From: David Laight > Sent: 13 January 2020 14:35 > > 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) ... > 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. Actually not setting the cpu affinity makes no difference. The process is woken up on the cpu it last ran on and sits 'waiting' until drm_cflush_sg() finishes - even though the other cpu become idle. No sign of sched_migrate_task event 'stealing' the process. Even worse, because 'ticket locks' are used no other user processes can acquire the same (user) mutex or be woken from cv_wait() until the process actually runs. This is a 5.4.0-rc7 kernel. I think I saw some recent scheduler patches, I can try them until I can't build with gcc 4.7.3 :-) 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
