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

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Date:	Fri, 22 Jun 2007 23:13:30 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

On Fri, 2007-06-22 at 22:00 +0100, Christoph Hellwig 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
> 
> This is rather inefficient due to all the (semi-)context switches already
> and not by far the worst setup given that a lot of dm modules can involve
> another thread in the process.
> 
> Now if just plain convert tasklets to a thread based abstraction this
> existing code becomes really dumb because we go from hardirq to process
> context to go back to softirq context to go back to process context.
> 
> Ouch!
> 
> I think we need to put a little more though into how we can optimize our
> irq path for the full stack.  Using irqthreads in an intelligent way might
> be one option, but we'll need a lot of heavy benchmarking whatever way
> we go.

Your above scenario screams for a threaded interrupt handler, where you
actually can unify a lot of this into one single context.

	tglx


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