Re: [PATCH v4] pseries: prevent free CPU ids to be reused on another node
From: Laurent Dufour
Date: Tue Apr 20 2021 - 12:35:13 EST
Le 07/04/2021 à 17:38, Laurent Dufour a écrit :
When a CPU is hot added, the CPU ids are taken from the available mask from
the lower possible set. If that set of values was previously used for CPU
attached to a different node, this seems to application like if these CPUs
have migrated from a node to another one which is not expected in real
life.
To prevent this, it is needed to record the CPU ids used for each node and
to not reuse them on another node. However, to prevent CPU hot plug to
fail, in the case the CPU ids is starved on a node, the capability to reuse
other nodes’ free CPU ids is kept. A warning is displayed in such a case
to warn the user.
A new CPU bit mask (node_recorded_ids_map) is introduced for each possible
node. It is populated with the CPU onlined at boot time, and then when a
CPU is hot plug to a node. The bits in that mask remain when the CPU is hot
unplugged, to remind this CPU ids have been used for this node.
The effect of this patch can be seen by removing and adding CPUs using the
Qemu monitor. In the following case, the first CPU from the node 2 is
removed, then the first one from the node 1 is removed too. Later, the
first CPU of the node 2 is added back. Without that patch, the kernel will
numbered these CPUs using the first CPU ids available which are the ones
freed when removing the second CPU of the node 0. This leads to the CPU ids
16-23 to move from the node 1 to the node 2. With the patch applied, the
CPU ids 32-39 are used since they are the lowest free ones which have not
been used on another node.
At boot time:
[root@vm40 ~]# numactl -H | grep cpus
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 1 cpus: 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
node 2 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Vanilla kernel, after the CPU hot unplug/plug operations:
[root@vm40 ~]# numactl -H | grep cpus
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 1 cpus: 24 25 26 27 28 29 30 31
node 2 cpus: 16 17 18 19 20 21 22 23 40 41 42 43 44 45 46 47
Patched kernel, after the CPU hot unplug/plug operations:
[root@vm40 ~]# numactl -H | grep cpus
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 1 cpus: 24 25 26 27 28 29 30 31
node 2 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Changes since V3, addressing Nathan's comment:
- Rename the local variable named 'nid' into 'assigned_node'
Changes since V2, addressing Nathan's comments:
- Remove the retry feature
- Reduce the number of local variables (removing 'i')
- Add comment about the cpu_add_remove_lock protecting the added CPU mask.
Changes since V1 (no functional changes):
- update the test's output in the commit's description
- node_recorded_ids_map should be static
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
I did further LPM tests with this patch applied and not allowing fall back
reusing free ids of another node is too strong.
This is easy to hit that limitation when a LPAR is running at the maximum number
of CPU it is configured for and when a LPAR migration leads to new node activation.
For instance, consider a dedicated LPAR configured with a max of 32 CPUs (4
cores SMT 8). At boot time, cpu_possible_mask is filled with CPU ids 0-31 in
smp_setup_cpu_maps() by reading the DT property "/rtas/ibm,lrdr-capacity", so
the higher CPU id for this LPAR is 31.
Departure box:
node 0 : CPU 0-31
Arrival box:
node 0 : CPU 0-15
node 1 : CPU 16-31 << need to reuse ids from node 0
Visualizing the CPU ids would have a big impact as it is used in several places
in the kernel as to index linear table.
But in the case the LPAR is migratable (DT property "ibm,migratable-partition"
is present), we may set the higher CPU ids to NR_CPUS (usually 2048), to limit
the case where CPU id has to be reused on a different node. Doing this will have
impact on some data allocation done in the kernel when the size is based on
num_possible_cpus.
Any better idea?
Thanks,
Laurent.