Re: [PATCH v6 3/3] cpus-common: implement dirty page limit on vCPU

From: Markus Armbruster <armbru@redhat.com>
To: huangy81@chinatelecom.cn
Cc: "Juan Quintela" <quintela@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	qemu-devel <qemu-devel@nongnu.org>,
	"Peter Xu" <peterx@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Philippe Mathieu-Daudé" <philmd@redhat.com>
Subject: Re: [PATCH v6 3/3] cpus-common: implement dirty page limit on vCPU
Date: Fri, 26 Nov 2021 08:03:43 +0100	[thread overview]
Message-ID: <87tufzbdps.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <91286fcfc24795385fe4df0d69a48c34f8eac942.1637857372.git.huangy81@chinatelecom.cn> (huangy's message of "Fri, 26 Nov 2021 00:27:52 +0800")

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();
>  }