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

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Message-ID: <CAMSQXEHjP5mzs9u5DUmuYgoEceM2OO=E8844-XQ7LPCSECoGRw@mail.gmail.com>
Date:	Wed, 16 Jan 2013 13:02:51 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>, 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

2013/1/16 Preeti U Murthy <preeti@...ux.vnet.ibm.com>:
>
> Yes.Thank you very much for the explanation :) But I dont see how the
> context switching goes away with your patch.With your patch, when the
> higher priority thread comes in when the lower priority thread is
> running in the critical section,it will see the wait queue empty and
> "continue its execution" without now wanting to enter the critical
> section.So this means it will preempt the lower priority thread because
> it is not waiting on a lock anyway.There is a context switch here right?
> I dont see any problem in scheduling due to this,but I do think your
> patch is essential.
>

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)

Regards,
Ivo Sieben
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