Re: Cache Allocation Technology Design

From: Vikas Shivappa
Date: Thu Nov 06 2014 - 12:21:58 EST




On Thu, 6 Nov 2014, Matt Fleming wrote:

On Thu, 30 Oct, at 11:47:40PM, Peter Zijlstra wrote:

Let me reply to just this one, I'll do the rest tomorrow, need sleeps.

On Thu, Oct 30, 2014 at 06:22:36PM -0400, Tejun Heo wrote:

This controller might not even require the distinction between
configured and effective tho? Can't a new child just inherit the
parent's configuration and never allow the config to become completely
empty?

It can do that. But that still has a problem, there is a mapping in
hardware which restricts the number of active configurations. The total
configuration space is larger than the supported active configurations.

So _something_ must fail. The initial proposal was mkdir failing when
there were more than the hardware supported active config cgroup
directories. The alternative was on-demand activation where we only
allocate the hardware resource when the first task gets moved into the
group -- which then clearly can fail.

Hmmm... why can't it just refuse creating a different configuration
when its config space is full? Make children inherit the parent's
configuration and refuse config writes which require it to create a
new one if the config space is full. Seems pretty straight-forward.
What am I missing?

We could do that I suppose, there is the one corner case that would not
allow, intermediate directories with a restricted config that also have
priv restrictions but no actual tasks. Not sure that makes sense though.

Could you elaborate on this configuration?

Are there any other cases I might have missed?

I don't think so.

So, for the specific CAT case what you're proposing is make the failure
case happen when writing to the cache bitmask file instead of failing
mkdir() or echo $tid > tasks ?

I think that's OK. If we've run out of CLOS ids I would expect to see
-ENOSPC returned, whereas if we try and set an invalid bitmask we'd get
-EINVAL.

Vikas, Will?

Yes that is correct. You can always create more cgroups and the new cgroup just inherits the mask from the parent and uses the same CLOSid as its parent , so it wont fail because of lack of CLOSids.

The only case of failure as you said is when user tries to modify a cbm to a different one.

>
--
Matt Fleming, Intel Open Source Technology Center

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/