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Date:   Wed, 2 Dec 2020 17:19:34 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Ian Rogers <irogers@google.com>,
        Andi Kleen <ak@linux.intel.com>
Subject: Re: [RFC 1/2] perf core: Add PERF_COUNT_SW_CGROUP_SWITCHES event
Message-ID: <20201202161934.GL3021@hirez.programming.kicks-ass.net>
References: <20201202150205.35750-1-namhyung@kernel.org>
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In-Reply-To: <20201202150205.35750-1-namhyung@kernel.org>
Precedence: bulk

On Thu, Dec 03, 2020 at 12:02:04AM +0900, Namhyung Kim wrote:

> +#ifdef CONFIG_CGROUP_PERF
> +static inline void
> +perf_sw_event_cgroup_switch(struct task_struct *prev, struct task_struct *next)
> +{
> +	struct cgroup *prev_cgrp, *next_cgrp;
> +
> +	rcu_read_lock();
> +
> +	prev_cgrp = task_css_check(prev, perf_event_cgrp_id, 1)->cgroup;
> +	next_cgrp = task_css_check(next, perf_event_cgrp_id, 1)->cgroup;
> +
> +	if (prev_cgrp != next_cgrp)
> +		perf_sw_event_sched(PERF_COUNT_SW_CGROUP_SWITCHES, 1, 0);
> +
> +	rcu_read_unlock();
> +}
> +#else
> +static inline void perf_sw_event_cgroup_switch(struct task_struct *prev,
> +					       struct task_struct *next) {}
> +#endif  /* CONFIG_CGROUP_PERF */
> +
>  extern struct static_key_false perf_sched_events;
>  
>  static __always_inline bool
> @@ -1220,6 +1241,7 @@ static inline void perf_event_task_sched_out(struct task_struct *prev,
>  					     struct task_struct *next)
>  {
>  	perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
> +	perf_sw_event_cgroup_switch(prev, next);
>  
>  	if (static_branch_unlikely(&perf_sched_events))
>  		__perf_event_task_sched_out(prev, next);

Urgh.. that's horrible, try something like this.

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 9a38f579bc76..5eb284819ee5 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1174,25 +1174,19 @@ DECLARE_PER_CPU(struct pt_regs, __perf_regs[4]);
  * which is guaranteed by us not actually scheduling inside other swevents
  * because those disable preemption.
  */
-static __always_inline void
-perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)
+static __always_inline void __perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)
 {
-	if (static_key_false(&perf_swevent_enabled[event_id])) {
-		struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
+	struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
 
-		perf_fetch_caller_regs(regs);
-		___perf_sw_event(event_id, nr, regs, addr);
-	}
+	perf_fetch_caller_regs(regs);
+	___perf_sw_event(event_id, nr, regs, addr);
 }
 
 extern struct static_key_false perf_sched_events;
 
-static __always_inline bool
-perf_sw_migrate_enabled(void)
+static __always_inline bool __perf_sw_enabled(int swevt)
 {
-	if (static_key_false(&perf_swevent_enabled[PERF_COUNT_SW_CPU_MIGRATIONS]))
-		return true;
-	return false;
+	return static_key_false(&perf_swevent_enabled[swevt]);
 }
 
 static inline void perf_event_task_migrate(struct task_struct *task)
@@ -1207,11 +1201,9 @@ static inline void perf_event_task_sched_in(struct task_struct *prev,
 	if (static_branch_unlikely(&perf_sched_events))
 		__perf_event_task_sched_in(prev, task);
 
-	if (perf_sw_migrate_enabled() && task->sched_migrated) {
-		struct pt_regs *regs = this_cpu_ptr(&__perf_regs[0]);
-
-		perf_fetch_caller_regs(regs);
-		___perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, regs, 0);
+	if (__perf_sw_enabled(PERF_COUNT_SW_CPU_MIGRATIONS) &&
+	    task->sched_migrated) {
+		__perf_sw_event_sched(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 0);
 		task->sched_migrated = 0;
 	}
 }
@@ -1219,7 +1211,13 @@ static inline void perf_event_task_sched_in(struct task_struct *prev,
 static inline void perf_event_task_sched_out(struct task_struct *prev,
 					     struct task_struct *next)
 {
-	perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
+	if (__perf_sw_enabled(PERF_COUNT_SW_CONTEXT_SWITCHES))
+		__perf_sw_event_sched(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 0);
+
+	if (__perf_sw_enabled(PERF_COUNT_SW_CGROUP_SWITCHES) &&
+	    (task_css_check(prev, perf_event_cgrp_id, 1)->cgroup !=
+	     task_css_check(next, perf_event_cgrp_id, 1)->cgroup))
+		__perf_sw_event_sched(PERF_COUNT_SW_CGROUP_SWITCHES, 1, 0);
 
 	if (static_branch_unlikely(&perf_sched_events))
 		__perf_event_task_sched_out(prev, next);
@@ -1475,8 +1473,6 @@ static inline int perf_event_refresh(struct perf_event *event, int refresh)
 static inline void
 perf_sw_event(u32 event_id, u64 nr, struct pt_regs *regs, u64 addr)	{ }
 static inline void
-perf_sw_event_sched(u32 event_id, u64 nr, u64 addr)			{ }
-static inline void
 perf_bp_event(struct perf_event *event, void *data)			{ }
 
 static inline int perf_register_guest_info_callbacks