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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] xen:rtds:fix bug in accounting budget
[cc. Tianyang...] On Wed, Oct 19, 2016 at 11:45 AM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote: > On Wed, Oct 19, 2016 at 11:13:54AM -0400, Meng Xu wrote: >> The bug is introduced in Xen 4.7 when we converted RTDS scheduler >> from quantum-driven model to event-driven model. >> We assumed rt_schedule() is always called for a VCPU >> before the VCPUs budget replenished handler. >> This assumption does not hold, when system is overloaded, or >> when the VCPU budget is almost equal its period. >> >> Buggy behavior: >> 1) A VCPU may get less budget that assigned in a period. >> 2) A full capacity VCPU, i.e., a VCPU whose period is equal to budget, >> may not get any budget in some period. >> >> Bug analysis: >> 1) A VCPU deadline can be fast-forwarded by more than one period. >> However, the VCPU last_start time was not updated immediately. >> If rt_schedule() is called after rt_update_deadline(), which happens >> when VCPU budget is equal to period or when VCPU has deadline miss, >> burn_budget() will burn the budget that was just replenished, >> although the replenished budget should be used in the most recent period >> only. >> >> We should update VCPU last_start time to the start of the current period >> when rt_update_deadline() updates a VCPU period. >> >> 2) When a full capacity VCPU depletes its budget and is context switching >> out, >> but has not updated the cores current running VCPU, >> the budget replenish timer may be triggerred. >> The replenish handler failed to re-schedule the full capacity VCPU >> because it thought the VCPU is running. >> >> When a VCPU budget is replenished, we try to tickle a CPU. >> When we find a core for a VCPU to tickle and the VCPU is context >> switching out, >> we will always tickle the core where the VCPU was running, >> if the VCPU cannot find another core to tickle >> >> This bug was reported by Dagaen Golomb >> >> Signed-off-by: Meng Xu > > You missed you email address here. > Oh, sorry! :-( Thank you very much for pointing it out, Wei! I will add my email address <mengxu@xxxxxxxxxxxxx> in the next version. Let's see if Dario and George have some comments on the fix. I can incorporate them in the next version. Meng ----------- Meng Xu PhD Student in Computer and Information Science University of Pennsylvania http://www.cis.upenn.edu/~mengxu/ _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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