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Date: Fri, 7 Aug 2020 12:46:18 +0200 From: peterz@...radead.org To: Juri Lelli <juri.lelli@...hat.com> Cc: mingo@...hat.com, rostedt@...dmis.org, tglx@...utronix.de, linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it, tommaso.cucinotta@...tannapisa.it, alessio.balsini@...il.com, bristot@...hat.com, dietmar.eggemann@....com, linux-rt-users@...r.kernel.org, mtosatti@...hat.com, williams@...hat.com, valentin.schneider@....com Subject: Re: [RFC PATCH v2 6/6] sched/fair: Implement starvation monitor On Fri, Aug 07, 2020 at 11:56:04AM +0200, Juri Lelli wrote: > Starting deadline server for lower priority classes right away when > first task is enqueued might break guarantees, as tasks belonging to > intermediate priority classes could be uselessly preempted. E.g., a well > behaving (non hog) FIFO task can be preempted by NORMAL tasks even if > there are still CPU cycles available for NORMAL tasks to run, as they'll > be running inside the fair deadline server for some period of time. > > To prevent this issue, implement a starvation monitor mechanism that > starts the deadline server only if a (fair in this case) task hasn't > been scheduled for some interval of time after it has been enqueued. > Use pick/put functions to manage starvation monitor status. One thing I considerd was scheduling this as a least-laxity entity -- such that it runs late, not early -- and start the server when rq->nr_running != rq->cfs.h_nr_running, IOW when there's !fair tasks around. Not saying we should do it like that, but that's perhaps more deterministic than this.
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