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

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Date:	Mon, 25 Jun 2007 18:46:39 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"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>,
	"Christoph Hellwig" <hch@...radead.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

> so how about the following, different approach: anyone who has a tasklet
> in any performance-sensitive codepath, please yell now. We'll also do a
> proactive search for such places. We can convert those places to
> softirqs, or move them back into hardirq context. Once this is done -
> and i doubt it will go beyond 1-2 places - we can just mass-convert the
> other 110 places to the lame but compatible solution of doing them in a
> global thread context.
>

I have a driver / testcase that reacts negatively to a workqueue
conversion.  This is with the iop-adma driver on an ARM based platform
re-syncing a degraded raid5 array.  The driver is currently in -mm and
it uses tasklets to run a short callback routine upon completion of an
offloaded memcpy or xor operation.  Quick tests show that
write-throughput does not go down too much, but resync speed, as
reported by /proc/mdstat, drops from ~50MB/s to ~30MB/s.

Context switches on this platform flush the L1 cache so bouncing
between a workqueue and the MD thread is painful.

The conversion patch is attached.

--
Dan

View attachment "iop-adma-workq-conv.patch" of type "text/x-patch" (3739 bytes)