linux-kernel - Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

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Date:	Fri, 22 Jun 2007 23:10:32 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, matthew.wilcox@...com,
	kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues


* Christoph Hellwig <hch@...radead.org> wrote:

> Note that we also have a lot of inefficiency in the way we do deferred 
> processing.  Think of a setup where you run a XFS filesystem runs over 
> a megaraid adapter.
> 
>  (1) we get a real hardirq, which just clears the interrupt and then
>      deferes to a tasklet
>  (2) tasklet walks the producer / consumer queue and then calls scsi_done
>      for each completeted scsi command which ends up doing
>      raise_softirq_irqoff(BLOCK_SOFTIRQ);
>  (3) block softirq does the heavy lifting for command completion and finally
>      calls back into the bio's completion routine
>  (4) xfs wants to avoid irq safe locking and thus deferes the command to a
>      kthread

i dont understand - why is a tasklet used at all? Why not do it straight 
in the BLOCK_SOFTIRQ? Using tasklets there is extra, unnecessary 
overhead already.

	Ingo
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