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perf/core: Fix perf_cgroup_switch()
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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 a8d757e ("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: a8d757e ("perf events: Fix slow and broken cgroup context switch code")
Signed-off-by: Chengming Zhou <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
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Chengming Zhou authored and Peter Zijlstra committed Apr 5, 2022
1 parent 6875186 commit 96492a6
Showing 1 changed file with 25 additions and 107 deletions.
132 changes: 25 additions & 107 deletions kernel/events/core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -845,94 +840,38 @@ 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);
}

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)
{
Expand Down Expand Up @@ -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)
Expand All @@ -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;
Expand All @@ -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

/*
Expand Down Expand Up @@ -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);
}

/*
Expand Down Expand Up @@ -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))
Expand Down Expand Up @@ -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;
}
Expand Down

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