Hi, On 14/03/19 4:45 PM, Grygorii Strashko wrote: > > > On 12.03.19 19:30, Rizvi, Mohammad Faiz Abbas wrote: >> Hi Adrian, >> >> On 3/8/2019 7:06 PM, Adrian Hunter wrote: >>> On 6/03/19 12:00 PM, Faiz Abbas wrote: >>>> Adrian, >>>> >>>> On 25/02/19 1:47 PM, Adrian Hunter wrote: >>>>> On 15/02/19 9:20 PM, Faiz Abbas wrote: >>>>>> sdhci.c has two bottom halves implemented. A threaded_irq for handling >>>>>> card insert/remove operations and a tasklet for finishing mmc requests. >>>>>> With the addition of external dma support, dmaengine APIs need to >>>>>> terminate in non-atomic context before unmapping the dma buffers. >>>>>> >>>>>> To facilitate this, remove the finish_tasklet and move the call of >>>>>> sdhci_request_done() to the threaded_irq() callback. >>>>> >>>>> The irq thread has a higher latency than the tasklet. The performance drop >>>>> is measurable on the system I tried: >>>>> >>>>> Before: >>>>> >>>>> # dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 & >>>>> 1+0 records in >>>>> 1+0 records out >>>>> 1073741824 bytes (1.1 GB) copied, 4.44502 s, 242 MB/s >>>>> >>>>> After: >>>>> >>>>> # dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 & >>>>> 1+0 records in >>>>> 1+0 records out >>>>> 1073741824 bytes (1.1 GB) copied, 4.50898 s, 238 MB/s >>>>> >>>>> So we only want to resort to the thread for the error case. >>>>> >>>> >>>> Sorry for the late response here, but this is about 1.6% decrease. I >>>> tried out the same commands on a dra7xx board here (with about 5 >>>> consecutive dd of 1GB) and the average decrease was 0.3%. I believe you >>>> will also find a lesser percentage change if you average over multiple >>>> dd commands. >>>> >>>> Is this really so significant that we have to maintain two different >>>> bottom halves and keep having difficulty with adding APIs that can sleep? >>> >>> It is a performance drop that can be avoided, so it might as well be. >>> Splitting the success path from the failure path is common for I/O drivers >>> for similar reasons as here: the success path can be optimized whereas the >>> failure path potentially needs to sleep. >> >> Understood. You wanna keep the success path as fast as possible. > > Sry, I've not completely followed this series, but I'd like to add 5c > > It's good thing to get rid of tasklets hence RT Linux kernel is actively moving towards to LKML > and there everything handled in threads (even networking trying to get rid of softirqs). > > Performance is pretty relative thing here - just try to run network traffic in parallel, and > there are no control over it comparing to threads. Now way to assign priority or pin to CPU. There is a 2007 LWN article(https://lwn.net/Articles/239633/) which talks about removing tasklets altogether. I wonder what happened after that. Thanks, Faiz
