I've been looking at why some RT processes don't get scheduled promptly. In my test the RT process's affinity ties it to a single cpu (this may not be such a good idea as it seems). What I've found is that the Intel i915 graphics driver uses the 'events_unbound' kernel worker thread to periodically execute drm_cflush_sg(). (see https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/drm_cache.c) I'm guessing this is to ensure that any writes to graphics memory become visible is a semi-timely manner. This loop takes about 1us per iteration split fairly evenly between whatever is in for_each_sg_page() and drm_cflush_page(). With a 2560x1440 display the loop count is 3600 (4 bytes/pixel) and the whole function takes around 3.3ms. Since the kernel isn't pre-emptive (I though that wasn't much harder than SMP) nothing else can run on that cpu until the loop finishes. Adding a cond_resched() to the loop (maybe every 64 iterations) will allow higher priority processes to run. But really the code needs to be a lot faster. I actually suspect that the (I assume IPI based) wbinv_on_all_cpus() would be a lot faster - especially done by a per-cpu work queue? I had moderate difficulty getting from the process (kworker/u8:3) to the name of the worker thread pool, never mind the actual work. Fortunately it runs so long that some of the output from 'echo t >/proc/sysrq-trigger' still linked the pid (which I knew from ftrace scheduler events (and schedviz)) to the actual work item name. (Oh, after I'd written a program to tidy up the raw ftrace output so schedviz didn't barf on a trace that had wrapped.) Is there anything in /proc (etc) that shows all the work queues and their current work? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
