linux-kernel - Re: [PATCH 1/2] lib: more scalable list

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Date:	Fri, 22 Jan 2010 11:43:21 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Don Mullis <don.mullis@...il.com>
Cc:	linux-kernel@...r.kernel.org, airlied@...hat.com,
	david@...morbit.com, dedekind@...radead.org
Subject: Re: [PATCH 1/2] lib: more scalable list_sort()

Don Mullis <don.mullis@...il.com> writes:
>
> Being just a dumb library routine, list_sort() has no idea what context
> it's been called in, how long a list a particular client could pass in,
> nor how expensive the client's cmp() callback might be.
>
> The cmp() callback already passes back a client-private pointer.
> Hanging off of this could be a count of calls, or timing information,
> maintained by the client.  Whenever some threshold is reached, the
> client's cmp() could do whatever good CPU-sharing citizenship required.

need_resched() does all the timing/thresholding (it checks the 
reschedule flag set by the timer interrupt). You just have to call it.
But preferable not in the inner loop, but in a outer one. It's
not hyper-expensive, but it's not free either.

The drawback is that if it's called the context always has to
allow sleeping, so it might need to be optional.

Anyways a better fix might be simply to ensure in the caller
that lists never get as long that they become a scheduling
hazard. But you indicated that ubifs would pass very long lists?
Perhaps ubifs (and other calls who might have that problem) simply
needs to be fixed.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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