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SubjectRe: [PATCH 01/14] x86/cqm: Intel Resource Monitoring Documentation

On Tue, 27 Dec 2016, Andi Kleen wrote:

> Shivappa Vikas <> writes:
>> Ok , looks like the interface is the problem. Will try to fix
>> this. We are just trying to have a light weight monitoring
>> option so that its reasonable to monitor for a
>> very long time (like lifetime of process etc). Mainly to not have all
>> the perf scheduling overhead.
> That seems like an odd reason to define a completely new user interface.
> This is to avoid one MSR write for a RMID change per context switch
> in/out cgroup or is it other code too?
> Is there some number you can put to the overhead?
> Or is there some other overhead other than the MSR write
> you're concerned about?

Yes, seems like the interface of having a file is odd as even Peterz thinks.

Its the perf overhead actually we are trying to avoid.

The MSR writes(the driver/cqm overhead
really not perf..) we try to optimize by having a per cpu cache/group the rmids/
have a common write for rmid/closid etc.

The perf overhead i was thinking atleast was during the context switch which is
the more constant overhead (the event creation is just one time).

-I was trying to see an alternative where
1.user specifies the continuous monitor with perf-attr in open
2.driver allocates the task/cgroup RMID and stores the RMID in cgroup or
3.turns off the event. (hence no perf ctx switch overhead? (all the perf hook
calls for start/stop/add we dont need any of those -
i was still finding out if this route works basically if i turn off the event
there is minimal overhead for the event and not start/stop/add calls for the
4.but during switch_to driver writes the RMID MSR, so we still monitor. -> calls the driver -> driver just returns the count by reading the

> Do you have an ftrace or better PT trace with the overhead before-after?
> Perhaps some optimization could be done in the code to make it faster,
> then the new interface wouldn't be needed.
> FWIW there are some pending changes to context switch that will
> eliminate at least one common MSR write [1]. If that was fixed
> you could do the RMID MSR write "for free"

I see, thats good to know..


> -Andi
> [1]

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