Re: [PATCH 5/9] x86/intel_rdt: Add new cgroup and Class of service management

From: Vikas Shivappa
Date: Thu Aug 20 2015 - 20:14:14 EST



On Thu, 20 Aug 2015, Vikas Shivappa wrote:



On Mon, 17 Aug 2015, Marcelo Tosatti wrote:

Vikas, Tejun,

This is an updated interface. It addresses all comments made
so far and also covers all use-cases the cgroup interface
covers.

Let me know what you think. I'll proceed to writing
the test applications.

Usage model:
------------

This document details how CAT technology is
exposed to userspace.

Each task has a list of task cache reservation entries (TCRE list).

The init process is created with empty TCRE list.

There is a system-wide unique ID space, each TCRE is assigned
an ID from this space. ID's can be reused (but no two TCREs
have the same ID at one time).

The interface accomodates transient and independent cache allocation
adjustments from applications, as well as static cache partitioning
schemes.

Allocation:
Usage of the system calls require CAP_SYS_CACHE_RESERVATION capability.

A configurable percentage is reserved to tasks with empty TCRE list.

And how do you think you will do this without a system controlled mechanism ? Everytime in your proposal you include these caveats which actually mean to include a system controlled interface in the background ,
and your below interfaces make no mention of this really ! Why do we want to confuse ourselves like this ?

syscall only interface does not seem to work on its own for the cache allocation scenario. This can only be a nice to have interface on top of a system controlled mechanism like cgroup interface. Sure you can do all the things you did with cgroup with the same with syscall interface but the point is what are the use cases that cant be done with this syscall only interface. (ex: to deal with cases you brought up earlier like when an app does cache intensive work for some time and later changes - it could use the syscall interface to quickly reqlinquish the cache lines or change a clos associated with it)

I have repeatedly listed the use cases that can be dealt with , with this
big typo - 'use cases that cannot be dealt with'

interface. How will you address the cases like 1.1 and 1.2 with your syscall only interface ? So we expect all the millions of apps like SAP, oracle etc and etc and all the millions of app developers to magically learn our new syscall interface and also cooperate between themselves to decide a cache allocation that is agreeable to all ? (which btw the interface doesnt list below how to do it) and then by some godly powers the noisly neighbour will decide himself to give up the cache ? (that should be first ever app to not request more resource in the world for himself and hurt his own performance - they surely dont want to do social service !)

And how do we do the case 1.5 where the administrator want to assign cache to specific VMs in a cloud etc - with the hypothetical syscall interface we now should expect all the apps to do the above and now they also need to know where they run (what VM , what socket etc) and then decide and cooperate an allocation : compare this to a container environment like rancher where today the admin can convinetly use docker underneath to allocate mem/storage/compute to containers and easily extend this to include shared l3.

http://marc.info/?l=linux-kernel&m=143889397419199

without addressing the above the details of the interface below is irrelavant -

Your initial request was to extend the cgroup interface to include rounding off the size of cache (which can easily be done with a bash script on top of cgroup interface !) and now you are proposing a syscall only interface ? this is very confusing and will only unnecessarily delay the process without adding any value.

however like i mentioned the syscall interface or user/app being able to modify the cache alloc could be used to address some very specific use cases on top an existing system managed interface. This is not really a common case in cloud or container environment and neither a feasible deployable solution.
Just consider the millions of apps that have to transition to such an interface to even use it - if thats the only way to do it, thats dead on arrival.

Also please donot include kernel automatically adjusting resources in your reply as thats totally irrelavent and again more confusing as we have already exchanged some >100 emails on this same patch version without meaning anything so far.

The debate is purely between a syscall only interface and a system manageable interface(like cgroup where admin or a central entity controls the resources). If not define what is it first before going into details.

Thanks,
Vikas


On fork, the child inherits the TCR from its parent.

Semantics:
Once a TCRE is created and assigned to a task, that task has
guaranteed reservation on any CPU where its scheduled in,
for the lifetime of the TCRE.

A task can have its TCR list modified without notification.

FIXME: Add a per-task flag to not copy the TCR list of a task but delete
all TCR's on fork.

Interface:

enum cache_rsvt_flags {
CACHE_RSVT_ROUND_DOWN = (1 << 0), /* round "kbytes" down */
};

enum cache_rsvt_type {
CACHE_RSVT_TYPE_CODE = 0, /* cache reservation is for code */
CACHE_RSVT_TYPE_DATA, /* cache reservation is for data */
CACHE_RSVT_TYPE_BOTH, /* cache reservation is for code and data */
};

struct cache_reservation {
unsigned long kbytes;
int type;
int flags;
int trcid;
};

The following syscalls modify the TCR of a task:

* int sys_create_cache_reservation(struct cache_reservation *rsvt);
DESCRIPTION: Creates a cache reservation entry, and assigns
it to the current task.

returns -ENOMEM if not enough space, -EPERM if no permission.
returns 0 if reservation has been successful, copying actual
number of kbytes reserved to "kbytes", type to type, and tcrid.

* int sys_delete_cache_reservation(struct cache_reservation *rsvt);
DESCRIPTION: Deletes a cache reservation entry, deassigning it
from any task.

Backward compatibility for processors with no support for code/data
differentiation: by default code and data cache allocation types
fallback to CACHE_RSVT_TYPE_BOTH on older processors (and return the
information that they done so via "flags").

* int sys_attach_cache_reservation(pid_t pid, unsigned int tcrid);
DESCRIPTION: Attaches cache reservation identified by "tcrid" to
task by identified by pid.
returns 0 if successful.

* int sys_detach_cache_reservation(pid_t pid, unsigned int tcrid);
DESCRIPTION: Detaches cache reservation identified by "tcrid" to
task by identified pid.

The following syscalls list the TCRs:
* int sys_get_cache_reservations(size_t size, struct cache_reservation list[]);
DESCRIPTION: Return all cache reservations in the system.
Size should be set to the maximum number of items that can be stored
in the buffer pointed to by list.

* int sys_get_tcrid_tasks(unsigned int tcrid, size_t size, pid_t list[]);
DESCRIPTION: Return which pids are associated to tcrid.

* sys_get_pid_cache_reservations(pid_t pid, size_t size,
struct cache_reservation list[]);
DESCRIPTION: Return all cache reservations associated with "pid".
Size should be set to the maximum number of items that can be stored
in the buffer pointed to by list.

* sys_get_cache_reservation_info()
DESCRIPTION: ioctl to retrieve hardware info: cache round size, whether
code/data separation is supported.



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