On Tue, May 10, 2016 at 04:39:39PM +0000, Luck, Tony wrote: > >> (3) Also we may not want to count at every sched_in and sched_out > >> because the MSR reads involve quite a bit of overhead. > > > > Every single other PMU driver just does this; why are you special? > > They just have to read a register. We have to write the IA32_EM_EVT_SEL MSR > and then read from the IA32_QM_CTR MSR ... if we are tracking both local > and total bandwidth, we have to do repeat and wrmr/rdmsr again to get the > other counter. That seems like it will noticeably affect the system if we do > it > on every sched_in and sched_out.
Right; but Vikas didn't say that did he ;-), he just mentioned msr-read. Also; I don't think you actually have to do it on every sched event, only when the event<->rmid association changes. As long as the event<->rmid association doesn't change, you can forgo updates. > But the more we make this complicated, the more I think that we should not > go through the pain of stealing/recycling RMIDs and just limit the number of > things that can be simultaneously monitored. If someone tries to monitor one > more thing when all the RMIDs are in use, we should just error out with > -ERUNOUTOFRMIDSTRYAGAINLATER (maybe -EAGAIN???) Possibly; but I would like to minimize churn at this point to let the Google guys get their patches in shape. They seem to have definite ideas about that as well :-)
