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Date: Tue, 2 Jul 2019 13:55:41 +0530
From: Mukesh Ojha <mojha@...eaurora.org>
To: linux-kernel@...r.kernel.org
Cc: Raghavendra Rao Ananta <rananta@...eaurora.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...hat.com>,
Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH RESEND V4 1/1] perf: event preserve and create across cpu
hotplug
Friendly Ping.
More explanation of the usecase added in the coverletter
[PATCH RESEND V4 0/1] perf: Add CPU hotplug support for events.
-Mukesh
On 6/18/2019 7:16 PM, Mukesh Ojha wrote:
> Perf framework doesn't allow preserving CPU events across
> CPU hotplugs. The events are scheduled out as and when the
> CPU walks offline. Moreover, the framework also doesn't
> allow the clients to create events on an offline CPU. As
> a result, the clients have to keep on monitoring the CPU
> state until it comes back online.
>
> Therefore, introducing the perf framework to support creation
> and preserving of (CPU) events for offline CPUs. Through
> this, the CPU's online state would be transparent to the
> client and it not have to worry about monitoring the CPU's
> state. Success would be returned to the client even while
> creating the event on an offline CPU. If during the lifetime
> of the event the CPU walks offline, the event would be
> preserved and would continue to count as soon as (and if) the
> CPU comes back online.
>
> Co-authored-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Raghavendra Rao Ananta <rananta@...eaurora.org>
> Signed-off-by: Mukesh Ojha <mojha@...eaurora.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> ---
> Change in V4:
> =============
> - Released, __get_cpu_context would not be correct way to get the
> cpu context of the cpu which is offline, instead use
> container_of to get the cpuctx from ctx.
>
> - Changed the goto label name inside event_function_call from
> 'remove_event_from_context' to 'out'.
>
> Change in V3:
> =============
> - Jiri has tried perf stat -a and removed one of the cpu from the other
> terminal. This resulted in a crash. Crash was because in
> event_function_call(), we were passing NULL as cpuctx in
> func(event, NULL, ctx, data).Fixed it in this patch.
>
> Change in V2:
> =============
> As per long back discussion happened at
> https://lkml.org/lkml/2018/2/15/1324
>
> Peter.Z. has suggested to do thing in different way and shared
> patch as well. This patch fixes the issue seen while trying
> to achieve the purpose.
>
> Fixed issue on top of Peter's patch:
> ===================================
> 1. Added a NULL check on task to avoid crash in __perf_install_in_context.
>
> 2. while trying to add event to context when cpu is offline.
> Inside add_event_to_ctx() to make consistent state machine while hotplug.
>
> -event->state += PERF_EVENT_STATE_HOTPLUG_OFFSET;
> +event->state = PERF_EVENT_STATE_HOTPLUG_OFFSET;
>
> 3. In event_function_call(), added a label 'remove_event_from_ctx' to
> delete events from context list while cpu is offline.
>
> include/linux/perf_event.h | 1 +
> kernel/events/core.c | 123 ++++++++++++++++++++++++++++-----------------
> 2 files changed, 79 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 3dc01cf..52b14b2 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -511,6 +511,7 @@ enum perf_event_state {
> PERF_EVENT_STATE_OFF = -1,
> PERF_EVENT_STATE_INACTIVE = 0,
> PERF_EVENT_STATE_ACTIVE = 1,
> + PERF_EVENT_STATE_HOTPLUG_OFFSET = -32,
> };
>
> struct file;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 118ad1a..82b5106 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -248,6 +248,8 @@ static int event_function(void *info)
> static void event_function_call(struct perf_event *event, event_f func, void *data)
> {
> struct perf_event_context *ctx = event->ctx;
> + struct perf_cpu_context *cpuctx =
> + container_of(ctx, struct perf_cpu_context, ctx);
> struct task_struct *task = READ_ONCE(ctx->task); /* verified in event_function */
> struct event_function_struct efs = {
> .event = event,
> @@ -264,17 +266,18 @@ static void event_function_call(struct perf_event *event, event_f func, void *da
> lockdep_assert_held(&ctx->mutex);
> }
>
> - if (!task) {
> - cpu_function_call(event->cpu, event_function, &efs);
> - return;
> - }
> -
> if (task == TASK_TOMBSTONE)
> return;
>
> again:
> - if (!task_function_call(task, event_function, &efs))
> - return;
> + if (task) {
> + if (!task_function_call(task, event_function, &efs))
> + return;
> + } else {
> + if (!cpu_function_call(event->cpu, event_function, &efs))
> + return;
> + }
> +
>
> raw_spin_lock_irq(&ctx->lock);
> /*
> @@ -286,11 +289,17 @@ static void event_function_call(struct perf_event *event, event_f func, void *da
> raw_spin_unlock_irq(&ctx->lock);
> return;
> }
> +
> + if (!task)
> + goto out;
> +
> if (ctx->is_active) {
> raw_spin_unlock_irq(&ctx->lock);
> goto again;
> }
> - func(event, NULL, ctx, data);
> +
> +out:
> + func(event, cpuctx, ctx, data);
> raw_spin_unlock_irq(&ctx->lock);
> }
>
> @@ -2310,7 +2319,7 @@ static void perf_set_shadow_time(struct perf_event *event,
> struct perf_event *event, *partial_group = NULL;
> struct pmu *pmu = ctx->pmu;
>
> - if (group_event->state == PERF_EVENT_STATE_OFF)
> + if (group_event->state <= PERF_EVENT_STATE_OFF)
> return 0;
>
> pmu->start_txn(pmu, PERF_PMU_TXN_ADD);
> @@ -2389,6 +2398,14 @@ static int group_can_go_on(struct perf_event *event,
> static void add_event_to_ctx(struct perf_event *event,
> struct perf_event_context *ctx)
> {
> + if (!ctx->task) {
> + struct perf_cpu_context *cpuctx =
> + container_of(ctx, struct perf_cpu_context, ctx);
> +
> + if (!cpuctx->online)
> + event->state = PERF_EVENT_STATE_HOTPLUG_OFFSET;
> + }
> +
> list_add_event(event, ctx);
> perf_group_attach(event);
> }
> @@ -2576,11 +2593,6 @@ static int __perf_install_in_context(void *info)
> */
> smp_store_release(&event->ctx, ctx);
>
> - if (!task) {
> - cpu_function_call(cpu, __perf_install_in_context, event);
> - return;
> - }
> -
> /*
> * Should not happen, we validate the ctx is still alive before calling.
> */
> @@ -2619,8 +2631,14 @@ static int __perf_install_in_context(void *info)
> */
> smp_mb();
> again:
> - if (!task_function_call(task, __perf_install_in_context, event))
> - return;
> +
> + if (task) {
> + if (!task_function_call(task, __perf_install_in_context, event))
> + return;
> + } else {
> + if (!cpu_function_call(cpu, __perf_install_in_context, event))
> + return;
> + }
>
> raw_spin_lock_irq(&ctx->lock);
> task = ctx->task;
> @@ -2637,7 +2655,7 @@ static int __perf_install_in_context(void *info)
> * If the task is not running, ctx->lock will avoid it becoming so,
> * thus we can safely install the event.
> */
> - if (task_curr(task)) {
> + if (task && task_curr(task)) {
> raw_spin_unlock_irq(&ctx->lock);
> goto again;
> }
> @@ -11022,16 +11040,7 @@ static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id)
> }
>
> if (!task) {
> - /*
> - * Check if the @cpu we're creating an event for is online.
> - *
> - * We use the perf_cpu_context::ctx::mutex to serialize against
> - * the hotplug notifiers. See perf_event_{init,exit}_cpu().
> - */
> - struct perf_cpu_context *cpuctx =
> - container_of(ctx, struct perf_cpu_context, ctx);
> -
> - if (!cpuctx->online) {
> + if (!cpu_possible(cpu)) {
> err = -ENODEV;
> goto err_locked;
> }
> @@ -11213,15 +11222,7 @@ struct perf_event *
> }
>
> if (!task) {
> - /*
> - * Check if the @cpu we're creating an event for is online.
> - *
> - * We use the perf_cpu_context::ctx::mutex to serialize against
> - * the hotplug notifiers. See perf_event_{init,exit}_cpu().
> - */
> - struct perf_cpu_context *cpuctx =
> - container_of(ctx, struct perf_cpu_context, ctx);
> - if (!cpuctx->online) {
> + if (!cpu_possible(cpu)) {
> err = -ENODEV;
> goto err_unlock;
> }
> @@ -11949,17 +11950,48 @@ static void perf_swevent_init_cpu(unsigned int cpu)
> }
>
> #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC_CORE
> +static void __perf_event_init_cpu_context(void *__info)
> +{
> + struct perf_cpu_context *cpuctx = __info;
> + struct perf_event_context *ctx = &cpuctx->ctx;
> + struct perf_event_context *task_ctx = cpuctx->task_ctx;
> + struct perf_event *event;
> +
> + perf_ctx_lock(cpuctx, task_ctx);
> + ctx_sched_out(ctx, cpuctx, EVENT_ALL);
> + if (task_ctx)
> + ctx_sched_out(task_ctx, cpuctx, EVENT_ALL);
> +
> + list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
> + perf_event_set_state(event, event->state - PERF_EVENT_STATE_HOTPLUG_OFFSET);
> +
> + perf_event_sched_in(cpuctx, task_ctx, current);
> + perf_ctx_unlock(cpuctx, task_ctx);
> +}
> +
> +static void _perf_event_init_cpu_context(int cpu, struct perf_cpu_context *cpuctx)
> +{
> + smp_call_function_single(cpu, __perf_event_init_cpu_context, cpuctx, 1);
> +}
> +
> static void __perf_event_exit_context(void *__info)
> {
> - struct perf_event_context *ctx = __info;
> - struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
> + struct perf_cpu_context *cpuctx = __info;
> + struct perf_event_context *ctx = &cpuctx->ctx;
> + struct perf_event_context *task_ctx = cpuctx->task_ctx;
> struct perf_event *event;
>
> - raw_spin_lock(&ctx->lock);
> - ctx_sched_out(ctx, cpuctx, EVENT_TIME);
> - list_for_each_entry(event, &ctx->event_list, event_entry)
> - __perf_remove_from_context(event, cpuctx, ctx, (void *)DETACH_GROUP);
> - raw_spin_unlock(&ctx->lock);
> + perf_ctx_lock(cpuctx, task_ctx);
> + ctx_sched_out(ctx, cpuctx, EVENT_ALL);
> + if (task_ctx)
> + ctx_sched_out(task_ctx, cpuctx, EVENT_ALL);
> +
> + list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
> + perf_event_set_state(event,
> + event->state + PERF_EVENT_STATE_HOTPLUG_OFFSET);
> +
> + perf_event_sched_in(cpuctx, task_ctx, current);
> + perf_ctx_unlock(cpuctx, task_ctx);
> }
>
> static void perf_event_exit_cpu_context(int cpu)
> @@ -11974,7 +12006,7 @@ static void perf_event_exit_cpu_context(int cpu)
> ctx = &cpuctx->ctx;
>
> mutex_lock(&ctx->mutex);
> - smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
> + smp_call_function_single(cpu, __perf_event_exit_context, cpuctx, 1);
> cpuctx->online = 0;
> mutex_unlock(&ctx->mutex);
> }
> @@ -11982,7 +12014,7 @@ static void perf_event_exit_cpu_context(int cpu)
> mutex_unlock(&pmus_lock);
> }
> #else
> -
> +static void _perf_event_init_cpu_context(int cpu, struct perf_cpu_context *cpuctx) { }
> static void perf_event_exit_cpu_context(int cpu) { }
>
> #endif
> @@ -12003,6 +12035,7 @@ int perf_event_init_cpu(unsigned int cpu)
>
> mutex_lock(&ctx->mutex);
> cpuctx->online = 1;
> + _perf_event_init_cpu_context(cpu, cpuctx);
> mutex_unlock(&ctx->mutex);
> }
> mutex_unlock(&pmus_lock);
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