[PATCH 1/2] perf_counter: dynamically allocate tasks' perf_counter_context struct [v2]

Paul Mackerras
2009-05-22T04:28:41Z

This replaces the struct perf_counter_context in the task_struct with
a pointer to a dynamically allocated perf_counter_context struct. The
main reason for doing is this is to allow us to transfer a
perf_counter_context from one task to another when we do lazy PMU
switching in a later patch.

This has a few side-benefits: the task_struct becomes a little smaller,
we save some memory because only tasks that have perf_counters attached
get a perf_counter_context allocated for them, and we can remove the
inclusion of <linux/perf_counter.h> in sched.h, meaning that we don't
end up recompiling nearly everything whenever perf_counter.h changes.

The perf_counter_context structures are reference-counted and freed
when the last reference is dropped. A context can have references
from its task and the counters on its task. Counters can outlive the
task so it is possible that a context will be freed well after its
task has exited.

Contexts are allocated on fork if the parent had a context, or
otherwise the first time that a per-task counter is created on a task.
In the latter case, we set the context pointer in the task struct
locklessly using an atomic compare-and-exchange operation in case we
raced with some other task in creating a context for the subject task.

This also removes the task pointer from the perf_counter struct. The
task pointer was not used anywhere and would make it harder to move a
context from one task to another. Anything that needed to know which
task a counter was attached to was already using counter->ctx->task.

The __perf_counter_init_context function moves up in perf_counter.c
so that it can be called from find_get_context, and now initializes
the refcount, but is otherwise unchanged.

We were potentially calling list_del_counter twice: once from
__perf_counter_exit_task when the task exits and once from
__perf_counter_remove_from_context when the counter's fd gets closed.
This adds a check in list_del_counter so it doesn't do anything if
the counter has already been removed from the lists.

Since perf_counter_task_sched_in doesn't do anything if the task doesn't
have a context, and leaves cpuctx->task_ctx = NULL, this adds code to
__perf_install_in_context to set cpuctx->task_ctx if necessary, i.e. in
the case where the current task adds the first counter to itself and
thus creates a context for itself.

This also adds similar code to __perf_counter_enable to handle a
similar situation which can arise when the counters have been disabled
using prctl; that also leaves cpuctx->task_ctx = NULL.

Signed-off-by: Paul Mackerras <paulus@samba.org>
---
arch/x86/kernel/apic/apic.c | 1 +
include/linux/init_task.h | 13 ---
include/linux/perf_counter.h | 4 +-
include/linux/sched.h | 6 +-
kernel/exit.c | 3 +-
kernel/fork.c | 1 +
kernel/perf_counter.c | 218 ++++++++++++++++++++++++++----------------
7 files changed, 145 insertions(+), 101 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index e9021a9..b4f6440 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -14,6 +14,7 @@
* Mikael Pettersson : PM converted to driver model.
*/

+#include <linux/perf_counter.h>
#include <linux/kernel_stat.h>
#include <linux/mc146818rtc.h>
#include <linux/acpi_pmtmr.h>
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 503afaa..d87247d 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -108,18 +108,6 @@ extern struct group_info init_groups;

extern struct cred init_cred;

-#ifdef CONFIG_PERF_COUNTERS
-# define INIT_PERF_COUNTERS(tsk) \
- .perf_counter_ctx.counter_list = \
- LIST_HEAD_INIT(tsk.perf_counter_ctx.counter_list), \
- .perf_counter_ctx.event_list = \
- LIST_HEAD_INIT(tsk.perf_counter_ctx.event_list), \
- .perf_counter_ctx.lock = \
- __SPIN_LOCK_UNLOCKED(tsk.perf_counter_ctx.lock),
-#else
-# define INIT_PERF_COUNTERS(tsk)
-#endif
-
/*
* INIT_TASK is used to set up the first task table, touch at
* your own risk!. Base=0, limit=0x1fffff (=2MB)
@@ -183,7 +171,6 @@ extern struct cred init_cred;
}, \
.dirties = INIT_PROP_LOCAL_SINGLE(dirties), \
INIT_IDS \
- INIT_PERF_COUNTERS(tsk) \
INIT_TRACE_IRQFLAGS \
INIT_LOCKDEP \
INIT_FTRACE_GRAPH \
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index f612941..0713090 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -449,7 +449,6 @@ struct perf_counter {
struct hw_perf_counter hw;

struct perf_counter_context *ctx;
- struct task_struct *task;
struct file *filp;

struct perf_counter *parent;
@@ -498,7 +497,6 @@ struct perf_counter {
* Used as a container for task counters and CPU counters as well:
*/
struct perf_counter_context {
-#ifdef CONFIG_PERF_COUNTERS
/*
* Protect the states of the counters in the list,
* nr_active, and the list:
@@ -516,6 +514,7 @@ struct perf_counter_context {
int nr_counters;
int nr_active;
int is_active;
+ atomic_t refcount;
struct task_struct *task;

/*
@@ -523,7 +522,6 @@ struct perf_counter_context {
*/
u64 time;
u64 timestamp;
-#endif
};

/**
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ff59d12..9714d45 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -71,7 +71,6 @@ struct sched_param {
#include <linux/path.h>
#include <linux/compiler.h>
#include <linux/completion.h>
-#include <linux/perf_counter.h>
#include <linux/pid.h>
#include <linux/percpu.h>
#include <linux/topology.h>
@@ -99,6 +98,7 @@ struct robust_list_head;
struct bio;
struct bts_tracer;
struct fs_struct;
+struct perf_counter_context;

/*
* List of flags we want to share for kernel threads,
@@ -1387,7 +1387,9 @@ struct task_struct {
struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache;
#endif
- struct perf_counter_context perf_counter_ctx;
+#ifdef CONFIG_PERF_COUNTERS
+ struct perf_counter_context *perf_counter_ctxp;
+#endif
#ifdef CONFIG_NUMA
struct mempolicy *mempolicy;
short il_next;
diff --git a/kernel/exit.c b/kernel/exit.c
index f9dfedd..99ad406 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -48,6 +48,7 @@
#include <linux/tracehook.h>
#include <linux/fs_struct.h>
#include <linux/init_task.h>
+#include <linux/perf_counter.h>
#include <trace/sched.h>

#include <asm/uaccess.h>
@@ -159,7 +160,7 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);

#ifdef CONFIG_PERF_COUNTERS
- WARN_ON_ONCE(!list_empty(&tsk->perf_counter_ctx.counter_list));
+ WARN_ON_ONCE(tsk->perf_counter_ctxp);
#endif
trace_sched_process_free(tsk);
put_task_struct(tsk);
diff --git a/kernel/fork.c b/kernel/fork.c
index d32fef4..e72a09f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -63,6 +63,7 @@
#include <linux/fs_struct.h>
#include <trace/sched.h>
#include <linux/magic.h>
+#include <linux/perf_counter.h>

#include <asm/pgtable.h>
#include <asm/pgalloc.h>
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 08584c1..06ea3ea 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -97,6 +97,17 @@ void perf_enable(void)
hw_perf_enable();
}

+static void get_ctx(struct perf_counter_context *ctx)
+{
+ atomic_inc(&ctx->refcount);
+}
+
+static void put_ctx(struct perf_counter_context *ctx)
+{
+ if (atomic_dec_and_test(&ctx->refcount))
+ kfree(ctx);
+}
+
static void
list_add_counter(struct perf_counter *counter, struct perf_counter_context *ctx)
{
@@ -118,11 +129,17 @@ list_add_counter(struct perf_counter *counter, struct perf_counter_context *ctx)
ctx->nr_counters++;
}

+/*
+ * Remove a counter from the lists for its context.
+ * Must be called with counter->mutex and ctx->mutex held.
+ */
static void
list_del_counter(struct perf_counter *counter, struct perf_counter_context *ctx)
{
struct perf_counter *sibling, *tmp;

+ if (list_empty(&counter->list_entry))
+ return;
ctx->nr_counters--;

list_del_init(&counter->list_entry);
@@ -216,8 +233,6 @@ static void __perf_counter_remove_from_context(void *info)

counter_sched_out(counter, cpuctx, ctx);

- counter->task = NULL;
-
list_del_counter(counter, ctx);

if (!ctx->task) {
@@ -279,7 +294,6 @@ retry:
*/
if (!list_empty(&counter->list_entry)) {
list_del_counter(counter, ctx);
- counter->task = NULL;
}
spin_unlock_irq(&ctx->lock);
}
@@ -568,11 +582,17 @@ static void __perf_install_in_context(void *info)
* If this is a task context, we need to check whether it is
* the current task context of this cpu. If not it has been
* scheduled out before the smp call arrived.
+ * Or possibly this is the right context but it isn't
+ * on this cpu because it had no counters.
*/
- if (ctx->task && cpuctx->task_ctx != ctx)
- return;
+ if (ctx->task && cpuctx->task_ctx != ctx) {
+ if (cpuctx->task_ctx || ctx->task != current)
+ return;
+ cpuctx->task_ctx = ctx;
+ }

spin_lock_irqsave(&ctx->lock, flags);
+ ctx->is_active = 1;
update_context_time(ctx);

/*
@@ -653,7 +673,6 @@ perf_install_in_context(struct perf_counter_context *ctx,
return;
}

- counter->task = task;
retry:
task_oncpu_function_call(task, __perf_install_in_context,
counter);
@@ -693,10 +712,14 @@ static void __perf_counter_enable(void *info)
* If this is a per-task counter, need to check whether this
* counter's task is the current task on this cpu.
*/
- if (ctx->task && cpuctx->task_ctx != ctx)
- return;
+ if (ctx->task && cpuctx->task_ctx != ctx) {
+ if (cpuctx->task_ctx || ctx->task != current)
+ return;
+ cpuctx->task_ctx = ctx;
+ }

spin_lock_irqsave(&ctx->lock, flags);
+ ctx->is_active = 1;
update_context_time(ctx);

counter->prev_state = counter->state;
@@ -852,10 +875,10 @@ void __perf_counter_sched_out(struct perf_counter_context *ctx,
void perf_counter_task_sched_out(struct task_struct *task, int cpu)
{
struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
- struct perf_counter_context *ctx = &task->perf_counter_ctx;
+ struct perf_counter_context *ctx = task->perf_counter_ctxp;
struct pt_regs *regs;

- if (likely(!cpuctx->task_ctx))
+ if (likely(!ctx || !cpuctx->task_ctx))
return;

update_context_time(ctx);
@@ -871,6 +894,8 @@ static void __perf_counter_task_sched_out(struct perf_counter_context *ctx)
{
struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);

+ if (!cpuctx->task_ctx)
+ return;
__perf_counter_sched_out(ctx, cpuctx);
cpuctx->task_ctx = NULL;
}
@@ -969,8 +994,10 @@ __perf_counter_sched_in(struct perf_counter_context *ctx,
void perf_counter_task_sched_in(struct task_struct *task, int cpu)
{
struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
- struct perf_counter_context *ctx = &task->perf_counter_ctx;
+ struct perf_counter_context *ctx = task->perf_counter_ctxp;

+ if (likely(!ctx))
+ return;
__perf_counter_sched_in(ctx, cpuctx, cpu);
cpuctx->task_ctx = ctx;
}
@@ -985,11 +1012,11 @@ static void perf_counter_cpu_sched_in(struct perf_cpu_context *cpuctx, int cpu)
int perf_counter_task_disable(void)
{
struct task_struct *curr = current;
- struct perf_counter_context *ctx = &curr->perf_counter_ctx;
+ struct perf_counter_context *ctx = curr->perf_counter_ctxp;
struct perf_counter *counter;
unsigned long flags;

- if (likely(!ctx->nr_counters))
+ if (!ctx || !ctx->nr_counters)
return 0;

local_irq_save(flags);
@@ -1020,12 +1047,12 @@ int perf_counter_task_disable(void)
int perf_counter_task_enable(void)
{
struct task_struct *curr = current;
- struct perf_counter_context *ctx = &curr->perf_counter_ctx;
+ struct perf_counter_context *ctx = curr->perf_counter_ctxp;
struct perf_counter *counter;
unsigned long flags;
int cpu;

- if (likely(!ctx->nr_counters))
+ if (!ctx || !ctx->nr_counters)
return 0;

local_irq_save(flags);
@@ -1128,19 +1155,23 @@ void perf_counter_task_tick(struct task_struct *curr, int cpu)
return;

cpuctx = &per_cpu(perf_cpu_context, cpu);
- ctx = &curr->perf_counter_ctx;
+ ctx = curr->perf_counter_ctxp;

perf_adjust_freq(&cpuctx->ctx);
- perf_adjust_freq(ctx);
+ if (ctx)
+ perf_adjust_freq(ctx);

perf_counter_cpu_sched_out(cpuctx);
- __perf_counter_task_sched_out(ctx);
+ if (ctx)
+ __perf_counter_task_sched_out(ctx);

rotate_ctx(&cpuctx->ctx);
- rotate_ctx(ctx);
+ if (ctx)
+ rotate_ctx(ctx);

perf_counter_cpu_sched_in(cpuctx, cpu);
- perf_counter_task_sched_in(curr, cpu);
+ if (ctx)
+ perf_counter_task_sched_in(curr, cpu);
}

/*
@@ -1176,6 +1207,22 @@ static u64 perf_counter_read(struct perf_counter *counter)
return atomic64_read(&counter->count);
}

+/*
+ * Initialize the perf_counter context in a task_struct:
+ */
+static void
+__perf_counter_init_context(struct perf_counter_context *ctx,
+ struct task_struct *task)
+{
+ memset(ctx, 0, sizeof(*ctx));
+ spin_lock_init(&ctx->lock);
+ mutex_init(&ctx->mutex);
+ INIT_LIST_HEAD(&ctx->counter_list);
+ INIT_LIST_HEAD(&ctx->event_list);
+ atomic_set(&ctx->refcount, 1);
+ ctx->task = task;
+}
+
static void put_context(struct perf_counter_context *ctx)
{
if (ctx->task)
@@ -1186,6 +1233,7 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
{
struct perf_cpu_context *cpuctx;
struct perf_counter_context *ctx;
+ struct perf_counter_context *tctx;
struct task_struct *task;

/*
@@ -1225,15 +1273,36 @@ static struct perf_counter_context *find_get_context(pid_t pid, int cpu)
if (!task)
return ERR_PTR(-ESRCH);

- ctx = &task->perf_counter_ctx;
- ctx->task = task;
-
/* Reuse ptrace permission checks for now. */
if (!ptrace_may_access(task, PTRACE_MODE_READ)) {
- put_context(ctx);
+ put_task_struct(task);
return ERR_PTR(-EACCES);
}

+ ctx = task->perf_counter_ctxp;
+ if (!ctx) {
+ ctx = kmalloc(sizeof(struct perf_counter_context), GFP_KERNEL);
+ if (!ctx) {
+ put_task_struct(task);
+ return ERR_PTR(-ENOMEM);
+ }
+ __perf_counter_init_context(ctx, task);
+ /*
+ * Make sure other cpus see correct values for *ctx
+ * once task->perf_counter_ctxp is visible to them.
+ */
+ smp_wmb();
+ tctx = cmpxchg(&task->perf_counter_ctxp, NULL, ctx);
+ if (tctx) {
+ /*
+ * We raced with some other task; use
+ * the context they set.
+ */
+ kfree(ctx);
+ ctx = tctx;
+ }
+ }
+
return ctx;
}

@@ -1242,6 +1311,7 @@ static void free_counter_rcu(struct rcu_head *head)
struct perf_counter *counter;

counter = container_of(head, struct perf_counter, rcu_head);
+ put_ctx(counter->ctx);
kfree(counter);
}

@@ -2247,7 +2317,7 @@ static void perf_counter_comm_event(struct perf_comm_event *comm_event)
perf_counter_comm_ctx(&cpuctx->ctx, comm_event);
put_cpu_var(perf_cpu_context);

- perf_counter_comm_ctx(&current->perf_counter_ctx, comm_event);
+ perf_counter_comm_ctx(current->perf_counter_ctxp, comm_event);
}

void perf_counter_comm(struct task_struct *task)
@@ -2256,7 +2326,9 @@ void perf_counter_comm(struct task_struct *task)

if (!atomic_read(&nr_comm_tracking))
return;
-
+ if (!current->perf_counter_ctxp)
+ return;
+
comm_event = (struct perf_comm_event){
.task = task,
.event = {
@@ -2372,7 +2444,7 @@ got_name:
perf_counter_mmap_ctx(&cpuctx->ctx, mmap_event);
put_cpu_var(perf_cpu_context);

- perf_counter_mmap_ctx(&current->perf_counter_ctx, mmap_event);
+ perf_counter_mmap_ctx(current->perf_counter_ctxp, mmap_event);

kfree(buf);
}
@@ -2384,6 +2456,8 @@ void perf_counter_mmap(unsigned long addr, unsigned long len,

if (!atomic_read(&nr_mmap_tracking))
return;
+ if (!current->perf_counter_ctxp)
+ return;

mmap_event = (struct perf_mmap_event){
.file = file,
@@ -2985,6 +3059,7 @@ perf_counter_alloc(struct perf_counter_hw_event *hw_event,
counter->group_leader = group_leader;
counter->pmu = NULL;
counter->ctx = ctx;
+ get_ctx(ctx);

counter->state = PERF_COUNTER_STATE_INACTIVE;
if (hw_event->disabled)
@@ -3150,21 +3225,6 @@ err_put_context:
}

/*
- * Initialize the perf_counter context in a task_struct:
- */
-static void
-__perf_counter_init_context(struct perf_counter_context *ctx,
- struct task_struct *task)
-{
- memset(ctx, 0, sizeof(*ctx));
- spin_lock_init(&ctx->lock);
- mutex_init(&ctx->mutex);
- INIT_LIST_HEAD(&ctx->counter_list);
- INIT_LIST_HEAD(&ctx->event_list);
- ctx->task = task;
-}
-
-/*
* inherit a counter from parent task to child task:
*/
static struct perf_counter *
@@ -3195,7 +3255,6 @@ inherit_counter(struct perf_counter *parent_counter,
/*
* Link it up in the child's context:
*/
- child_counter->task = child;
add_counter_to_ctx(child_counter, child_ctx);

child_counter->parent = parent_counter;
@@ -3294,40 +3353,15 @@ __perf_counter_exit_task(struct task_struct *child,
struct perf_counter *parent_counter;

/*
- * If we do not self-reap then we have to wait for the
- * child task to unschedule (it will happen for sure),
- * so that its counter is at its final count. (This
- * condition triggers rarely - child tasks usually get
- * off their CPU before the parent has a chance to
- * get this far into the reaping action)
+ * Protect against concurrent operations on child_counter
+ * due its fd getting closed, etc.
*/
- if (child != current) {
- wait_task_inactive(child, 0);
- update_counter_times(child_counter);
- list_del_counter(child_counter, child_ctx);
- } else {
- struct perf_cpu_context *cpuctx;
- unsigned long flags;
-
- /*
- * Disable and unlink this counter.
- *
- * Be careful about zapping the list - IRQ/NMI context
- * could still be processing it:
- */
- local_irq_save(flags);
- perf_disable();
-
- cpuctx = &__get_cpu_var(perf_cpu_context);
+ mutex_lock(&child_counter->mutex);

- group_sched_out(child_counter, cpuctx, child_ctx);
- update_counter_times(child_counter);
+ update_counter_times(child_counter);
+ list_del_counter(child_counter, child_ctx);

- list_del_counter(child_counter, child_ctx);
-
- perf_enable();
- local_irq_restore(flags);
- }
+ mutex_unlock(&child_counter->mutex);

parent_counter = child_counter->parent;
/*
@@ -3346,19 +3380,29 @@ __perf_counter_exit_task(struct task_struct *child,
*
* Note: we may be running in child context, but the PID is not hashed
* anymore so new counters will not be added.
+ * (XXX not sure that is true when we get called from flush_old_exec.
+ * -- paulus)
*/
void perf_counter_exit_task(struct task_struct *child)
{
struct perf_counter *child_counter, *tmp;
struct perf_counter_context *child_ctx;
+ unsigned long flags;

WARN_ON_ONCE(child != current);

- child_ctx = &child->perf_counter_ctx;
+ child_ctx = child->perf_counter_ctxp;

- if (likely(!child_ctx->nr_counters))
+ if (likely(!child_ctx))
return;

+ local_irq_save(flags);
+ __perf_counter_task_sched_out(child_ctx);
+ child->perf_counter_ctxp = NULL;
+ local_irq_restore(flags);
+
+ mutex_lock(&child_ctx->mutex);
+
again:
list_for_each_entry_safe(child_counter, tmp, &child_ctx->counter_list,
list_entry)
@@ -3371,6 +3415,10 @@ again:
*/
if (!list_empty(&child_ctx->counter_list))
goto again;
+
+ mutex_unlock(&child_ctx->mutex);
+
+ put_ctx(child_ctx);
}

/*
@@ -3382,19 +3430,25 @@ void perf_counter_init_task(struct task_struct *child)
struct perf_counter *counter;
struct task_struct *parent = current;

- child_ctx = &child->perf_counter_ctx;
- parent_ctx = &parent->perf_counter_ctx;
-
- __perf_counter_init_context(child_ctx, child);
+ child->perf_counter_ctxp = NULL;

/*
* This is executed from the parent task context, so inherit
- * counters that have been marked for cloning:
+ * counters that have been marked for cloning.
+ * First allocate and initialize a context for the child.
*/

- if (likely(!parent_ctx->nr_counters))
+ child_ctx = kmalloc(sizeof(struct perf_counter_context), GFP_KERNEL);
+ if (!child_ctx)
+ return;
+
+ parent_ctx = parent->perf_counter_ctxp;
+ if (likely(!parent_ctx || !parent_ctx->nr_counters))
return;

+ __perf_counter_init_context(child_ctx, child);
+ child->perf_counter_ctxp = child_ctx;
+
/*
* Lock the parent list. No need to lock the child - not PID
* hashed yet and not running, so nobody can access it.
--
1.6.0.4