Messages in this thread | | | Subject | Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy | From | Rik van Riel <> | Date | Tue, 09 Feb 2016 16:20:48 -0500 |
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On Tue, 2016-02-09 at 18:11 +0100, Frederic Weisbecker wrote: > > So for any T_slice being a given cpu timeslice (in secs) executed > between > two ring switch (user <-> kernel), we are going to account: 1 * > P(T_slice*HZ) > (P() stand for probability here). > > Now after this patch, the scenario is rather different. We are > accounting the > real time spent in a slice with a similar probablity. > This becomes: T_slice * P(T_slice*HZ). > > So it seems it could result into logarithmic accounting: timeslices > of 1 second > will be accounted right whereas repeating tiny timeslices may result > in much lower > values than expected. > > To fix this we should instead account jiffies_to_nsecs(jiffies - t- > >vtime_jiffies). > Well, that would drop the use of finegrained clock and even the need > of nsecs based > cputime. But why not if we still have acceptable result for much more > performances.
Looking over the code some more, you are right.
My changes to vtime_account_idle and vtime_account_system will cause them to do nothing a lot of the time, when called from vtime_common_task_switch.
This causes a discrepancy between the time accounted at task switch time, and the time delta accounted later on.
I see two ways to fix this: 1) Always do fine granularity accounting at task switch time, and revert to coarser granularity at syscall time only. This may or may not be more accurate (not sure how much, or whether we care). 2) Always account at coarser granularity, like you suggest above. This has the advantage of leading to faster context switch times (no TSC reads).
I am happy to implement either.
Frederic, Thomas, Ingo, do you have a preference between these approaches?
Would you like me to opt for the higher performance option, or go for the potentially higher accuracy one?
As an aside, I am wondering whether the call to vtime_account_user() from vtime_common_task_switch() ever does anything.
After all, the preceding call to vtime_account_system would have already accounted all the CPU time that passed to system time, and there will be no time left to account to userspace.
Unless I am missing something, I suspect that line can just go.
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