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Re: [PATCH v6 3/3] cpus-common: implement dirty page limit on vCPU
From: |
Markus Armbruster |
Subject: |
Re: [PATCH v6 3/3] cpus-common: implement dirty page limit on vCPU |
Date: |
Fri, 26 Nov 2021 08:03:43 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) |
huangy81@chinatelecom.cn writes:
> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>
> Implement dirtyrate calculation periodically basing on
> dirty-ring and throttle vCPU until it reachs the quota
> dirty page rate given by user.
>
> Introduce qmp commands set-dirty-limit/cancel-dirty-limit to
> set/cancel dirty page limit on vCPU.
>
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> ---
> cpus-common.c | 41 +++++++++++++++++++++++++++++++++++++++++
> include/hw/core/cpu.h | 9 +++++++++
> qapi/migration.json | 47 +++++++++++++++++++++++++++++++++++++++++++++++
> softmmu/vl.c | 1 +
> 4 files changed, 98 insertions(+)
>
> diff --git a/cpus-common.c b/cpus-common.c
> index 6e73d3e..3c156b3 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -23,6 +23,11 @@
> #include "hw/core/cpu.h"
> #include "sysemu/cpus.h"
> #include "qemu/lockable.h"
> +#include "sysemu/dirtylimit.h"
> +#include "sysemu/cpu-throttle.h"
> +#include "sysemu/kvm.h"
> +#include "qapi/error.h"
> +#include "qapi/qapi-commands-migration.h"
>
> static QemuMutex qemu_cpu_list_lock;
> static QemuCond exclusive_cond;
> @@ -352,3 +357,39 @@ void process_queued_cpu_work(CPUState *cpu)
> qemu_mutex_unlock(&cpu->work_mutex);
> qemu_cond_broadcast(&qemu_work_cond);
> }
> +
> +void qmp_set_dirty_limit(int64_t idx,
> + uint64_t dirtyrate,
> + Error **errp)
> +{
> + if (!kvm_enabled() || !kvm_dirty_ring_enabled()) {
> + error_setg(errp, "setting a dirty page limit requires support from
> dirty ring");
Can we phrase the message in a way that gives the user a chance to guess
what he needs to do to avoid it?
Perhaps: "setting a dirty page limit requires KVM with accelerator
property 'dirty-ring-size' set".
> + return;
> + }
> +
> + dirtylimit_calc();
> + dirtylimit_vcpu(idx, dirtyrate);
> +}
> +
> +void qmp_cancel_dirty_limit(int64_t idx,
> + Error **errp)
> +{
Three cases:
Case 1: enable is impossible, so nothing to do.
Case 2: enable is possible and we actually enabled.
Case 3: enable is possible, but we didn't. Nothing to do.
> + if (!kvm_enabled() || !kvm_dirty_ring_enabled()) {
> + error_setg(errp, "no need to cancel a dirty page limit as dirty ring
> not enabled");
> + return;
This is case 1. We error out.
> + }
> +
> + if (unlikely(!dirtylimit_cancel_vcpu(idx))) {
I don't think unlikely() matters here.
> + dirtylimit_calc_quit();
> + }
In case 2, dirtylimit_calc_quit() returns zero if this was the last
limit, else non-zero. If the former, we request the thread to stop.
In case 3, dirtylimit_calc_quit() returns zero, and we do nothing.
Why is case 1 and error, but case 3 isn't?
Both could silently do nothing, like case 3 does now.
Both could error out, like case 1 does now. A possible common error
message: "there is no dirty page limit to cancel".
I'd be okay with consistently doing nothing, and with consistently
erroring out.
> +}
> +
> +void dirtylimit_setup(int max_cpus)
> +{
> + if (!kvm_enabled() || !kvm_dirty_ring_enabled()) {
> + return;
> + }
> +
> + dirtylimit_calc_state_init(max_cpus);
> + dirtylimit_state_init(max_cpus);
> +}
> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> index e948e81..11df012 100644
> --- a/include/hw/core/cpu.h
> +++ b/include/hw/core/cpu.h
> @@ -881,6 +881,15 @@ void end_exclusive(void);
> */
> void qemu_init_vcpu(CPUState *cpu);
>
> +/**
> + * dirtylimit_setup:
> + *
> + * Initializes the global state of dirtylimit calculation and
> + * dirtylimit itself. This is prepared for vCPU dirtylimit which
> + * could be triggered during vm lifecycle.
> + */
> +void dirtylimit_setup(int max_cpus);
> +
> #define SSTEP_ENABLE 0x1 /* Enable simulated HW single stepping */
> #define SSTEP_NOIRQ 0x2 /* Do not use IRQ while single stepping */
> #define SSTEP_NOTIMER 0x4 /* Do not Timers while single stepping */
> diff --git a/qapi/migration.json b/qapi/migration.json
> index bbfd48c..2b0fe19 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1850,6 +1850,53 @@
> { 'command': 'query-dirty-rate', 'returns': 'DirtyRateInfo' }
>
> ##
> +# @set-dirty-limit:
> +#
> +# Set the upper limit of dirty page rate for a vCPU.
> +#
> +# This command could be used to cap the vCPU memory load, which is also
"Could be used" suggests there are other uses. I don't think there are.
> +# refered as "dirty page rate". Users can use set-dirty-limit
> unconditionally,
> +# but if one want to know which vCPU is in high memory load and which vCPU
"one wants"
> +# should be limited, using calc-dirty-rate command with dirty-ring mode maybe
"may be"
> +# an availiable method. Note that this command requires support from dirty
"available". Consider using a spell checker.
> +# ring, which means property "dirty-ring-size" of accelerator object "kvm"
> +# must be set.
Please limit doc comment line length to approximately 70 characters for
legibility.
Here's my try:
##
# @set-dirty-limit:
#
# Set the upper limit of dirty page rate for a virtual CPU.
#
# Requires KVM with accelerator property "dirty-ring-size" set.
# A virtual CPU's dirty page rate is a measure of its memory load.
# To observe dirty page rates, use @calc-dirty-rate.
> +#
> +# @cpu-index: vCPU index to set dirty page limit.
I'd go with
# @cpu-index: index of the virtual CPU.
> +#
> +# @dirty-rate: upper limit for the specified vCPU's dirty page rate (MB/s)
> +#
> +# Since: 7.0
> +#
> +# Example:
> +# {"execute": "set-dirty-limit"}
> +# "arguments": { "cpu-index": 0,
> +# "dirty-rate": 200 } }
> +#
> +##
> +{ 'command': 'set-dirty-limit',
> + 'data': { 'cpu-index': 'int', 'dirty-rate': 'uint64' } }
> +
> +##
> +# @cancel-dirty-limit:
> +#
> +# Cancel the dirty page limit for the vCPU which has been set with
> +# set-dirty-limit command. Note that this command requires support from
> +# dirty ring, same as the "set-dirty-limit" command.
> +#
> +# @cpu-index: vCPU index to cancel the dirty page limit
> +#
> +# Since: 7.0
> +#
> +# Example:
> +# {"execute": "cancel-dirty-limit"}
> +# "arguments": { "cpu-index": 0 } }
> +#
> +##
> +{ 'command': 'cancel-dirty-limit',
> + 'data': { 'cpu-index': 'int' } }
> +
> +##
> # @snapshot-save:
> #
> # Save a VM snapshot
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index 620a1f1..0f83ce3 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -3777,5 +3777,6 @@ void qemu_init(int argc, char **argv, char **envp)
> qemu_init_displays();
> accel_setup_post(current_machine);
> os_setup_post();
> + dirtylimit_setup(current_machine->smp.max_cpus);
> resume_mux_open();
> }