linux-kernel - Re: [PATCH] Remove useless test during exchanging values

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Date:	Thu, 21 Apr 2016 15:27:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Minfei Huang <mnghuan@...il.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Remove useless test during exchanging values

On Thu, Apr 21, 2016 at 08:28:28PM +0800, Minfei Huang wrote:
> The value delta is correct as well, although calc_load_idle[idx] is
> equal to 0. Remove this useless test to improve performance, since this
> function is called more frequently.
> 
> Signed-off-by: Minfei Huang <mnghuan@...il.com>
> ---
>  kernel/sched/loadavg.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index ef71590..5a5d7ae 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -216,10 +216,9 @@ void calc_load_exit_idle(void)
>  static long calc_load_fold_idle(void)
>  {
>  	int idx = calc_load_read_idx();
> -	long delta = 0;
> +	long delta;
>  
> -	if (atomic_long_read(&calc_load_idle[idx]))
> -		delta = atomic_long_xchg(&calc_load_idle[idx], 0);
> +	delta = atomic_long_xchg(&calc_load_idle[idx], 0);

This will actually degrade performance.

The read can have the cacheline in shared mode, while the xchg will
force it into exclusive mode. Also, the xchg atomic operation is a
really expensive instruction.