linux-kernel - Re: [RFC PATCH 2/2] pipe: use pipe busy wait

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Date:   Mon, 17 Sep 2018 14:05:40 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, dhaval.giani@...cle.com,
        steven.sistare@...cle.com
Subject: Re: [RFC PATCH 2/2] pipe: use pipe busy wait



On 09/07/2018 05:25 AM, Peter Zijlstra wrote:
> On Thu, Aug 30, 2018 at 01:24:58PM -0700, subhra mazumdar wrote:
>> +void pipe_busy_wait(struct pipe_inode_info *pipe)
>> +{
>> +	unsigned long wait_flag = pipe->pipe_wait_flag;
>> +	unsigned long start_time = pipe_busy_loop_current_time();
>> +
>> +	pipe_unlock(pipe);
>> +	preempt_disable();
>> +	for (;;) {
>> +		if (pipe->pipe_wait_flag > wait_flag) {
>> +			preempt_enable();
>> +			pipe_lock(pipe);
>> +			return;
>> +		}
>> +		if (pipe_busy_loop_timeout(pipe, start_time))
>> +			break;
>> +		cpu_relax();
>> +	}
>> +	preempt_enable();
>> +	pipe_lock(pipe);
>> +	if (pipe->pipe_wait_flag > wait_flag)
>> +		return;
>> +	pipe_wait(pipe);
>> +}
>> +
>> +void wake_up_busy_poll(struct pipe_inode_info *pipe)
>> +{
>> +	pipe->pipe_wait_flag++;
>> +}
> Why not just busy wait on current->state ? A little something like:
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index bdc5d3c0977d..8d9f1c95ff99 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -106,6 +106,7 @@ void pipe_double_lock(struct pipe_inode_info *pipe1,
>   void pipe_wait(struct pipe_inode_info *pipe)
>   {
>   	DEFINE_WAIT(wait);
> +	u64 start;
>   
>   	/*
>   	 * Pipes are system-local resources, so sleeping on them
> @@ -113,7 +114,15 @@ void pipe_wait(struct pipe_inode_info *pipe)
>   	 */
>   	prepare_to_wait(&pipe->wait, &wait, TASK_INTERRUPTIBLE);
>   	pipe_unlock(pipe);
> -	schedule();
> +
> +	preempt_disable();
> +	start = local_clock();
> +	while (!need_resched() && current->state != TASK_RUNNING &&
> +			(local_clock() - start) < pipe->poll_usec)
> +		cpu_relax();
> +	schedule_preempt_disabled();
> +	preempt_enable();
> +
>   	finish_wait(&pipe->wait, &wait);
>   	pipe_lock(pipe);
>   }
This will make the current thread always spin and block as it itself does
the state change to TASK_RUNNING in finish_wait.