linux-kernel - Re: [PATCH v3] softlockup: remove hung_task_check

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Message-ID: <20090124015513.GA31189@google.com>
Date:	Fri, 23 Jan 2009 17:55:14 -0800
From:	Mandeep Singh Baines <msb@...gle.com>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	rientjes@...gle.com, mbligh@...gle.com, thockin@...gle.com
Subject: Re: [PATCH v3] softlockup: remove hung_task_check_count

Frédéric Weisbecker (fweisbec@...il.com) wrote:
> 2009/1/23 Ingo Molnar <mingo@...e.hu>:
> >
> > not sure i like the whole idea of removing the max iterations check. In
> > theory if there's a _ton_ of tasks, we could spend a lot of time looping
> > there. So it always looked prudent to limit it somewhat.
> >
> 
> Which means we can loose several of them. Would it hurt to iterate as
> much as possible along the task list,
> keeping some care about writers starvation and latency?
> BTW I thought about the slow work framework, but I can't retrieve
> it....  But this thread has already a slow priority.
> 
> Would it be interesting to provide a way for rwlocks to know if there
> is writer waiting for the lock?

Would be cool if that API existed. You could release the CPU and/or lock as
soon as either was contended for. You'd have the benefits of fine-grained
locking without the overhead of locking and unlocking multiple time.

Currently, there is no bit that can tell you there is a writer waiting. You'd
probably need to change the write_lock() implementation at a minimum. Maybe
if the first writer left the RW_LOCK_BIAS bit clear and then waited for the
readers to leave instead of re-trying? That would actually make write_lock()
more efficient for the 1-writer case since you'd only need to spin doing
a read in the failure case instead of an atomic_dec and atomic_inc.

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