Re: [PATCH 9/9] mmc: Convert from tasklet to BH workqueue

From: Linus Walleij <linus.walleij@linaro.org>
To: Ulf Hansson <ulf.hansson@linaro.org>
Cc: imx@lists.linux.dev, linux-hyperv@vger.kernel.org,
	oneukum@suse.com, duncan.sands@free.fr,
	hayashi.kunihiko@socionext.com, linux-mmc@vger.kernel.org,
	aubin.constans@microchip.com, Frank.Li@nxp.com,
	linux-usb@vger.kernel.org, HaraldWelte@viatech.com,
	paul@crapouillou.net, linux-tegra@vger.kernel.org,
	netdev@vger.kernel.org, maintainers@bluecherrydvr.com,
	peter.ujfalusi@gmail.com, manivannan.sadhasivam@linaro.org,
	linux-riscv@lists.infradead.org, kys@microsoft.com,
	robert.jarzmik@free.fr, haijie1@huawei.com,
	linux-renesas-soc@vger.kernel.org, wei.liu@kernel.org,
	linux-omap@vger.kernel.org, florian.fainelli@broadcom.com,
	linux-rdma@vger.kernel.org, vireshk@kernel.org,
	jassisinghbrar@gmail.com, decui@microsoft.com,
	wangzhou1@hisilicon.com, jernej.skrabec@gmail.com,
	jh80.chung@samsung.com, zw@zh-kernel.org, wens@csie.org,
	stern@rowland.harvard.edu, linux-arm-msm@vger.kernel.org,
	orsonzhai@gmail.com, pierre@ossman.eu,
	linux-mips@vger.kernel.org, Eugeniy.Paltsev@synopsys.com,
	patrice.ch otard@foss.st.com, asahi@lists.linux.dev,
	brucechang@via.com.tw, keescook@chromium.org, oakad@yahoo.com,
	sven@svenpeter.dev, rjui@broadcom.com, s.hauer@pengutronix.de,
	sean.wang@mediatek.com, linux-actions@lists.infradead.org,
	linuxppc-dev@lists.ozlabs.org, haojian.zhuang@gmail.com,
	mirq-linux@rere.qmqm.pl, dmaengine@vger.kernel.org,
	linux-mediatek@lists.infradead.org,
	linux-rpi-kernel@lists.infradead.org,
	baolin.wang@linux.alibaba.com, matthias.bgg@gmail.com,
	openipmi-developer@lists.sourceforge.net, mchehab@kernel.org,
	Allen Pais <apais@linux.microsoft.com>,
	linux-arm-kernel@lists.infradead.org,
	angelogioacchino.delregno@collabora.com, sbranden@broadcom.com,
	logang@deltatee.com, andersson@kernel.org, marcan@marcan.st,
	haiyangz@microsoft.com, linux-kernel@vger.kernel.org,
	leoyang.li@nxp.com, konrad.dybcio@linaro.org,
	linux-sunxi@lists.linux.dev, vkoul@kernel.org,
	linux-s390@vger.kernel.org, mhiramat@kernel.org,
	zhang.lyra@gmail.com, tj@kernel.org, manuel.lauss@gmail.com,
	linux-medi a@vger.kernel.org, shawnguo@kernel.org,
	afaerber@suse.de, daniel@zonque.org
Subject: Re: [PATCH 9/9] mmc: Convert from tasklet to BH workqueue
Date: Thu, 28 Mar 2024 14:37:09 +0100	[thread overview]
Message-ID: <CACRpkdZ7wAbtTUmmLCef7KnATmfZeAL26Q-gLqnGe3CdZ3+O3A@mail.gmail.com> (raw)
In-Reply-To: <CAPDyKFpuKadPQv6+61C2pE4x4FE-DUC5W6WCCPu9Nb2DnDB56g@mail.gmail.com>

On Thu, Mar 28, 2024 at 1:54 PM Ulf Hansson <ulf.hansson@linaro.org> wrote:

> At this point we have suggested to drivers to switch to use threaded
> irq handlers (and regular work queues if needed too). That said,
> what's the benefit of using the BH work queue?

Context:
https://lwn.net/Articles/960041/
"Tasklets, in particular, remain because they offer lower latency than
workqueues which, since they must go through the CPU scheduler,
can take longer to execute a deferred-work item."

The BH WQ is controlled by a software IRQ and quicker than an
ordinary work item.

I don't know if this little latency could actually affect any MMC
device, I doubt it.

The other benefit IIUC is that it is easy to mechanically rewrite tasklets
to BH workqueues and be sure that it is as fast as the tasklet, if you want
to switch to threaded IRQ handlers or proper work, you need to write a
lot of elaborate code and test it (preferably on real hardware).

Yours,
Linus Walleij