Messages in this thread | | | Date | Fri, 14 Jan 2005 11:50:14 +1100 | From | Con Kolivas <> | Subject | Re: [PATCH] [request for inclusion] Realtime LSM |
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Jack O'Quin wrote: > Arjan van de Ven <arjanv@redhat.com> writes: > > >>On Thu, Jan 13, 2005 at 04:25:08PM -0500, Lee Revell wrote: >> >>>The basic issue is that the current semantics of SCHED_FIFO seem make >>>the deadlock/data corruption due to runaway RT thread issue difficult. >>>The obvious solution is a new scheduling class equivalent to SCHED_FIFO >>>but with a mechanism for the kernel to demote the offending thread to >>>SCHED_OTHER in an emergency. >> >>and this is getting really close to the original "counter proposal" to the >>LSM module that was basically "lets make lower nice limit an rlimit, and >>have -20 mean "basically FIFO" *if* the task behaves itself". > > > Yes. However, my tests have so far shown a need for "actual FIFO as > long as the task behaves itself."
I should comment on this thread on lkml. After some investigation/discussion and testing I came up with a proposal for this problem. Since we are a general purpose operating system and not a hard rt system (although addon patches are clearly making that a future possibility) we need a solution that is satisfactory to a general...
There are two ways I suggested for this.
First, (and I am increasingly believing in the second) is to implement a new scheduling class for isochronous scheduling. This would be a class for unprivileged users, and behave like SCHED_RR (to avoid complications of QoS features we dont have infrastrucutre for) at a priority just above SCHED_NORMAL, but below all privileged SCHED_RR and SCHED_FIFO. Importantly, a soft cpu limit and rate period can be set by default for this scheduling class that provides good true SCHED_RR performance, and is configurable. Literature suggests that 70% is adequate cpu for good real time performance and would be starvation free. I believe setting 70% with 10% hysteresis (dropping to say 63% on hitting limit) would be a good start. Beyond this, however, to satisfy the needs of those with more demanding setups, a simple configurable runtime setting to set both the cpu% and the rate period could be available to something as simple as proc /proc/sys/kernel/iso_cpu /proc/sys/kernel/iso_cpu_period where iso_cpu is set to 70, and period to maybe 1 second. The actual mode of setting this tunable is not important, and could be in /sys or whatever
The second option is to not implement a new scheduling class at all, and allow unprivileged users to use either SCHED_FIFO or SCHED_RR, but to make the cpu constraints described for SCHED_ISO above apply to their use of those classes. Supporting priority settings for these could be possible, but in my opinion, it would work as a better class if they only had one priority level, as for the SCHED_ISO implementation above (better than any SCHED_NORMAL, but lower than privileged SCHED_RR/FIFO).
This latter approach to me seems the least invasive and most user and sysadmin friendly method.
What was amusing to me was that after I suggested the latter option, I discovered that was basically what OSX does, however being not a real multi-user operating system they had absurd limits for cpu at 90% by default. Theory suggests 70% should be a good default limit.
Cheers, Con [unhandled content-type:application/pgp-signature] | |