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There are many use cases where people will want to monitor more tasks than there exist RMIDs in the hardware, meaning that we have to perform some kind of multiplexing. We do this by "rotating" the RMIDs in a workqueue, and assigning an RMID to a waiting event when the RMID becomes unused. This scheme reserves one RMID at all times for rotation. When we need to schedule a new event we give it the reserved RMID, pick a victim event from the front of the global CQM list and wait for the victim's RMID to drop to zero occupancy, before it becomes the new reserved RMID. We put the victim's RMID onto the limbo list, where it resides for a "minimum queue time", which is intended to save ourselves an expensive smp IPI when the RMID is unlikely to have a occupancy value below __intel_cqm_threshold. If we fail to recycle an RMID, even after waiting the minimum queue time then we need to increment __intel_cqm_threshold. There is an upper bound on this threshold, __intel_cqm_max_threshold, which is programmable from userland as /sys/devices/intel_cqm/max_recycling_threshold. The comments above __intel_cqm_rmid_rotate() have more details. Signed-off-by: Matt Fleming <matt.fleming@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Kanaka Juvva <kanaka.d.juvva@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Vikas Shivappa <vikas.shivappa@linux.intel.com> Link: http://lkml.kernel.org/r/1422038748-21397-9-git-send-email-matt@codeblueprint.co.uk Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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