps can let you specify which cache ways ways the cache is allocated 
? - No , this is 
implementation specific as mentioned in the SDM. So when we configure a mask , 
you really dont know which ways or which exact lines are used on which SKUs .. 
We may not see any use case as well 
which is needed for apps to allocate cache in specific areas and the h/w does 
not support this as well.

3. Letting the user specify size in bytes instead of bitmap : we have already 
gone through this discussion in older versions. The user can simply check the 
size of the total cache and understand what map could be what size. I dont see a 
special need to specify an interface to enter the cache in bytes and then round 
off - user could instead use the roundoff values before hand or iow it 
automatically does when he specifies the bitmask.

ex: find cache size from /proc/cpuinfo. - say 20MB
bitmask max - 0xfffff.

This means the roundoff(chunk) size supported is only 1MB , so when you specify 
the mask say 0x3(2MB) thats already taken care of.
Same applies to percentage - the masks automatically round off the percentage.

Please note that this is quite different from the way we can allocate memory in 
bytes and needs to be treated differently given that the hardware provides interface 
in a particular way.

4. Letting the kernel automatically extend the bitmap may affect a lot of other 
things and will need a lot of heuristics - note that we have overlapping masks . 
This interface lets the super-user control the cache allocation and it may be 
very confusing for the user if he has allocated a cache mask and suddenly from 
under the floor the kernel changes it.

Thanks,
Vikas


On Fri, 31 Jul 2015, Marcelo Tosatti wrote:

> On Thu, Jul 30, 2015 at 04:03:07PM -0700, Vikas Shivappa wrote:
>>
>>
>> On Thu, 30 Jul 2015, Marcelo Tosatti wrote:
>>
>>> On Thu, Jul 30, 2015 at 10:47:23AM -0700, Vikas Shivappa wrote:
>>>>
>>>>
>>>> Marcello,
>>>>
>>>>
>>>> On Wed, 29 Jul 2015, Marcelo Tosatti wrote:
>>>>>
>>>>> How about this:
>>>>>
>>>>> desiredclos (closid  p1  p2  p3 p4)
>>>>> 	     1       1   0   0  0
>>>>> 	     2	     0	 0   0  1
>>>>> 	     3	     0   1   1  0
>>>>
>>>> #1 Currently in the rdt cgroup , the root cgroup always has all the
>>>> bits set and cant be changed (because the cgroup hierarchy would by
>>>> default make this to have all bits as all the children need to have
>>>> a subset of the root's bitmask). So if the user creates a cgroup and
>>>> not put any task in it , the tasks in the root cgroup could be still
>>>> using that part of the cache. Thats the reason i say we can have
>>>> really 'exclusive' masks.
>>>>
>>>> Or in other words - there is always a desired clos (0) which has all
>>>> parts set which acts like a default pool.
>>>>
>>>> Also the parts can overlap.  Please apply this for all the below
>>>> comments which will change the way they work.
>>>
>>>
>>>>
>>>>>
>>>>> p means part.
>>>>
>>>> I am assuming p = (a contiguous cache capacity bit mask)
>>>>
>>>>> closid 1 is a exclusive cgroup.
>>>>> closid 2 is a "cache hog" class.
>>>>> closid 3 is "default closid".
>>>>>
>>>>> Desiredclos is what user has specified.
>>>>>
>>>>> Transition 1: desiredclos --> effectiveclos
>>>>> Clean all bits of unused closid's
>>>>> (that must be updated whenever a
>>>>> closid1 cgroup goes from empty->nonempty
>>>>> and vice-versa).
>>>>>
>>>>> effectiveclos (closid  p1  p2  p3 p4)
>>>>> 	       1       0   0   0  0
>>>>> 	       2       0   0   0  1
>>>>> 	       3       0   1   1  0
>>>>
>>>>>
>>>>> Transition 2: effectiveclos --> expandedclos
>>>>> expandedclos (closid  p1  p2  p3 p4)
>>>>> 	       1       0   0   0  0
>>>>> 	       2       0   0   0  1
>>>>> 	       3       1   1   1  0
>>>>> Then you have different inplacecos for each
>>>>> CPU (see pseudo-code below):
>>>>>
>>>>> On the following events.
>>>>>
>>>>> - task migration to new pCPU:
>>>>> - task creation:
>>>>>
>>>>> 	id = smp_processor_id();
>>>>> 	for (part = desiredclos.p1; ...; part++)
>>>>> 		/* if my cosid is set and any other
>>>>> 	   	   cosid is clear, for the part,
>>>>> 		   synchronize desiredclos --> inplacecos */
>>>>> 		if (part[mycosid] == 1 &&
>>>>> 		    part[any_othercosid] == 0)
>>>>> 			wrmsr(part, desiredclos);
>>>>>
>>>>
>>>> Currently the root cgroup would have all the bits set which will act
>>>> like a default cgroup where all the otherwise unused parts (assuming
>>>> they are a set of contiguous cache capacity bits) will be used.
>>>
>>> Right, but we don't want to place tasks in there in case one cgroup
>>> wants exclusive cache access.
>>>
>>> So whenever you want an exclusive cgroup you'd do:
>>>
>>> create cgroup-exclusive; reserve desired part of the cache
>>> for it.
>>> create cgroup-default; reserved all cache minus that of cgroup-exclusive
>>> for it.
>>>
>>> place tasks that belong to cgroup-exclusive into it.
>>> place all other tasks (including init) into cgroup-default.
>>>
>>> Is that right?
>>
>> Yes you could do that.
>>
>> You can create cgroups to have masks which are exclusive in todays
>> implementation, just that you could also created more cgroups to
>> overlap the masks again.. iow we dont have an exclusive flag for the
>> cgroup mask.
>> Is that a common use case in the server environment that you need to
>> prevent other cgroups from using a certain mask ? (since the root
>> user should control these allocations .. he should know?)
>
> Yes, there are two known use-cases that have this characteristic:
>
> 1) High performance numeric application which has been optimized
> to a certain fraction of the cache.
>
> 2) Low latency application in multi-application OS.
>
> For both cases exclusive cache access is wanted.
>
>
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