Messages in this thread Patch in this message | | | From | kan.liang@linux ... | Subject | [PATCH V6 2/6] perf: attach/detach PMU specific data | Date | Tue, 20 Jul 2021 06:40:16 -0700 |
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From: Kan Liang <kan.liang@linux.intel.com>
The LBR call stack data has to be saved/restored during context switch to fix the shorter LBRs call stacks issue in the system-wide mode. Allocate PMU specific data and attach them to the corresponding task_struct during LBR call stack monitoring.
When a LBR call stack event is accounted, the perf_ctx_data for the related tasks will be allocated/attached by attach_perf_ctx_data(). When a LBR call stack event is unaccounted, the perf_ctx_data for related tasks will be detached/freed by detach_perf_ctx_data().
The LBR call stack event could be a per-task event or a system-wide event. - For a per-task event, perf only allocates the perf_ctx_data for the current task. If the allocation fails, perf will error out. - For a system-wide event, perf has to allocate the perf_ctx_data for both the existing tasks and the upcoming tasks. The allocation for the existing tasks is done in perf_event_alloc(). If any allocation fails, perf will error out. The allocation for the new tasks will be done in perf_event_fork(). A global reader/writer semaphore, global_ctx_data_rwsem, is added to address the global race. - The perf_ctx_data only be freed by the last LBR call stack event. The number of the per-task events is tracked by refcount of each task. Since the system-wide events impact all tasks, it's not practical to go through the whole task list to update the refcount for each system-wide event. The number of system-wide events is tracked by a global variable global_ctx_data_ref.
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Kan Liang <kan.liang@linux.intel.com> ---
Changes since V5: - Fix sparse warnings (LKP)
Changes since V4: - Remove the per-task lock and simplify all the attach_* and detach_* functions. (Peter) - Add a global RWSEM to protect the system-wide allocation and free. Add a refcount to track the global users. (Peter) - Modify the perf_event_alloc_task_data() to use the global RWSEM (Peter)
Changes since V3: - Rebase for the Arch LBR - Use kvcalloc to replace kcalloc (Andi)
Changes since V2: - Remove global spin lock task_data_sys_wide_events_lock Since the global spin lock has been removed, we cannot guarantee that the allocation/assignments for existing threads and free are serialized. To fix it, in V3, we go through the task list when accounting for each system-wide event, and assign the perf_ctx_data pointer if needed. (In V2, we only do the assignment for the first system-wide event). In V3, we also add a breaker in free process for system-wide event. If there is new system-wide event accounted, stop the free process immediately. - Add a macro TASK_DATA_SYS_WIDE to indicate the PMU specific data is used by system-wide events.
kernel/events/core.c | 284 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 284 insertions(+)
diff --git a/kernel/events/core.c b/kernel/events/core.c index dcdd164..e445f47 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -54,6 +54,7 @@ #include <linux/highmem.h> #include <linux/pgtable.h> #include <linux/buildid.h> +#include <linux/percpu-rwsem.h> #include "internal.h" @@ -4776,6 +4777,222 @@ static void unaccount_freq_event(void) atomic_dec(&nr_freq_events); } + +static struct perf_ctx_data * +alloc_perf_ctx_data(struct kmem_cache *ctx_cache, bool global) +{ + struct perf_ctx_data *cd; + + cd = kzalloc(sizeof(*cd), GFP_KERNEL); + if (!cd) + return NULL; + + cd->data = kmem_cache_zalloc(ctx_cache, GFP_KERNEL); + if (!cd->data) { + kfree(cd); + return NULL; + } + + cd->global = global; + cd->ctx_cache = ctx_cache; + refcount_set(&cd->refcount, 1); + + return cd; +} + +static void free_perf_ctx_data(struct perf_ctx_data *cd) +{ + kmem_cache_free(cd->ctx_cache, cd->data); + kfree(cd); +} + +static void __free_perf_ctx_data_rcu(struct rcu_head *rcu_head) +{ + struct perf_ctx_data *cd; + + cd = container_of(rcu_head, struct perf_ctx_data, rcu_head); + free_perf_ctx_data(cd); +} + +static inline void perf_free_ctx_data_rcu(struct perf_ctx_data *cd) +{ + call_rcu(&cd->rcu_head, __free_perf_ctx_data_rcu); +} + +static int +attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache, + bool global) +{ + struct perf_ctx_data *cd, *old = NULL; + + cd = alloc_perf_ctx_data(ctx_cache, global); + if (!cd) + return -ENOMEM; + + for (;;) { + if (try_cmpxchg((struct perf_ctx_data **)&task->perf_ctx_data, &old, cd)) { + if (old) + perf_free_ctx_data_rcu(old); + return 0; + } + + if (!old) { + /* + * After seeing a dead @old, we raced with + * removal and lost, try again to install @cd. + */ + continue; + } + + if (refcount_inc_not_zero(&old->refcount)) { + free_perf_ctx_data(cd); /* unused */ + return 0; + } + + /* + * @old is a dead object, refcount==0 is stable, try and + * replace it with @cd. + */ + } + return 0; +} + +static void __detach_global_ctx_data(void); +DEFINE_STATIC_PERCPU_RWSEM(global_ctx_data_rwsem); +static refcount_t global_ctx_data_ref; + +static int +attach_global_ctx_data(struct kmem_cache *ctx_cache) +{ + if (refcount_inc_not_zero(&global_ctx_data_ref)) + return 0; + + percpu_down_write(&global_ctx_data_rwsem); + if (!refcount_inc_not_zero(&global_ctx_data_ref)) { + struct task_struct *g, *p; + struct perf_ctx_data *cd; + int ret; + +again: + /* Allocate everything */ + rcu_read_lock(); + for_each_process_thread(g, p) { + cd = rcu_dereference(p->perf_ctx_data); + if (cd && !cd->global) { + cd->global = 1; + if (!refcount_inc_not_zero(&cd->refcount)) + cd = NULL; + } + if (!cd) { + get_task_struct(p); + rcu_read_unlock(); + + ret = attach_task_ctx_data(p, ctx_cache, true); + put_task_struct(p); + if (ret) { + __detach_global_ctx_data(); + return ret; + } + goto again; + } + } + rcu_read_unlock(); + + refcount_set(&global_ctx_data_ref, 1); + } + percpu_up_write(&global_ctx_data_rwsem); + + return 0; +} + +static int +attach_perf_ctx_data(struct perf_event *event) +{ + struct task_struct *task = event->hw.target; + struct kmem_cache *ctx_cache = event->pmu->task_ctx_cache; + + if (!ctx_cache) + return -ENOMEM; + + if (task) + return attach_task_ctx_data(task, ctx_cache, false); + else + return attach_global_ctx_data(ctx_cache); +} + +static void +detach_task_ctx_data(struct task_struct *p) +{ + struct perf_ctx_data *cd; + + rcu_read_lock(); + cd = rcu_dereference(p->perf_ctx_data); + if (!cd || !refcount_dec_and_test(&cd->refcount)) { + rcu_read_unlock(); + return; + } + rcu_read_unlock(); + + /* + * The old ctx_data may be lost because of the race. + * Nothing is required to do for the case. + * See attach_task_ctx_data(). + */ + if (try_cmpxchg((struct perf_ctx_data **)&p->perf_ctx_data, &cd, NULL)) + perf_free_ctx_data_rcu(cd); +} + +static void __detach_global_ctx_data(void) +{ + struct task_struct *g, *p; + struct perf_ctx_data *cd; + +again: + rcu_read_lock(); + for_each_process_thread(g, p) { + cd = rcu_dereference(p->perf_ctx_data); + if (!cd || !cd->global) + continue; + cd->global = 0; + get_task_struct(p); + rcu_read_unlock(); + + detach_task_ctx_data(p); + put_task_struct(p); + goto again; + } + rcu_read_unlock(); +} + +static void detach_global_ctx_data(void) +{ + if (refcount_dec_not_one(&global_ctx_data_ref)) + return; + + percpu_down_write(&global_ctx_data_rwsem); + if (!refcount_dec_and_test(&global_ctx_data_ref)) + goto unlock; + + /* remove everything */ + __detach_global_ctx_data(); + +unlock: + percpu_up_write(&global_ctx_data_rwsem); +} + +static void detach_perf_ctx_data(struct perf_event *event) +{ + struct task_struct *task = event->hw.target; + + if (!event->pmu->task_ctx_cache) + return; + + if (task) + detach_task_ctx_data(task); + else + detach_global_ctx_data(); +} + static void unaccount_event(struct perf_event *event) { bool dec = false; @@ -4813,6 +5030,8 @@ static void unaccount_event(struct perf_event *event) atomic_dec(&nr_bpf_events); if (event->attr.text_poke) atomic_dec(&nr_text_poke_events); + if (event->attach_state & PERF_ATTACH_TASK_DATA) + detach_perf_ctx_data(event); if (dec) { if (!atomic_add_unless(&perf_sched_count, -1, 1)) @@ -7849,10 +8068,62 @@ static void perf_event_task(struct task_struct *task, task_ctx); } +/* + * Allocate data for a new task when profiling system-wide + * events which require PMU specific data + */ +static void +perf_event_alloc_task_data(struct task_struct *child, + struct task_struct *parent) +{ + struct kmem_cache *ctx_cache = NULL; + struct perf_ctx_data *cd; + + if (!refcount_read(&global_ctx_data_ref)) + return; + + rcu_read_lock(); + cd = rcu_dereference(parent->perf_ctx_data); + if (cd) + ctx_cache = cd->ctx_cache; + rcu_read_unlock(); + + if (!ctx_cache) + return; + + percpu_down_read(&global_ctx_data_rwsem); + + rcu_read_lock(); + cd = rcu_dereference(child->perf_ctx_data); + + if (!cd) { + /* + * A system-wide event may be unaccount, + * when attaching the perf_ctx_data. + */ + if (!refcount_read(&global_ctx_data_ref)) + goto rcu_unlock; + rcu_read_unlock(); + attach_task_ctx_data(child, ctx_cache, true); + goto up_rwsem; + } + + if (!cd->global) { + cd->global = 1; + refcount_inc(&cd->refcount); + } + +rcu_unlock: + rcu_read_unlock(); +up_rwsem: + percpu_up_read(&global_ctx_data_rwsem); +} + void perf_event_fork(struct task_struct *task) { perf_event_task(task, NULL, 1); perf_event_namespaces(task); + perf_event_alloc_task_data(task, current); } /* @@ -11622,11 +11893,17 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu, if (err) goto err_callchain_buffer; + if ((event->attach_state & PERF_ATTACH_TASK_DATA) && + attach_perf_ctx_data(event)) + goto err_task_ctx_data; + /* symmetric to unaccount_event() in _free_event() */ account_event(event); return event; +err_task_ctx_data: + security_perf_event_free(event); err_callchain_buffer: if (!event->parent) { if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) @@ -12707,6 +12984,13 @@ void perf_event_exit_task(struct task_struct *child) * At this point we need to send EXIT events to cpu contexts. */ perf_event_task(child, NULL, 0); + + /* + * Detach the perf_ctx_data for the system-wide event. + */ + percpu_down_read(&global_ctx_data_rwsem); + detach_task_ctx_data(child); + percpu_up_read(&global_ctx_data_rwsem); } static void perf_free_event(struct perf_event *event, -- 2.7.4
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