linux-hardening - RE: [RFC 1/1] drivers/dma/*: replace tasklets with workqueue

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Date:   Wed, 25 May 2022 15:14:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>
CC:     Allen Pais <apais@...ux.microsoft.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "olivier.dautricourt@...lia.com" <olivier.dautricourt@...lia.com>,
        Stefan Roese <sr@...x.de>, Vinod Koul <vkoul@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "Ludovic Desroches" <ludovic.desroches@...rochip.com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "Ray Jui" <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        "Nicolas Saenz Julienne" <nsaenz@...nel.org>,
        Paul Cercueil <paul@...pouillou.net>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>,
        "Gustavo Pimentel" <gustavo.pimentel@...opsys.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Leo Li <leoyang.li@....com>,
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        Samuel Holland <samuel@...lland.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [RFC 1/1] drivers/dma/*: replace tasklets with workqueue

From: Arnd Bergmann
> Sent: 25 May 2022 12:04
> 
> On Wed, May 25, 2022 at 11:24 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Tue, Apr 19, 2022 at 11:17 PM Allen Pais <apais@...ux.microsoft.com> wrote:
> >
> > > The tasklet is an old API which will be deprecated, workqueue API
> > > cab be used instead of them.
> > >
> > > This patch replaces the tasklet usage in drivers/dma/* with a
> > > simple work.
> > >
> > > Github: https://github.com/KSPP/linux/issues/94
> > >
> > > Signed-off-by: Allen Pais <apais@...ux.microsoft.com>
> >
> > Paging Vincent Guittot and Arnd Bergmann on the following question
> > on this patch set:
> >
> > - Will replacing tasklets with workque like this negatively impact the
> >   performance on DMA engine bottom halves?
> 
> I think it will in some cases but not others. The problem I see is that
> the short patch description makes it sound like a trivial conversion of a
> single subsystem, but in reality this interacts with all the drivers using
> DMA engines, including tty/serial, sound, mmc and spi.
> 
> In many cases, the change is an improvement, but I can see a number
> of ways this might go wrong:

If the 'tasklet' API is based on the softint (or similar)
then changing to workqueue will cause serious grief in many cases
unless the workqueue process runs at a high priority.

Currently softint callbacks are usually higher priority than
any task/process.
So on a busy system they almost always run.
(They can get caught out by a need_resched() call and suddenly
be fighting with normal user processes for cpu time.)

As arnd said, I suspect this will break anything using tasklets
to chain together audio or video buffers.
Any process code doing that would need probbaly to run at a
'middling' RT priority.

	David

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