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Date:   Wed, 6 Mar 2019 11:09:07 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yafang Shao <laoar.shao@gmail.com>
Cc:     mingo@redhat.com, linux-kernel@vger.kernel.org,
        shaoyafang@didiglobal.com
Subject: Re: [PATCH] sched: fair: fix missed CONFIG_SCHEDSTATS
Message-ID: <20190306100907.GT32477@hirez.programming.kicks-ass.net>
References: <1551861826-12592-1-git-send-email-laoar.shao@gmail.com>
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On Wed, Mar 06, 2019 at 04:43:46PM +0800, Yafang Shao wrote:
> When I'm using trace_sched_stat_{iowait, blocked, wait, sleep} to
> measure how long the processes are stalled, there's always no output from
> trace_pipe while there're really some tasks in uninterruptible sleep
> state. That makes me confused, so I try to investigate why.
> Finally I find the reason is that CONFIG_SCHEDSTATS is not set.
> 
> To avoid such kind of confusion, we should not expose these tracepoints
> if CONFIG_SCHEDSTATS is not set.

Yeah, lets not sprinkle #ifdef. Big fat NAK.

Also, the below seem to indicate your compiler is stupid. Without
CONFIG_SCHEDSTAT, schedstat_enabled() should be a constant 0 and DCE
should delete all code.

> @@ -976,6 +982,7 @@ static void update_curr_fair(struct rq *rq)
>  static inline void
>  update_stats_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  {
> +#ifdef CONFIG_SCHEDSTATS
>  	if (!schedstat_enabled())
>  		return;
>  
> @@ -988,12 +995,13 @@ static void update_curr_fair(struct rq *rq)
>  
>  	if (flags & ENQUEUE_WAKEUP)
>  		update_stats_enqueue_sleeper(cfs_rq, se);
> +#endif
>  }
>  
>  static inline void
>  update_stats_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  {
> -
> +#ifdef CONFIG_SCHEDSTATS
>  	if (!schedstat_enabled())
>  		return;
>  
> @@ -1014,6 +1022,7 @@ static void update_curr_fair(struct rq *rq)
>  			__schedstat_set(se->statistics.block_start,
>  				      rq_clock(rq_of(cfs_rq)));
>  	}
> +#endif
>  }
>  
>  /*
> @@ -4090,6 +4099,7 @@ static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  	update_stats_curr_start(cfs_rq, se);
>  	cfs_rq->curr = se;
>  
> +#ifdef CONFIG_SCHEDSTATS
>  	/*
>  	 * Track our maximum slice length, if the CPU's load is at
>  	 * least twice that of our own weight (i.e. dont track it
> @@ -4100,6 +4110,7 @@ static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  			max((u64)schedstat_val(se->statistics.slice_max),
>  			    se->sum_exec_runtime - se->prev_sum_exec_runtime));
>  	}
> +#endif
>  
>  	se->prev_sum_exec_runtime = se->sum_exec_runtime;
>  }
> -- 
> 1.8.3.1
>