The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 96492a6c558acb56124844d1409d9ef8624a0322 Mon Sep 17 00:00:00 2001 From: Chengming Zhou zhouchengming@bytedance.com Date: Tue, 29 Mar 2022 23:45:22 +0800 Subject: [PATCH] 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@bytedance.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lore.kernel.org/r/20220329154523.86438-4-zhouchengming@bytedance.com
diff --git a/kernel/events/core.c b/kernel/events/core.c index a08fb92b3934..bdeb41fe7f15 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; }