Re: [PATCH V15 11/11] x86,cgroup/intel_rdt : Add a cgroup interface to manage Intel cache allocation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Oct 01, 2015 at 11:09:45PM -0700, Fenghua Yu wrote:
> Add a new cgroup 'intel_rdt' to manage cache allocation. Each cgroup
> directory is associated with a class of service id(closid). To map a
> task with closid during scheduling, this patch removes the closid field
> from task_struct and uses the already existing 'cgroups' field in
> task_struct.
> 
> The cgroup has a file 'l3_cbm' which represents the L3 cache capacity
> bitmask(CBM). The CBM is global for the whole system currently. The
> capacity bitmask needs to have only contiguous bits set and number of
> bits that can be set is less than the max bits that can be set. The
> tasks belonging to a cgroup get to fill in the L3 cache represented by
> the capacity bitmask of the cgroup. For ex: if the max bits in the CBM
> is 10 and the cache size is 10MB, each bit represents 1MB of cache
> capacity.
> 
> Root cgroup always has all the bits set in the l3_cbm. User can create
> more cgroups with mkdir syscall. By default the child cgroups inherit
> the capacity bitmask(CBM) from parent. User can change the CBM specified
> in hex for each cgroup. Each unique bitmask is associated with a class
> of service ID and an -ENOSPC is returned once we run out of
> closids.
> 
> Signed-off-by: Vikas Shivappa <vikas.shivappa@linux.intel.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>


+	clos_cbm_table_read(ir->closid, &ccbm);
+	if (cbmvalue == ccbm)
+		goto out;
+
+	err = cbm_validate_rdt_cgroup(ir, cbmvalue);
+	if (err)
+		goto out;
+
+	/*
+	 * Try to get a reference for a different CLOSid and release the
+	 * reference to the current CLOSid.
+	 * Need to put down the reference here and get it back in case we
+	 * run out of closids. Otherwise we run into a problem when
+	 * we could be using the last closid that could have been available.
+	 */
+	closid_put(ir->closid);
+	if (cbm_search(cbmvalue, &closid)) {

Can't you move closid_put here?

+		ir->closid = closid;
+		closid_get(closid);
+	} else {
+		closid = ir->closid;

Variable unused.

+		err = closid_alloc(&ir->closid);
+		if (err) {
+			closid_get(ir->closid);
+			goto out;
+		}

This makes you cycle closid when changing the cbm, not necessary.
(not very important, but closid_put is nerving because it can possibly 
set l3_cbm to zero).