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

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Date:   Tue, 4 Sep 2018 17:20:11 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        dhaval.giani@...cle.com, steven.sistare@...cle.com
Subject: Re: [RFC PATCH 2/2] pipe: use pipe busy wait



On 09/04/2018 02:54 PM, Thomas Gleixner wrote:
> On Thu, 30 Aug 2018, 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();
> You are not really serious about busy looping with preemption disabled?
>
> That's just wrong. Why do you want to block others from getting on the CPU
> if there is nothing in the pipe?
>
> There is no point in doing so, really. If the wait loop is preempted
> because there is more important work to do, then it will come back and
> either see new data, or leave due to wait time reached.
Makes sense. I will remove it.

Thanks,
Subhra
>
> Thanks,
>
> 	tglx
>