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SubjectRe: [PATCH] [request for inclusion] Realtime LSM
Jack O'Quin wrote:
> Arjan van de Ven <> 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
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

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.

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