linux-kernel - Re: [PATCH] sched: fix task and run queue run

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Message-ID: <20151001094313.GA3281@worktop.programming.kicks-ass.net>
Date:	Thu, 1 Oct 2015 11:43:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Meyer, Mike" <Mike.Meyer@...adata.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: [PATCH] sched: fix task and run queue run_delay inconsistencies

On Thu, Oct 01, 2015 at 08:37:32AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 30, 2015 at 08:28:41PM +0000, Meyer, Mike wrote:

> > Yes that will also address the issue.
> > 
> > The reason I approached the way I did was to avoid adding code path to
> > the far more common uses of {en,de}queue_task() but I doubt anyone is
> > going to notice a difference with the addition of some register
> > save/restores and a compare in that path.  Overall the code does
> > shrink with the alternative which is good.
> 
> In most cases the flags should be compile time constants, and with the
> inline we can determine the branch at compile time, avoiding emitting
> that branch instruction entirely.
> 
> But let me double check the asm for a few important sites.

It looks like the sites in the wakeup path do indeed not get any
additional conditionals.

> > My only comment is I am not sure about the naming of the flag
> > ENQUEUE_TEMP which implies (to me) the enqueue is temporary which
> > clearly it isn't.    Maybe something like DEQUEUE_MOVE/ENQUEUE_MOVE
> > would be a bit more descriptive of the use case.
> 
> Yes, I ran out of creative juices, let me attempt a better name once
> I've woken up a bit.

How about DEQUEUE_SAVE, ENQUEUE_RESTORE ? Ideally I'd wrap the whole
pattern into a helper but C isn't really supportive of pre+post patterns
like this.

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