On 06/11/2015 05:38 AM, Jakub Kiciński wrote: > On Wed, 10 Jun 2015 07:32:52 -0400, Peter Hurley wrote: >> On 06/09/2015 02:05 PM, Steven Walter wrote: >>> Use a dedicated kthread for handling ports marked as low_latency. Since >>> this thread is RT_FIFO, it is not subject to the same types of >>> starvation as the normal priority kworker threads. >> >> This is not a problem unique to the tty subsystem; many subsystems use >> kworkers to handle i/o after the initial ISR. >> >> Without careful design, high-prio userspace RT threads can effectively starve >> themselves of i/o. >> >> In any event, solutions to this problem belong either in the core workqueue >> (for example, an i/o-specific unbounded workqueue) or in the CONFIG_PREEMPT_RT >> patch. > > But kthread_worker *is* the workqueue subsystem's answer to users who > require low-latency processing. Not really; you even note below the lack of RT support in wq. > SPI subsystem has been using it successfully for message pump handling > for last few releases. SPI is trivial compared to tty. > Lack of RT functionality in workqueue subsystem and prevalence of wq use makes > them a huge pain on embedded/rt systems. I would like to see more > generic solution to this problem as well but I can't think of one :/ Direct support from workqueue is the generic solution, insofar as there is a generic solution. Ultimately, RT priority inversion is a upward spiral to 99. tty uses the system_unbound_wq; as long as the substitute wq has similar guarantees (guaranteed forward progress and unbounded worker running times) using a different wq for any given tty_port would be ok. I'd like to see whatever solution go through CONFIG_PREEMPT_RT patch first though, since there is no linux-rt list/tree, especially for proposals that are pushing down into the device subsystems. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
