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We want these fixes. Signed-off-by: Greg Kroah-Hartman <[email protected]>
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Documentation/devicetree/bindings/video/simple-framebuffer.txt
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Simple Framebuffer | ||
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A simple frame-buffer describes a raw memory region that may be rendered to, | ||
with the assumption that the display hardware has already been set up to scan | ||
out from that buffer. | ||
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Required properties: | ||
- compatible: "simple-framebuffer" | ||
- reg: Should contain the location and size of the framebuffer memory. | ||
- width: The width of the framebuffer in pixels. | ||
- height: The height of the framebuffer in pixels. | ||
- stride: The number of bytes in each line of the framebuffer. | ||
- format: The format of the framebuffer surface. Valid values are: | ||
- r5g6b5 (16-bit pixels, d[15:11]=r, d[10:5]=g, d[4:0]=b). | ||
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Example: | ||
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framebuffer { | ||
compatible = "simple-framebuffer"; | ||
reg = <0x1d385000 (1600 * 1200 * 2)>; | ||
width = <1600>; | ||
height = <1200>; | ||
stride = <(1600 * 2)>; | ||
format = "r5g6b5"; | ||
}; |
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REDUCING OS JITTER DUE TO PER-CPU KTHREADS | ||
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This document lists per-CPU kthreads in the Linux kernel and presents | ||
options to control their OS jitter. Note that non-per-CPU kthreads are | ||
not listed here. To reduce OS jitter from non-per-CPU kthreads, bind | ||
them to a "housekeeping" CPU dedicated to such work. | ||
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REFERENCES | ||
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o Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs. | ||
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o Documentation/cgroups: Using cgroups to bind tasks to sets of CPUs. | ||
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o man taskset: Using the taskset command to bind tasks to sets | ||
of CPUs. | ||
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o man sched_setaffinity: Using the sched_setaffinity() system | ||
call to bind tasks to sets of CPUs. | ||
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o /sys/devices/system/cpu/cpuN/online: Control CPU N's hotplug state, | ||
writing "0" to offline and "1" to online. | ||
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o In order to locate kernel-generated OS jitter on CPU N: | ||
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cd /sys/kernel/debug/tracing | ||
echo 1 > max_graph_depth # Increase the "1" for more detail | ||
echo function_graph > current_tracer | ||
# run workload | ||
cat per_cpu/cpuN/trace | ||
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KTHREADS | ||
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Name: ehca_comp/%u | ||
Purpose: Periodically process Infiniband-related work. | ||
To reduce its OS jitter, do any of the following: | ||
1. Don't use eHCA Infiniband hardware, instead choosing hardware | ||
that does not require per-CPU kthreads. This will prevent these | ||
kthreads from being created in the first place. (This will | ||
work for most people, as this hardware, though important, is | ||
relatively old and is produced in relatively low unit volumes.) | ||
2. Do all eHCA-Infiniband-related work on other CPUs, including | ||
interrupts. | ||
3. Rework the eHCA driver so that its per-CPU kthreads are | ||
provisioned only on selected CPUs. | ||
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Name: irq/%d-%s | ||
Purpose: Handle threaded interrupts. | ||
To reduce its OS jitter, do the following: | ||
1. Use irq affinity to force the irq threads to execute on | ||
some other CPU. | ||
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Name: kcmtpd_ctr_%d | ||
Purpose: Handle Bluetooth work. | ||
To reduce its OS jitter, do one of the following: | ||
1. Don't use Bluetooth, in which case these kthreads won't be | ||
created in the first place. | ||
2. Use irq affinity to force Bluetooth-related interrupts to | ||
occur on some other CPU and furthermore initiate all | ||
Bluetooth activity on some other CPU. | ||
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Name: ksoftirqd/%u | ||
Purpose: Execute softirq handlers when threaded or when under heavy load. | ||
To reduce its OS jitter, each softirq vector must be handled | ||
separately as follows: | ||
TIMER_SOFTIRQ: Do all of the following: | ||
1. To the extent possible, keep the CPU out of the kernel when it | ||
is non-idle, for example, by avoiding system calls and by forcing | ||
both kernel threads and interrupts to execute elsewhere. | ||
2. Build with CONFIG_HOTPLUG_CPU=y. After boot completes, force | ||
the CPU offline, then bring it back online. This forces | ||
recurring timers to migrate elsewhere. If you are concerned | ||
with multiple CPUs, force them all offline before bringing the | ||
first one back online. Once you have onlined the CPUs in question, | ||
do not offline any other CPUs, because doing so could force the | ||
timer back onto one of the CPUs in question. | ||
NET_TX_SOFTIRQ and NET_RX_SOFTIRQ: Do all of the following: | ||
1. Force networking interrupts onto other CPUs. | ||
2. Initiate any network I/O on other CPUs. | ||
3. Once your application has started, prevent CPU-hotplug operations | ||
from being initiated from tasks that might run on the CPU to | ||
be de-jittered. (It is OK to force this CPU offline and then | ||
bring it back online before you start your application.) | ||
BLOCK_SOFTIRQ: Do all of the following: | ||
1. Force block-device interrupts onto some other CPU. | ||
2. Initiate any block I/O on other CPUs. | ||
3. Once your application has started, prevent CPU-hotplug operations | ||
from being initiated from tasks that might run on the CPU to | ||
be de-jittered. (It is OK to force this CPU offline and then | ||
bring it back online before you start your application.) | ||
BLOCK_IOPOLL_SOFTIRQ: Do all of the following: | ||
1. Force block-device interrupts onto some other CPU. | ||
2. Initiate any block I/O and block-I/O polling on other CPUs. | ||
3. Once your application has started, prevent CPU-hotplug operations | ||
from being initiated from tasks that might run on the CPU to | ||
be de-jittered. (It is OK to force this CPU offline and then | ||
bring it back online before you start your application.) | ||
TASKLET_SOFTIRQ: Do one or more of the following: | ||
1. Avoid use of drivers that use tasklets. (Such drivers will contain | ||
calls to things like tasklet_schedule().) | ||
2. Convert all drivers that you must use from tasklets to workqueues. | ||
3. Force interrupts for drivers using tasklets onto other CPUs, | ||
and also do I/O involving these drivers on other CPUs. | ||
SCHED_SOFTIRQ: Do all of the following: | ||
1. Avoid sending scheduler IPIs to the CPU to be de-jittered, | ||
for example, ensure that at most one runnable kthread is present | ||
on that CPU. If a thread that expects to run on the de-jittered | ||
CPU awakens, the scheduler will send an IPI that can result in | ||
a subsequent SCHED_SOFTIRQ. | ||
2. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y, | ||
CONFIG_NO_HZ_FULL=y, and, in addition, ensure that the CPU | ||
to be de-jittered is marked as an adaptive-ticks CPU using the | ||
"nohz_full=" boot parameter. This reduces the number of | ||
scheduler-clock interrupts that the de-jittered CPU receives, | ||
minimizing its chances of being selected to do the load balancing | ||
work that runs in SCHED_SOFTIRQ context. | ||
3. To the extent possible, keep the CPU out of the kernel when it | ||
is non-idle, for example, by avoiding system calls and by | ||
forcing both kernel threads and interrupts to execute elsewhere. | ||
This further reduces the number of scheduler-clock interrupts | ||
received by the de-jittered CPU. | ||
HRTIMER_SOFTIRQ: Do all of the following: | ||
1. To the extent possible, keep the CPU out of the kernel when it | ||
is non-idle. For example, avoid system calls and force both | ||
kernel threads and interrupts to execute elsewhere. | ||
2. Build with CONFIG_HOTPLUG_CPU=y. Once boot completes, force the | ||
CPU offline, then bring it back online. This forces recurring | ||
timers to migrate elsewhere. If you are concerned with multiple | ||
CPUs, force them all offline before bringing the first one | ||
back online. Once you have onlined the CPUs in question, do not | ||
offline any other CPUs, because doing so could force the timer | ||
back onto one of the CPUs in question. | ||
RCU_SOFTIRQ: Do at least one of the following: | ||
1. Offload callbacks and keep the CPU in either dyntick-idle or | ||
adaptive-ticks state by doing all of the following: | ||
a. Build with CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_ALL=y, | ||
CONFIG_NO_HZ_FULL=y, and, in addition ensure that the CPU | ||
to be de-jittered is marked as an adaptive-ticks CPU using | ||
the "nohz_full=" boot parameter. Bind the rcuo kthreads | ||
to housekeeping CPUs, which can tolerate OS jitter. | ||
b. To the extent possible, keep the CPU out of the kernel | ||
when it is non-idle, for example, by avoiding system | ||
calls and by forcing both kernel threads and interrupts | ||
to execute elsewhere. | ||
2. Enable RCU to do its processing remotely via dyntick-idle by | ||
doing all of the following: | ||
a. Build with CONFIG_NO_HZ=y and CONFIG_RCU_FAST_NO_HZ=y. | ||
b. Ensure that the CPU goes idle frequently, allowing other | ||
CPUs to detect that it has passed through an RCU quiescent | ||
state. If the kernel is built with CONFIG_NO_HZ_FULL=y, | ||
userspace execution also allows other CPUs to detect that | ||
the CPU in question has passed through a quiescent state. | ||
c. To the extent possible, keep the CPU out of the kernel | ||
when it is non-idle, for example, by avoiding system | ||
calls and by forcing both kernel threads and interrupts | ||
to execute elsewhere. | ||
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Name: rcuc/%u | ||
Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels. | ||
To reduce its OS jitter, do at least one of the following: | ||
1. Build the kernel with CONFIG_PREEMPT=n. This prevents these | ||
kthreads from being created in the first place, and also obviates | ||
the need for RCU priority boosting. This approach is feasible | ||
for workloads that do not require high degrees of responsiveness. | ||
2. Build the kernel with CONFIG_RCU_BOOST=n. This prevents these | ||
kthreads from being created in the first place. This approach | ||
is feasible only if your workload never requires RCU priority | ||
boosting, for example, if you ensure frequent idle time on all | ||
CPUs that might execute within the kernel. | ||
3. Build with CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y, | ||
which offloads all RCU callbacks to kthreads that can be moved | ||
off of CPUs susceptible to OS jitter. This approach prevents the | ||
rcuc/%u kthreads from having any work to do, so that they are | ||
never awakened. | ||
4. Ensure that the CPU never enters the kernel, and, in particular, | ||
avoid initiating any CPU hotplug operations on this CPU. This is | ||
another way of preventing any callbacks from being queued on the | ||
CPU, again preventing the rcuc/%u kthreads from having any work | ||
to do. | ||
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Name: rcuob/%d, rcuop/%d, and rcuos/%d | ||
Purpose: Offload RCU callbacks from the corresponding CPU. | ||
To reduce its OS jitter, do at least one of the following: | ||
1. Use affinity, cgroups, or other mechanism to force these kthreads | ||
to execute on some other CPU. | ||
2. Build with CONFIG_RCU_NOCB_CPUS=n, which will prevent these | ||
kthreads from being created in the first place. However, please | ||
note that this will not eliminate OS jitter, but will instead | ||
shift it to RCU_SOFTIRQ. | ||
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Name: watchdog/%u | ||
Purpose: Detect software lockups on each CPU. | ||
To reduce its OS jitter, do at least one of the following: | ||
1. Build with CONFIG_LOCKUP_DETECTOR=n, which will prevent these | ||
kthreads from being created in the first place. | ||
2. Echo a zero to /proc/sys/kernel/watchdog to disable the | ||
watchdog timer. | ||
3. Echo a large number of /proc/sys/kernel/watchdog_thresh in | ||
order to reduce the frequency of OS jitter due to the watchdog | ||
timer down to a level that is acceptable for your workload. |
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