[tip: perf/urgent] perf/core: Fix perf_cgroup_switch()

From: tip-bot2 for Chengming Zhou
Date: Tue Apr 05 2022 - 06:17:54 EST

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The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 96492a6c558acb56124844d1409d9ef8624a0322
Gitweb: https://git.kernel.org/tip/96492a6c558acb56124844d1409d9ef8624a0322
Author: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
AuthorDate: Tue, 29 Mar 2022 23:45:22 +08:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 05 Apr 2022 09:59:45 +02:00

perf/core: Fix perf_cgroup_switch()

There is a race problem that can trigger WARN_ON_ONCE(cpuctx->cgrp)
in perf_cgroup_switch().

CPU1 CPU2
perf_cgroup_sched_out(prev, next)
cgrp1 = perf_cgroup_from_task(prev)
cgrp2 = perf_cgroup_from_task(next)
if (cgrp1 != cgrp2)
perf_cgroup_switch(prev, PERF_CGROUP_SWOUT)
cgroup_migrate_execute()
task->cgroups = ?
perf_cgroup_attach()
task_function_call(task, __perf_cgroup_move)
perf_cgroup_sched_in(prev, next)
cgrp1 = perf_cgroup_from_task(prev)
cgrp2 = perf_cgroup_from_task(next)
if (cgrp1 != cgrp2)
perf_cgroup_switch(next, PERF_CGROUP_SWIN)
__perf_cgroup_move()
perf_cgroup_switch(task, PERF_CGROUP_SWOUT | PERF_CGROUP_SWIN)

The commit a8d757ef076f ("perf events: Fix slow and broken cgroup
context switch code") want to skip perf_cgroup_switch() when the
perf_cgroup of "prev" and "next" are the same.

But task->cgroups can change in concurrent with context_switch()
in cgroup_migrate_execute(). If cgrp1 == cgrp2 in sched_out(),
cpuctx won't do sched_out. Then task->cgroups changed cause
cgrp1 != cgrp2 in sched_in(), cpuctx will do sched_in. So trigger
WARN_ON_ONCE(cpuctx->cgrp).

Even though __perf_cgroup_move() will be synchronized as the context
switch disables the interrupt, context_switch() still can see the
task->cgroups is changing in the middle, since task->cgroups changed
before sending IPI.

So we have to combine perf_cgroup_sched_in() into perf_cgroup_sched_out(),
unified into perf_cgroup_switch(), to fix the incosistency between
perf_cgroup_sched_out() and perf_cgroup_sched_in().

But we can't just compare prev->cgroups with next->cgroups to decide
whether to skip cpuctx sched_out/in since the prev->cgroups is changing
too. For example:

CPU1 CPU2
cgroup_migrate_execute()
prev->cgroups = ?
perf_cgroup_attach()
task_function_call(task, __perf_cgroup_move)
perf_cgroup_switch(task)
cgrp1 = perf_cgroup_from_task(prev)
cgrp2 = perf_cgroup_from_task(next)
if (cgrp1 != cgrp2)
cpuctx sched_out/in ...
task_function_call() will return -ESRCH

In the above example, prev->cgroups changing cause (cgrp1 == cgrp2)
to be true, so skip cpuctx sched_out/in. And later task_function_call()
would return -ESRCH since the prev task isn't running on cpu anymore.
So we would leave perf_events of the old prev->cgroups still sched on
the CPU, which is wrong.

The solution is that we should use cpuctx->cgrp to compare with
the next task's perf_cgroup. Since cpuctx->cgrp can only be changed
on local CPU, and we have irq disabled, we can read cpuctx->cgrp to
compare without holding ctx lock.

Fixes: a8d757ef076f ("perf events: Fix slow and broken cgroup context switch code")
Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220329154523.86438-4-zhouchengming@xxxxxxxxxxxxx
---
kernel/events/core.c | 132 +++++++-----------------------------------
1 file changed, 25 insertions(+), 107 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a08fb92..bdeb41f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -824,17 +824,12 @@ perf_cgroup_set_timestamp(struct perf_cpu_context *cpuctx)

static DEFINE_PER_CPU(struct list_head, cgrp_cpuctx_list);

-#define PERF_CGROUP_SWOUT 0x1 /* cgroup switch out every event */
-#define PERF_CGROUP_SWIN 0x2 /* cgroup switch in events based on task */
-
/*
* reschedule events based on the cgroup constraint of task.
- *
- * mode SWOUT : schedule out everything
- * mode SWIN : schedule in based on cgroup for next
*/
-static void perf_cgroup_switch(struct task_struct *task, int mode)
+static void perf_cgroup_switch(struct task_struct *task)
{
+ struct perf_cgroup *cgrp;
struct perf_cpu_context *cpuctx, *tmp;
struct list_head *list;
unsigned long flags;
@@ -845,35 +840,31 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
*/
local_irq_save(flags);

+ cgrp = perf_cgroup_from_task(task, NULL);
+
list = this_cpu_ptr(&cgrp_cpuctx_list);
list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
+ if (READ_ONCE(cpuctx->cgrp) == cgrp)
+ continue;

perf_ctx_lock(cpuctx, cpuctx->task_ctx);
perf_pmu_disable(cpuctx->ctx.pmu);

- if (mode & PERF_CGROUP_SWOUT) {
- cpu_ctx_sched_out(cpuctx, EVENT_ALL);
- /*
- * must not be done before ctxswout due
- * to event_filter_match() in event_sched_out()
- */
- cpuctx->cgrp = NULL;
- }
+ cpu_ctx_sched_out(cpuctx, EVENT_ALL);
+ /*
+ * must not be done before ctxswout due
+ * to update_cgrp_time_from_cpuctx() in
+ * ctx_sched_out()
+ */
+ cpuctx->cgrp = cgrp;
+ /*
+ * set cgrp before ctxsw in to allow
+ * perf_cgroup_set_timestamp() in ctx_sched_in()
+ * to not have to pass task around
+ */
+ cpu_ctx_sched_in(cpuctx, EVENT_ALL);

- if (mode & PERF_CGROUP_SWIN) {
- WARN_ON_ONCE(cpuctx->cgrp);
- /*
- * set cgrp before ctxsw in to allow
- * perf_cgroup_set_timestamp() in ctx_sched_in()
- * to not have to pass task around
- * we pass the cpuctx->ctx to perf_cgroup_from_task()
- * because cgorup events are only per-cpu
- */
- cpuctx->cgrp = perf_cgroup_from_task(task,
- &cpuctx->ctx);
- cpu_ctx_sched_in(cpuctx, EVENT_ALL);
- }
perf_pmu_enable(cpuctx->ctx.pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}
@@ -881,58 +872,6 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
local_irq_restore(flags);
}

-static inline void perf_cgroup_sched_out(struct task_struct *task,
- struct task_struct *next)
-{
- struct perf_cgroup *cgrp1;
- struct perf_cgroup *cgrp2 = NULL;
-
- rcu_read_lock();
- /*
- * we come here when we know perf_cgroup_events > 0
- * we do not need to pass the ctx here because we know
- * we are holding the rcu lock
- */
- cgrp1 = perf_cgroup_from_task(task, NULL);
- cgrp2 = perf_cgroup_from_task(next, NULL);
-
- /*
- * only schedule out current cgroup events if we know
- * that we are switching to a different cgroup. Otherwise,
- * do no touch the cgroup events.
- */
- if (cgrp1 != cgrp2)
- perf_cgroup_switch(task, PERF_CGROUP_SWOUT);
-
- rcu_read_unlock();
-}
-
-static inline void perf_cgroup_sched_in(struct task_struct *prev,
- struct task_struct *task)
-{
- struct perf_cgroup *cgrp1;
- struct perf_cgroup *cgrp2 = NULL;
-
- rcu_read_lock();
- /*
- * we come here when we know perf_cgroup_events > 0
- * we do not need to pass the ctx here because we know
- * we are holding the rcu lock
- */
- cgrp1 = perf_cgroup_from_task(task, NULL);
- cgrp2 = perf_cgroup_from_task(prev, NULL);
-
- /*
- * only need to schedule in cgroup events if we are changing
- * cgroup during ctxsw. Cgroup events were not scheduled
- * out of ctxsw out if that was not the case.
- */
- if (cgrp1 != cgrp2)
- perf_cgroup_switch(task, PERF_CGROUP_SWIN);
-
- rcu_read_unlock();
-}
-
static int perf_cgroup_ensure_storage(struct perf_event *event,
struct cgroup_subsys_state *css)
{
@@ -1096,16 +1035,6 @@ static inline void update_cgrp_time_from_cpuctx(struct perf_cpu_context *cpuctx,
{
}

-static inline void perf_cgroup_sched_out(struct task_struct *task,
- struct task_struct *next)
-{
-}
-
-static inline void perf_cgroup_sched_in(struct task_struct *prev,
- struct task_struct *task)
-{
-}
-
static inline int perf_cgroup_connect(pid_t pid, struct perf_event *event,
struct perf_event_attr *attr,
struct perf_event *group_leader)
@@ -1118,11 +1047,6 @@ perf_cgroup_set_timestamp(struct perf_cpu_context *cpuctx)
{
}

-static inline void
-perf_cgroup_switch(struct task_struct *task, struct task_struct *next)
-{
-}
-
static inline u64 perf_cgroup_event_time(struct perf_event *event)
{
return 0;
@@ -1142,6 +1066,10 @@ static inline void
perf_cgroup_event_disable(struct perf_event *event, struct perf_event_context *ctx)
{
}
+
+static void perf_cgroup_switch(struct task_struct *task)
+{
+}
#endif

/*
@@ -3661,7 +3589,7 @@ void __perf_event_task_sched_out(struct task_struct *task,
* cgroup event are system-wide mode only
*/
if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
- perf_cgroup_sched_out(task, next);
+ perf_cgroup_switch(next);
}

/*
@@ -3975,16 +3903,6 @@ void __perf_event_task_sched_in(struct task_struct *prev,
struct perf_event_context *ctx;
int ctxn;

- /*
- * If cgroup events exist on this CPU, then we need to check if we have
- * to switch in PMU state; cgroup event are system-wide mode only.
- *
- * Since cgroup events are CPU events, we must schedule these in before
- * we schedule in the task events.
- */
- if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
- perf_cgroup_sched_in(prev, task);
-
for_each_task_context_nr(ctxn) {
ctx = task->perf_event_ctxp[ctxn];
if (likely(!ctx))
@@ -13556,7 +13474,7 @@ static int __perf_cgroup_move(void *info)
{
struct task_struct *task = info;
rcu_read_lock();
- perf_cgroup_switch(task, PERF_CGROUP_SWOUT | PERF_CGROUP_SWIN);
+ perf_cgroup_switch(task);
rcu_read_unlock();
return 0;
}

Next message: tip-bot2 for Christophe Leroy: "[tip: locking/urgent] static_call: Remove __DEFINE_STATIC_CALL macro"
Previous message: Greg Kroah-Hartman: "[PATCH 5.17 1049/1126] block: Fix the maximum minor value is blk_alloc_ext_minor()"
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