Re: [PATCH 9/9] contrib/plugins: add ips plugin example for cost modeling

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Alex Bennée (alex.bennee@linaro.org) wrote:
> From: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> 
> This plugin uses the new time control interface to make decisions
> about the state of time during the emulation. The algorithm is
> currently very simple. The user specifies an ips rate which applies
> per core. If the core runs ahead of its allocated execution time the
> plugin sleeps for a bit to let real time catch up. Either way time is
> updated for the emulation as a function of total executed instructions
> with some adjustments for cores that idle.

A few random thoughts:
  a) Are there any definitions of what a plugin that controls time
     should do with a live migration?
  b) The sleep in migration/dirtyrate.c points out g_usleep might
     sleep for longer, so reads the actual wall clock time to
     figure out a new 'now'.
  c) A fun thing to do with this would be to follow an external simulation
     or 2nd qemu, trying to keep the two from running too far past
     each other.

Dave

> Examples
> --------
> 
> Slow down execution of /bin/true:
> $ num_insn=$(./build/qemu-x86_64 -plugin ./build/tests/plugin/libinsn.so -d plugin /bin/true |& grep total | sed -e 's/.*: //')
> $ time ./build/qemu-x86_64 -plugin ./build/contrib/plugins/libips.so,ips=$(($num_insn/4)) /bin/true
> real 4.000s
> 
> Boot a Linux kernel simulating a 250MHz cpu:
> $ /build/qemu-system-x86_64 -kernel /boot/vmlinuz-6.1.0-21-amd64 -append "console=ttyS0" -plugin ./build/contrib/plugins/libips.so,ips=$((250*1000*1000)) -smp 1 -m 512
> check time until kernel panic on serial0
> 
> Tested in system mode by booting a full debian system, and using:
> $ sysbench cpu run
> Performance decrease linearly with the given number of ips.
> 
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> Message-Id: <20240530220610.1245424-7-pierrick.bouvier@linaro.org>
> ---
>  contrib/plugins/ips.c    | 164 +++++++++++++++++++++++++++++++++++++++
>  contrib/plugins/Makefile |   1 +
>  2 files changed, 165 insertions(+)
>  create mode 100644 contrib/plugins/ips.c
> 
> diff --git a/contrib/plugins/ips.c b/contrib/plugins/ips.c
> new file mode 100644
> index 0000000000..db77729264
> --- /dev/null
> +++ b/contrib/plugins/ips.c
> @@ -0,0 +1,164 @@
> +/*
> + * ips rate limiting plugin.
> + *
> + * This plugin can be used to restrict the execution of a system to a
> + * particular number of Instructions Per Second (ips). This controls
> + * time as seen by the guest so while wall-clock time may be longer
> + * from the guests point of view time will pass at the normal rate.
> + *
> + * This uses the new plugin API which allows the plugin to control
> + * system time.
> + *
> + * Copyright (c) 2023 Linaro Ltd
> + *
> + * SPDX-License-Identifier: GPL-2.0-or-later
> + */
> +
> +#include <stdio.h>
> +#include <glib.h>
> +#include <qemu-plugin.h>
> +
> +QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
> +
> +/* how many times do we update time per sec */
> +#define NUM_TIME_UPDATE_PER_SEC 10
> +#define NSEC_IN_ONE_SEC (1000 * 1000 * 1000)
> +
> +static GMutex global_state_lock;
> +
> +static uint64_t max_insn_per_second = 1000 * 1000 * 1000; /* ips per core, per second */
> +static uint64_t max_insn_per_quantum; /* trap every N instructions */
> +static int64_t virtual_time_ns; /* last set virtual time */
> +
> +static const void *time_handle;
> +
> +typedef struct {
> +    uint64_t total_insn;
> +    uint64_t quantum_insn; /* insn in last quantum */
> +    int64_t last_quantum_time; /* time when last quantum started */
> +} vCPUTime;
> +
> +struct qemu_plugin_scoreboard *vcpus;
> +
> +/* return epoch time in ns */
> +static int64_t now_ns(void)
> +{
> +    return g_get_real_time() * 1000;
> +}
> +
> +static uint64_t num_insn_during(int64_t elapsed_ns)
> +{
> +    double num_secs = elapsed_ns / (double) NSEC_IN_ONE_SEC;
> +    return num_secs * (double) max_insn_per_second;
> +}
> +
> +static int64_t time_for_insn(uint64_t num_insn)
> +{
> +    double num_secs = (double) num_insn / (double) max_insn_per_second;
> +    return num_secs * (double) NSEC_IN_ONE_SEC;
> +}
> +
> +static void update_system_time(vCPUTime *vcpu)
> +{
> +    int64_t elapsed_ns = now_ns() - vcpu->last_quantum_time;
> +    uint64_t max_insn = num_insn_during(elapsed_ns);
> +
> +    if (vcpu->quantum_insn >= max_insn) {
> +        /* this vcpu ran faster than expected, so it has to sleep */
> +        uint64_t insn_advance = vcpu->quantum_insn - max_insn;
> +        uint64_t time_advance_ns = time_for_insn(insn_advance);
> +        int64_t sleep_us = time_advance_ns / 1000;
> +        g_usleep(sleep_us);
> +    }
> +
> +    vcpu->total_insn += vcpu->quantum_insn;
> +    vcpu->quantum_insn = 0;
> +    vcpu->last_quantum_time = now_ns();
> +
> +    /* based on total number of instructions, what should be the new time? */
> +    int64_t new_virtual_time = time_for_insn(vcpu->total_insn);
> +
> +    g_mutex_lock(&global_state_lock);
> +
> +    /* Time only moves forward. Another vcpu might have updated it already. */
> +    if (new_virtual_time > virtual_time_ns) {
> +        qemu_plugin_update_ns(time_handle, new_virtual_time);
> +        virtual_time_ns = new_virtual_time;
> +    }
> +
> +    g_mutex_unlock(&global_state_lock);
> +}
> +
> +static void vcpu_init(qemu_plugin_id_t id, unsigned int cpu_index)
> +{
> +    vCPUTime *vcpu = qemu_plugin_scoreboard_find(vcpus, cpu_index);
> +    vcpu->total_insn = 0;
> +    vcpu->quantum_insn = 0;
> +    vcpu->last_quantum_time = now_ns();
> +}
> +
> +static void vcpu_exit(qemu_plugin_id_t id, unsigned int cpu_index)
> +{
> +    vCPUTime *vcpu = qemu_plugin_scoreboard_find(vcpus, cpu_index);
> +    update_system_time(vcpu);
> +}
> +
> +static void every_quantum_insn(unsigned int cpu_index, void *udata)
> +{
> +    vCPUTime *vcpu = qemu_plugin_scoreboard_find(vcpus, cpu_index);
> +    g_assert(vcpu->quantum_insn >= max_insn_per_quantum);
> +    update_system_time(vcpu);
> +}
> +
> +static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
> +{
> +    size_t n_insns = qemu_plugin_tb_n_insns(tb);
> +    qemu_plugin_u64 quantum_insn =
> +        qemu_plugin_scoreboard_u64_in_struct(vcpus, vCPUTime, quantum_insn);
> +    /* count (and eventually trap) once per tb */
> +    qemu_plugin_register_vcpu_tb_exec_inline_per_vcpu(
> +        tb, QEMU_PLUGIN_INLINE_ADD_U64, quantum_insn, n_insns);
> +    qemu_plugin_register_vcpu_tb_exec_cond_cb(
> +        tb, every_quantum_insn,
> +        QEMU_PLUGIN_CB_NO_REGS, QEMU_PLUGIN_COND_GE,
> +        quantum_insn, max_insn_per_quantum, NULL);
> +}
> +
> +static void plugin_exit(qemu_plugin_id_t id, void *udata)
> +{
> +    qemu_plugin_scoreboard_free(vcpus);
> +}
> +
> +QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
> +                                           const qemu_info_t *info, int argc,
> +                                           char **argv)
> +{
> +    for (int i = 0; i < argc; i++) {
> +        char *opt = argv[i];
> +        g_auto(GStrv) tokens = g_strsplit(opt, "=", 2);
> +        if (g_strcmp0(tokens[0], "ips") == 0) {
> +            max_insn_per_second = g_ascii_strtoull(tokens[1], NULL, 10);
> +            if (!max_insn_per_second && errno) {
> +                fprintf(stderr, "%s: couldn't parse %s (%s)\n",
> +                        __func__, tokens[1], g_strerror(errno));
> +                return -1;
> +            }
> +        } else {
> +            fprintf(stderr, "option parsing failed: %s\n", opt);
> +            return -1;
> +        }
> +    }
> +
> +    vcpus = qemu_plugin_scoreboard_new(sizeof(vCPUTime));
> +    max_insn_per_quantum = max_insn_per_second / NUM_TIME_UPDATE_PER_SEC;
> +
> +    time_handle = qemu_plugin_request_time_control();
> +    g_assert(time_handle);
> +
> +    qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
> +    qemu_plugin_register_vcpu_init_cb(id, vcpu_init);
> +    qemu_plugin_register_vcpu_exit_cb(id, vcpu_exit);
> +    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
> +
> +    return 0;
> +}
> diff --git a/contrib/plugins/Makefile b/contrib/plugins/Makefile
> index 0b64d2c1e3..449ead1130 100644
> --- a/contrib/plugins/Makefile
> +++ b/contrib/plugins/Makefile
> @@ -27,6 +27,7 @@ endif
>  NAMES += hwprofile
>  NAMES += cache
>  NAMES += drcov
> +NAMES += ips
>  
>  ifeq ($(CONFIG_WIN32),y)
>  SO_SUFFIX := .dll
> -- 
> 2.39.2
> 
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
 \ _________________________|_____ http://www.treblig.org   |_______/