The query-cpus command has an extremely serious side effect: it always interrupts all running vCPUs so that they can run ioctl calls. This can cause a huge performance degradation for some workloads. And most of the information retrieved by the ioctl calls are not even used by query-cpus.
This commit introduces a replacement for query-cpus called query-cpus-fast, which has the following features: o Never interrupt vCPUs threads. query-cpus-fast only returns vCPU information maintained by QEMU itself, which should be sufficient for most management software needs o Make "halted" field optional: we only return it if the halted state is maintained by QEMU. But this also gives the option of dropping the field in the future (see below) o Drop irrelevant fields such as "current", "pc" and "arch" o Rename some fields for better clarification & proper naming standard The "halted" field is somewhat controversial. On the one hand, it offers a convenient way to know if a guest CPU is idle or running. On the other hand, it's a field that can change many times a second. In fact, the halted state can change even before query-cpus-fast has returned. This makes one wonder if this field should be dropped all together. Having the "halted" field as optional gives a better option for dropping it in the future, since we can just stop returning it. Signed-off-by: Luiz Capitulino <lcapitul...@redhat.com> --- o v2 - Many English and naming corrections by Eric - Adopt query-cpus text from Daniel - squash query-cpus text change into this patch cpus.c | 44 ++++++++++++++++++++++++++++++ hmp-commands-info.hx | 14 ++++++++++ hmp.c | 24 ++++++++++++++++ hmp.h | 1 + qapi-schema.json | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 160 insertions(+) diff --git a/cpus.c b/cpus.c index 182caf764e..905b1f4d79 100644 --- a/cpus.c +++ b/cpus.c @@ -2150,6 +2150,50 @@ CpuInfoList *qmp_query_cpus(Error **errp) return head; } +/* + * fast means: we NEVER interrupt vCPU threads to retrieve + * information from KVM. + */ +CpuInfoFastList *qmp_query_cpus_fast(Error **errp) +{ + MachineState *ms = MACHINE(qdev_get_machine()); + MachineClass *mc = MACHINE_GET_CLASS(ms); + CpuInfoFastList *head = NULL, *cur_item = NULL; + CPUState *cpu; + + CPU_FOREACH(cpu) { + CpuInfoFastList *info = g_malloc0(sizeof(*info)); + info->value = g_malloc0(sizeof(*info->value)); + + info->value->cpu_index = cpu->cpu_index; + info->value->qom_path = object_get_canonical_path(OBJECT(cpu)); + info->value->thread_id = cpu->thread_id; + + info->value->has_props = !!mc->cpu_index_to_instance_props; + if (info->value->has_props) { + CpuInstanceProperties *props; + props = g_malloc0(sizeof(*props)); + *props = mc->cpu_index_to_instance_props(ms, cpu->cpu_index); + info->value->props = props; + } + + /* if in kernel irqchip is used, we don't have 'halted' */ + info->value->has_halted = !kvm_irqchip_in_kernel(); + if (info->value->has_halted) { + info->value->halted = cpu->halted; + } + + if (!cur_item) { + head = cur_item = info; + } else { + cur_item->next = info; + cur_item = info; + } + } + + return head; +} + void qmp_memsave(int64_t addr, int64_t size, const char *filename, bool has_cpu, int64_t cpu_index, Error **errp) { diff --git a/hmp-commands-info.hx b/hmp-commands-info.hx index ad590a4ffb..16ac60285f 100644 --- a/hmp-commands-info.hx +++ b/hmp-commands-info.hx @@ -157,6 +157,20 @@ STEXI @item info cpus @findex info cpus Show infos for each CPU. +ETEXI + + { + .name = "cpus_fast", + .args_type = "", + .params = "", + .help = "show information for each CPU without interrupting them", + .cmd = hmp_info_cpus_fast, + }, + +STEXI +@item info cpus_fast +@findex info cpus_fast +Show infos for each CPU without performance penalty. ETEXI { diff --git a/hmp.c b/hmp.c index b3de32d219..90717c13b2 100644 --- a/hmp.c +++ b/hmp.c @@ -404,6 +404,30 @@ void hmp_info_cpus(Monitor *mon, const QDict *qdict) qapi_free_CpuInfoList(cpu_list); } +void hmp_info_cpus_fast(Monitor *mon, const QDict *qdict) +{ + CpuInfoFastList *head, *cpu; + TargetInfo *target; + + target = qmp_query_target(NULL); + monitor_printf(mon, "CPU architecture is '%s'\n\n", target->arch); + qapi_free_TargetInfo(target); + + head = qmp_query_cpus_fast(NULL); + + for (cpu = head; cpu; cpu = cpu->next) { + monitor_printf(mon, "CPU%" PRId64 "\n", cpu->value->cpu_index); + monitor_printf(mon, " thread-id=%" PRId64 "\n", cpu->value->thread_id); + if (cpu->value->has_halted) { + monitor_printf(mon, " halted=%d\n", cpu->value->halted); + } + monitor_printf(mon, " qom-path=%s\n", cpu->value->qom_path); + monitor_printf(mon, "\n"); + } + + qapi_free_CpuInfoFastList(head); +} + static void print_block_info(Monitor *mon, BlockInfo *info, BlockDeviceInfo *inserted, bool verbose) { diff --git a/hmp.h b/hmp.h index 536cb91caa..f4ceba5cc8 100644 --- a/hmp.h +++ b/hmp.h @@ -31,6 +31,7 @@ void hmp_info_migrate_capabilities(Monitor *mon, const QDict *qdict); void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict); void hmp_info_migrate_cache_size(Monitor *mon, const QDict *qdict); void hmp_info_cpus(Monitor *mon, const QDict *qdict); +void hmp_info_cpus_fast(Monitor *mon, const QDict *qdict); void hmp_info_block(Monitor *mon, const QDict *qdict); void hmp_info_blockstats(Monitor *mon, const QDict *qdict); void hmp_info_vnc(Monitor *mon, const QDict *qdict); diff --git a/qapi-schema.json b/qapi-schema.json index 5c06745c79..9bc37ea604 100644 --- a/qapi-schema.json +++ b/qapi-schema.json @@ -526,6 +526,12 @@ # # Returns a list of information about each virtual CPU. # +# This command causes vCPU threads to exit to userspace, which causes +# an small interruption guest CPU execution. This will have a negative +# impact on realtime guests and other latency sensitive guest workloads. +# It is recommended to use @query-cpus-fast instead of this command to +# avoid the vCPU interruption. +# # Returns: a list of @CpuInfo for each virtual CPU # # Since: 0.14.0 @@ -558,6 +564,77 @@ ## { 'command': 'query-cpus', 'returns': ['CpuInfo'] } +## +# @CpuInfoFast: +# +# Information about a virtual CPU +# +# @cpu-index: index of the virtual CPU +# +# @halted: true if the virtual CPU is in the halt state. Halt usually refers +# to a processor specific low power mode. This field is optional, +# it is only present if the halted state can be retrieved without +# a performance penalty +# +# @qom-path: path to the CPU object in the QOM tree +# +# @thread-id: ID of the underlying host thread +# +# @props: properties describing to which node/socket/core/thread +# virtual CPU belongs to, provided if supported by board +# +# Since: 2.12 +# +# Notes: @halted is a transient state that changes frequently. By the time the +# data is sent to the client, the guest may no longer be halted. +## +{ 'struct': 'CpuInfoFast', + 'data': {'cpu-index': 'int', '*halted': 'bool', 'qom-path': 'str', + 'thread-id': 'int', '*props': 'CpuInstanceProperties' } } + +## +# @query-cpus-fast: +# +# Returns information about all virtual CPUs. This command does not +# incur a performance penalty and should be used in production +# instead of query-cpus. +# +# Returns: list of @CpuInfoFast +# +# Notes: The CPU architecture name is not returned by query-cpus-fast. +# Use query-target to retrieve that information. +# +# Since: 2.12 +# +# Example: +# +# -> { "execute": "query-cpus-fast" } +# <- { "return": [ +# { +# "thread-id": 25627, +# "props": { +# "core-id": 0, +# "thread-id": 0, +# "socket-id": 0 +# }, +# "qom-path": "/machine/unattached/device[0]", +# "cpu-index": 0 +# }, +# { +# "thread-id": 25628, +# "props": { +# "core-id": 0, +# "thread-id": 0, +# "socket-id": 1 +# }, +# "qom-path": "/machine/unattached/device[2]", +# "cpu-index": 1 +# } +# ] +# } +## +{ 'command': 'query-cpus-fast', 'returns': [ 'CpuInfoFast' ] } + ## # @IOThreadInfo: # -- 2.14.3