linux-kernel - Re: [RFC][PATCH 2/2] add a counter for writers spinning on a rwlock

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Date:	Mon, 26 Jan 2009 17:04:34 +0100
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...gle.com>
Subject: Re: [RFC][PATCH 2/2] add a counter for writers spinning on a rwlock

2009/1/26 Peter Zijlstra <peterz@...radead.org>:
> On Mon, 2009-01-26 at 16:25 +0100, Frédéric Weisbecker wrote:
>> 2009/1/26 Peter Zijlstra <peterz@...radead.org>:
>> > On Sun, 2009-01-25 at 12:50 -0800, Frederic Weisbecker wrote:
>> >> This patch adds a counter for writers that enter a rwlock slow path.
>> >> For example it can be useful for slow background tasks which perform some jobs
>> >> on the tasklist, such as the hung_task detector (kernel/hung_task.c).
>> >>
>> >> It adds a inc/dec pair on the slow path and 4 bytes for each rwlocks, so the overhead
>> >> is not null.
>> >>
>> >> Only x86 is supported for now, writers_spinning_lock() will return 0 on other archs (which
>> >> is perhaps not a good idea).
>> >>
>> >> Comments?
>> >
>> > _why_ ?
>>
>> The hung task detector runs a periodic loop through the task_list, and
>> currently it doesn't run
>> over an arbitrary threshold of tasks to not hold the task_list lock
>> for too long.
>>
>> So we thought about a way to detect if there are some writers waiting
>> for the lock, anf if so, release
>> the lock, schedule and retry.
>
> Ah, if it can do that, then it can also use RCU, no? Only users who
> really have to hold off new tasks need the read-task_lock. The rest can
> use RCU.


Really?
That sounds a good news. Mandeep, what do you think?
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