Re: [PATCH v7 24/24] x86/resctrl: Introduce interface to modify assignment states of the groups

[Moger, Babu]

Hi Reinette,

On 10/4/2024 11:52 AM, Reinette Chatre wrote:
> Hi Babu,
>
> On 10/4/24 9:38 AM, Moger, Babu wrote:
> > Hi Reinette,
> >
> > On 10/3/2024 9:17 PM, Reinette Chatre wrote:
> > > Hi Babu,
> > >
> > > On 10/3/24 6:11 PM, Moger, Babu wrote:
> > > > Hi Reinette,
> > > >
> > > > On 10/2/2024 1:19 PM, Reinette Chatre wrote:
> > > > > Hi Babu,
> > > > >
> > > > > On 9/27/24 10:47 AM, Moger, Babu wrote:
> > > > > > On 9/19/2024 12:59 PM, Reinette Chatre wrote:
> > > > > > > On 9/4/24 3:21 PM, Babu Moger wrote:
> > > > >
> > > > > > > > v7: Simplified the parsing (strsep(&token, "//") in rdtgroup_mbm_assign_control_write().
> > > > > > > >         Added mutex lock in rdtgroup_mbm_assign_control_write() while processing.
> > > > > > > >         Renamed rdtgroup_find_grp to rdtgroup_find_grp_by_name.
> > > > > > > >         Fixed rdtgroup_str_to_mon_state to return error for invalid flags.
> > > > > > > >         Simplified the calls rdtgroup_assign_cntr by merging few functions earlier.
> > > > > > > >         Removed ABMC reference in FS code.
> > > > > > > >         Reinette commented about handling the combination of flags like 'lt_' and '_lt'.
> > > > > > > >         Not sure if we need to change the behaviour here. Processed them sequencially right now.
> > > > > > > >         Users have the liberty to pass the flags. Restricting it might be a problem later.
> > > > > > >
> > > > > > > Could you please give an example of what problem may be encountered later? An assignment
> > > > > > > like "domain=_lt" seems like a contradiction to me since user space essentially asks
> > > > > > > for "None of the MBM events" as well as "MBM total event" and "MBM local event".
> > > > > >
> > > > > > I agree it is contradiction. But user is the one who decides to do that. I think we should allow it. Also, there is some value to it as well.
> > > > > >
> > > > > > "domain=_lt" This will also reset the counters if the total and local events are assigned earlier this action.
> > > > >
> > > > > The last sentence is not clear to me. Could you please elaborate what
> > > > > you mean with "are assigned earlier this action"?
> > > >
> > > > I think I confused you here. "domain=_lt" is equivalent to "domain=lt".  My reasoning is handling all the combination in the code adds code complexity and leave it the user what he wants to do.
> > >
> > > hmmm ... and how about "domain=lt_"? Do you think this should also be equivalent to
> > > "domain=lt" or perhaps an expectation that counters should be assigned to the two events
> > > and then immediately unassigned?
> >
> > Yes. "domain=lt_" should be "domain=lt".
> >
> > > Giving user such flexibility may be interpreted as the assignment seen as acting
> > > sequentially through the flags provided. Ideally the interface should behave in
> > > a predictable way if the goal is to provide flexibility to the user.
> >
> > My only concern is adding the check now and reverting it back later.
> > Basically process the flags sequentially and don't differentiate between the flags. I feel it fits the predictable behavior. No?
>
> This is the point I was trying to make. If flags are processed sequentially then it would be
> predictable behavior and if that is documented expectation then that should be ok. The problem
> that I want to highlight is that if predictable sequential processing is the goal then
> "domain=_lt" cannot be interpreted the same as "domain="lt_". When flags in "domain=lt_"
> are processed sequentially then final state should be "domain=_", no?

Yes. that is correct.
> If sequential processing is done then "domain=_lt" means "unassign all counters followed
> by assign of counter to local MBM monitoring, followed by assign of counter to total MBM
> monitoring". Similarly, "domain=lt_" means "assign a counter to local MBM monitoring, then
> assign a counter to total MBM monitoring, then unassign all counters".

Yes. That is correct.
> If this sequential processing is the goal then the implementation would still need to be
> adapted. Consider, for example, "domain=lt" ... with sequential processing the user
> indicates/expects that "local MBM monitoring" has priority if there is only one counter
> available, but the current implementation does not process it sequentially and would end up
> assigning counter to "total MBM monitoring" first.

Sure. Lets accommodate the sequential processing. Process the  flags in 
the order it is provided. I need to make few changes to 
rdtgroup_process_flags() to address it. Hopefully, it can be done 
without much complexity. Thanks
--
- Babu Moger