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Date: Tue, 21 May 2019 09:52:48 +0200 From: Enric Balletbo i Serra <enric.balletbo@...labora.com> To: Guenter Roeck <groeck@...gle.com>, Douglas Anderson <dianders@...omium.org> Cc: Mark Brown <broonie@...nel.org>, Benson Leung <bleung@...omium.org>, "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>, Nicolas Boichat <drinkcat@...omium.org>, Guenter Roeck <groeck@...omium.org>, Brian Norris <briannorris@...omium.org>, Matthias Kaehlcke <mka@...omium.org>, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers Hi, On 15/5/19 19:02, Guenter Roeck wrote: > On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@...omium.org> wrote: >> >> In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer >> messages at high priority") we moved transfers to a high priority >> workqueue. This helped make them much more reliable. >> >> ...but, we still saw failures. >> >> We were actually finding ourselves competing for time with dm-crypt >> which also scheduled work on HIGHPRI workqueues. While we can >> consider reverting the change that made dm-crypt run its work at >> HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use >> WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling. >> It does make sense for IO to be scheduled at a priority that's higher >> than the default user priority. It also turns out that dm-crypt isn't >> alone in using high priority like this. loop_prepare_queue() does >> something similar for loopback devices. >> >> Looking in more detail, it can be seen that the high priority >> workqueue isn't actually that high of a priority. It runs at MIN_NICE >> which is _fairly_ high priority but still below all real time >> priority. >> >> Should we move cros_ec_spi to real time priority to fix our problems, >> or is this just escalating a priority war? I'll argue here that >> cros_ec_spi _does_ belong at real time priority. Specifically >> cros_ec_spi actually needs to run quickly for correctness. As I >> understand this is exactly what real time priority is for. >> >> There currently doesn't appear to be any way to use the standard >> workqueue APIs with a real time priority, so we'll switch over to >> using using a kthread worker. We'll match the priority that the SPI >> core uses when it wants to do things on a realtime thread and just use >> "MAX_RT_PRIO - 1". >> >> This commit plus the patch ("platform/chrome: cros_ec_spi: Request the >> SPI thread be realtime") are enough to get communications very close >> to 100% reliable (the only known problem left is when serial console >> is turned on, which isn't something that happens in shipping devices). >> Specifically this test case now passes (tested on rk3288-veyron-jerry): >> >> dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512& >> while true; do >> ectool version > /dev/null; >> done >> >> It should be noted that "/var/log" is encrypted (and goes through >> dm-crypt) and also passes through a loopback device. >> >> Signed-off-by: Douglas Anderson <dianders@...omium.org> > > Reviewed-by: Guenter Roeck <groeck@...omium.org> > Added to the for-next branch for the autobuilders to play with, if all goes well will be queued in chrome-platform-5.3 Thanks, Enric
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