linux-kernel - Re: [PATCH] tty: Only wakeup the line discipline idle queue when queue is active

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Message-ID: <20130118154504.GA28072@redhat.com>
Date:	Fri, 18 Jan 2013 16:45:04 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Ivo Sieben <meltedpianoman@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>, Jiri Slaby <jslaby@...e.cz>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-serial@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] tty: Only wakeup the line discipline idle queue when
	queue is active

On 01/17, Preeti U Murthy wrote:
>
> On 01/16/2013 05:32 PM, Ivo Sieben wrote:
> >
> > I don't have a problem that there is a context switch to the high
> > priority process: it has a higher priority, so it probably is more
> > important.
> > My problem is that even when the waitqueue is empty, the high priority
> > thread has a risk to block on the spinlock needlessly (causing context
> > switches to low priority task and back to the high priority task)
> >
> Fair enough Ivo.I think you should go ahead with merging the
> waitqueue_active()
>   wake_up()
> logic into the wake_up() variants.

This is not easy. We can't simply change wake_up*() helpers or modify
__wake_up().

I can't understand why do you dislike Ivo's simple patch. There are
a lot of "if (waitqueue_active) wake_up" examples. Even if we add the
new helpers (personally I don't think this makes sense) , we can do
this later. Why should we delay this fix?

Oleg.

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