265 lines
13 KiB
Text
265 lines
13 KiB
Text
REDUCING OS JITTER DUE TO PER-CPU KTHREADS
|
|
|
|
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.
|
|
|
|
|
|
REFERENCES
|
|
|
|
o Documentation/IRQ-affinity.txt: Binding interrupts to sets of CPUs.
|
|
|
|
o Documentation/cgroups: Using cgroups to bind tasks to sets of CPUs.
|
|
|
|
o man taskset: Using the taskset command to bind tasks to sets
|
|
of CPUs.
|
|
|
|
o man sched_setaffinity: Using the sched_setaffinity() system
|
|
call to bind tasks to sets of CPUs.
|
|
|
|
o /sys/devices/system/cpu/cpuN/online: Control CPU N's hotplug state,
|
|
writing "0" to offline and "1" to online.
|
|
|
|
o In order to locate kernel-generated OS jitter on CPU N:
|
|
|
|
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
|
|
|
|
|
|
KTHREADS
|
|
|
|
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.
|
|
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
Name: kworker/%u:%d%s (cpu, id, priority)
|
|
Purpose: Execute workqueue requests
|
|
To reduce its OS jitter, do any of the following:
|
|
1. Run your workload at a real-time priority, which will allow
|
|
preempting the kworker daemons.
|
|
2. A given workqueue can be made visible in the sysfs filesystem
|
|
by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
|
|
Such a workqueue can be confined to a given subset of the
|
|
CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
|
|
files. The set of WQ_SYSFS workqueues can be displayed using
|
|
"ls sys/devices/virtual/workqueue". That said, the workqueues
|
|
maintainer would like to caution people against indiscriminately
|
|
sprinkling WQ_SYSFS across all the workqueues. The reason for
|
|
caution is that it is easy to add WQ_SYSFS, but because sysfs is
|
|
part of the formal user/kernel API, it can be nearly impossible
|
|
to remove it, even if its addition was a mistake.
|
|
3. Do any of the following needed to avoid jitter that your
|
|
application cannot tolerate:
|
|
a. Build your kernel with CONFIG_SLUB=y rather than
|
|
CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
|
|
use of each CPU's workqueues to run its cache_reap()
|
|
function.
|
|
b. Avoid using oprofile, thus avoiding OS jitter from
|
|
wq_sync_buffer().
|
|
c. Limit your CPU frequency so that a CPU-frequency
|
|
governor is not required, possibly enlisting the aid of
|
|
special heatsinks or other cooling technologies. If done
|
|
correctly, and if you CPU architecture permits, you should
|
|
be able to build your kernel with CONFIG_CPU_FREQ=n to
|
|
avoid the CPU-frequency governor periodically running
|
|
on each CPU, including cs_dbs_timer() and od_dbs_timer().
|
|
WARNING: Please check your CPU specifications to
|
|
make sure that this is safe on your particular system.
|
|
d. It is not possible to entirely get rid of OS jitter
|
|
from vmstat_update() on CONFIG_SMP=y systems, but you
|
|
can decrease its frequency by writing a large value
|
|
to /proc/sys/vm/stat_interval. The default value is
|
|
HZ, for an interval of one second. Of course, larger
|
|
values will make your virtual-memory statistics update
|
|
more slowly. Of course, you can also run your workload
|
|
at a real-time priority, thus preempting vmstat_update(),
|
|
but if your workload is CPU-bound, this is a bad idea.
|
|
However, there is an RFC patch from Christoph Lameter
|
|
(based on an earlier one from Gilad Ben-Yossef) that
|
|
reduces or even eliminates vmstat overhead for some
|
|
workloads at https://lkml.org/lkml/2013/9/4/379.
|
|
e. If running on high-end powerpc servers, build with
|
|
CONFIG_PPC_RTAS_DAEMON=n. This prevents the RTAS
|
|
daemon from running on each CPU every second or so.
|
|
(This will require editing Kconfig files and will defeat
|
|
this platform's RAS functionality.) This avoids jitter
|
|
due to the rtas_event_scan() function.
|
|
WARNING: Please check your CPU specifications to
|
|
make sure that this is safe on your particular system.
|
|
f. If running on Cell Processor, build your kernel with
|
|
CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
|
|
spu_gov_work().
|
|
WARNING: Please check your CPU specifications to
|
|
make sure that this is safe on your particular system.
|
|
g. If running on PowerMAC, build your kernel with
|
|
CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
|
|
avoiding OS jitter from rackmeter_do_timer().
|
|
|
|
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.
|
|
|
|
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_CPU=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.
|
|
|
|
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.
|