linux-kernel - Re: [PATCH 04/21] sched: implement scheduler notifiers

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Message-ID: <4B019D40.4010108@kernel.org>
Date:	Tue, 17 Nov 2009 03:43:12 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Avi Kivity <avi@...hat.com>
CC:	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	andi@...stfloor.org, fweisbec@...il.com
Subject: Re: [PATCH 04/21] sched: implement scheduler notifiers

Hello,

Avi Kivity wrote:
> Four hlist_heads (64 bytes) is pretty heavy for this.

hlist_head is one pointer, so it will be 32bytes on 64bit machines.

> I having all members present in sched_notifier (instead of a union)
> and calling a callback if it is not NULL.  This reduces the overhead
> to 16 bytes at the expense of an extra check for sched_notifier
> users.

And it will reduce the overhead to 8 bytes.  Anyways, Linus was
against walking the list multiple times for different callbacks and
the way kvm uses these notifiers doesn't work very well with
allocating separate table on demand, so I just went with four
pointers.  Given that these notifiers are quite unpopular yet, I lean
toward Avi's suggestion.  Linus?

> Besides this, is there any difference to preempt_notifiers?  if not we
> can just add the new members and rename.

Yeap, if we're gonna add things to ops table, I agree that would be
better.

Thanks.

-- 
tejun
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