Commit a9cad3d4 authored by Vikas Shivappa's avatar Vikas Shivappa Committed by Thomas Gleixner

Documentation, x86: Intel Memory bandwidth allocation

Update the 'intel_rdt_ui' documentation to have Memory bandwidth(b/w)
allocation interface usage.
Signed-off-by: default avatarVikas Shivappa <vikas.shivappa@linux.intel.com>
Cc: ravi.v.shankar@intel.com
Cc: tony.luck@intel.com
Cc: fenghua.yu@intel.com
Cc: vikas.shivappa@intel.com
Link: http://lkml.kernel.org/r/1491611637-20417-2-git-send-email-vikas.shivappa@linux.intel.comSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 70a1ee92
......@@ -4,6 +4,7 @@ Copyright (C) 2016 Intel Corporation
Fenghua Yu <fenghua.yu@intel.com>
Tony Luck <tony.luck@intel.com>
Vikas Shivappa <vikas.shivappa@intel.com>
This feature is enabled by the CONFIG_INTEL_RDT_A Kconfig and the
X86 /proc/cpuinfo flag bits "rdt", "cat_l3" and "cdp_l3".
......@@ -22,19 +23,34 @@ Info directory
The 'info' directory contains information about the enabled
resources. Each resource has its own subdirectory. The subdirectory
names reflect the resource names. Each subdirectory contains the
following files:
names reflect the resource names.
Cache resource(L3/L2) subdirectory contains the following files:
"num_closids": The number of CLOSIDs which are valid for this
resource. The kernel uses the smallest number of
CLOSIDs of all enabled resources as limit.
"num_closids": The number of CLOSIDs which are valid for this
resource. The kernel uses the smallest number of
CLOSIDs of all enabled resources as limit.
"cbm_mask": The bitmask which is valid for this resource. This
mask is equivalent to 100%.
"cbm_mask": The bitmask which is valid for this resource.
This mask is equivalent to 100%.
"min_cbm_bits": The minimum number of consecutive bits which must be
set when writing a mask.
"min_cbm_bits": The minimum number of consecutive bits which
must be set when writing a mask.
Memory bandwitdh(MB) subdirectory contains the following files:
"min_bandwidth": The minimum memory bandwidth percentage which
user can request.
"bandwidth_gran": The granularity in which the memory bandwidth
percentage is allocated. The allocated
b/w percentage is rounded off to the next
control step available on the hardware. The
available bandwidth control steps are:
min_bandwidth + N * bandwidth_gran.
"delay_linear": Indicates if the delay scale is linear or
non-linear. This field is purely informational
only.
Resource groups
---------------
......@@ -110,6 +126,22 @@ and 0xA are not. On a system with a 20-bit mask each bit represents 5%
of the capacity of the cache. You could partition the cache into four
equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
Memory bandwidth(b/w) percentage
--------------------------------
For Memory b/w resource, user controls the resource by indicating the
percentage of total memory b/w.
The minimum bandwidth percentage value for each cpu model is predefined
and can be looked up through "info/MB/min_bandwidth". The bandwidth
granularity that is allocated is also dependent on the cpu model and can
be looked up at "info/MB/bandwidth_gran". The available bandwidth
control steps are: min_bw + N * bw_gran. Intermediate values are rounded
to the next control step available on the hardware.
The bandwidth throttling is a core specific mechanism on some of Intel
SKUs. Using a high bandwidth and a low bandwidth setting on two threads
sharing a core will result in both threads being throttled to use the
low bandwidth.
L3 details (code and data prioritization disabled)
--------------------------------------------------
......@@ -132,6 +164,13 @@ schemata format is always:
L2:<cache_id0>=<cbm>;<cache_id1>=<cbm>;...
Memory b/w Allocation details
-----------------------------
Memory b/w domain is L3 cache.
MB:<cache_id0>=bandwidth0;<cache_id1>=bandwidth1;...
Reading/writing the schemata file
---------------------------------
Reading the schemata file will show the state of all resources
......@@ -149,13 +188,14 @@ L3CODE:0=fffff;1=fffff;2=fffff;3=fffff
Example 1
---------
On a two socket machine (one L3 cache per socket) with just four bits
for cache bit masks
for cache bit masks, minimum b/w of 10% with a memory bandwidth
granularity of 10%
# mount -t resctrl resctrl /sys/fs/resctrl
# cd /sys/fs/resctrl
# mkdir p0 p1
# echo "L3:0=3;1=c" > /sys/fs/resctrl/p0/schemata
# echo "L3:0=3;1=3" > /sys/fs/resctrl/p1/schemata
# echo "L3:0=3;1=c\nMB:0=50;1=50" > /sys/fs/resctrl/p0/schemata
# echo "L3:0=3;1=3\nMB:0=50;1=50" > /sys/fs/resctrl/p1/schemata
The default resource group is unmodified, so we have access to all parts
of all caches (its schemata file reads "L3:0=f;1=f").
......@@ -164,6 +204,14 @@ Tasks that are under the control of group "p0" may only allocate from the
"lower" 50% on cache ID 0, and the "upper" 50% of cache ID 1.
Tasks in group "p1" use the "lower" 50% of cache on both sockets.
Similarly, tasks that are under the control of group "p0" may use a
maximum memory b/w of 50% on socket0 and 50% on socket 1.
Tasks in group "p1" may also use 50% memory b/w on both sockets.
Note that unlike cache masks, memory b/w cannot specify whether these
allocations can overlap or not. The allocations specifies the maximum
b/w that the group may be able to use and the system admin can configure
the b/w accordingly.
Example 2
---------
Again two sockets, but this time with a more realistic 20-bit mask.
......@@ -177,9 +225,10 @@ of L3 cache on socket 0.
# cd /sys/fs/resctrl
First we reset the schemata for the default group so that the "upper"
50% of the L3 cache on socket 0 cannot be used by ordinary tasks:
50% of the L3 cache on socket 0 and 50% of memory b/w cannot be used by
ordinary tasks:
# echo "L3:0=3ff;1=fffff" > schemata
# echo "L3:0=3ff;1=fffff\nMB:0=50;1=100" > schemata
Next we make a resource group for our first real time task and give
it access to the "top" 25% of the cache on socket 0.
......@@ -202,6 +251,20 @@ Ditto for the second real time task (with the remaining 25% of cache):
# echo 5678 > p1/tasks
# taskset -cp 2 5678
For the same 2 socket system with memory b/w resource and CAT L3 the
schemata would look like(Assume min_bandwidth 10 and bandwidth_gran is
10):
For our first real time task this would request 20% memory b/w on socket
0.
# echo -e "L3:0=f8000;1=fffff\nMB:0=20;1=100" > p0/schemata
For our second real time task this would request an other 20% memory b/w
on socket 0.
# echo -e "L3:0=f8000;1=fffff\nMB:0=20;1=100" > p0/schemata
Example 3
---------
......@@ -215,18 +278,22 @@ the tasks.
# cd /sys/fs/resctrl
First we reset the schemata for the default group so that the "upper"
50% of the L3 cache on socket 0 cannot be used by ordinary tasks:
50% of the L3 cache on socket 0, and 50% of memory bandwidth on socket 0
cannot be used by ordinary tasks:
# echo "L3:0=3ff" > schemata
# echo "L3:0=3ff\nMB:0=50" > schemata
Next we make a resource group for our real time cores and give
it access to the "top" 50% of the cache on socket 0.
Next we make a resource group for our real time cores and give it access
to the "top" 50% of the cache on socket 0 and 50% of memory bandwidth on
socket 0.
# mkdir p0
# echo "L3:0=ffc00;" > p0/schemata
# echo "L3:0=ffc00\nMB:0=50" > p0/schemata
Finally we move core 4-7 over to the new group and make sure that the
kernel and the tasks running there get 50% of the cache.
kernel and the tasks running there get 50% of the cache. They should
also get 50% of memory bandwidth assuming that the cores 4-7 are SMT
siblings and only the real time threads are scheduled on the cores 4-7.
# echo C0 > p0/cpus
......
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