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

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Message-ID: <Pine.LNX.4.58.0706280823020.19012@gandalf.stny.rr.com>
Date:	Thu, 28 Jun 2007 08:37:23 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dan Williams <dan.j.williams@...el.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.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>, kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

Hi Dan,

On Mon, 25 Jun 2007, Dan Williams wrote:

> Yes you are right, ARM does not flush L1 when prev==next in switch_mm.
>
> > Perhaps something else is at fault here.
> >
> I'll try and dig a bit deeper...

BTW:

 static int __init iop_adma_init (void)
 {
+       iop_adma_workqueue = create_workqueue("iop-adma");
+       if (!iop_adma_workqueue)
+               return -ENODEV;
+

Could you also try upping the prio of all the "iop-adma" threads?

You should see thread names such as (on SMP) "iop-adma/0", "iop-adma/1"
... "iop-adma/N" where N = # of CPUs - 1.

do a "chrt -p -f 98 <pid>"  once for each of the thread's PIDs.  The
chrt can be found in the package "util-linux" on Red Hat / Fedora, and in
schedutils on Debian.

It just dawned on me that workqueues don't run at a high priority by
default.  So it's funny that I'm running all my current tasklets as a low
priority work queues :-)

But that can certainly be a cause of high latency.  I need to update my
patches to make the workqueue thread a higher priority. All benchmarks on
this patch have been using a low priority work queue.

I also don't see any nice API to have the priority set for a workqueue
thread from within the kernel. Looks like one needs to be added,
otherwise, I need to have the wrapper dig into the workqueue structs to
find the thread that handles the workqueue.

Thanks,

-- Steve
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