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Date:   Tue, 18 Sep 2018 00:43:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Subhra Mazumdar <subhra.mazumdar@oracle.com>
Cc:     linux-kernel@vger.kernel.org, dhaval.giani@oracle.com,
        steven.sistare@oracle.com
Subject: Re: [RFC PATCH 2/2] pipe: use pipe busy wait
Message-ID: <20180917224344.GB3284@worktop.programming.kicks-ass.net>
References: <20180830202458.32579-1-subhra.mazumdar@oracle.com>
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 <20180907122557.GY24106@hirez.programming.kicks-ass.net>
 <304ffc06-1ab9-19d5-d5ca-e63e2abe2900@oracle.com>
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On Mon, Sep 17, 2018 at 02:05:40PM -0700, Subhra Mazumdar wrote:
> On 09/07/2018 05:25 AM, Peter Zijlstra wrote:

> >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.

Nah, the actual wakeup will also do that state change. The one in
finish_wait() is for the case where the wait condition became true
without wakeup, such that we don't 'leak' the INTERRUPTIBLE state.