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Date: Mon, 27 Mar 2017 16:26:33 +0200 From: Peter Zijlstra <peterz@...radead.org> To: luca abeni <luca.abeni@...tannapisa.it> Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@....com>, Claudio Scordino <claudio@...dence.eu.com>, Steven Rostedt <rostedt@...dmis.org>, Tommaso Cucinotta <tommaso.cucinotta@...up.it>, Daniel Bristot de Oliveira <bristot@...hat.com>, Joel Fernandes <joelaf@...gle.com>, Mathieu Poirier <mathieu.poirier@...aro.org> Subject: Re: [RFC v5 8/9] sched/deadline: base GRUB reclaiming on the inactive utilization On Fri, Mar 24, 2017 at 04:53:01AM +0100, luca abeni wrote: > From: Luca Abeni <luca.abeni@...tannapisa.it> > > Instead of decreasing the runtime as "dq = -Uact dt" (eventually > divided by the maximum utilization available for deadline tasks), > decrease it as "dq = -(1 - Uinact) dt", where Uinact is the "inactive > utilization". > In this way, the maximum fraction of CPU time that can be reclaimed > is given by the total utilization of deadline tasks. > This approach solves some fairness issues that have been noticed with > "traditional" global GRUB reclaiming. I think the Changelog could do with explicit enumeration of what "some" is. > Signed-off-by: Luca Abeni <luca.abeni@...tannapisa.it> > Tested-by: Daniel Bristot de Oliveira <bristot@...hat.com> > --- > kernel/sched/deadline.c | 23 ++++++++++++++++------- > 1 file changed, 16 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index d70a7b9..c393c3d 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -900,14 +900,23 @@ extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq); > /* > * This function implements the GRUB accounting rule: > * according to the GRUB reclaiming algorithm, the runtime is > + * not decreased as "dq = -dt", but as "dq = (1 - Uinact) dt", where Changelog had it right I think: dq = -(1 - Uinact) dt > + * Uinact is the (per-runqueue) inactive utilization, computed as the > + * difference between the "total runqueue utilization" and the runqueue > + * active utilization. > + * Since rq->dl.running_bw and rq->dl.this_bw contain utilizations > + * multiplied by 2^20, the result has to be shifted right by 20. > */ > -u64 grub_reclaim(u64 delta, struct rq *rq) > +u64 grub_reclaim(u64 delta, struct rq *rq, u64 u) > { > + u64 u_act; > + > + if (rq->dl.this_bw - rq->dl.running_bw > (1 << 20) - u) > + u_act = u; > + else > + u_act = (1 << 20) - rq->dl.this_bw + rq->dl.running_bw; > + > + return (delta * u_act) >> 20; But that's not what is done here I think, something like this instead: Uinact = Utot - Uact -t_u dt ; Uinact > (1 - t_u) dq = { -(1 - Uinact) dt And nowhere do we have an explanation for that. Now, I suspect we can write that like: dq = -max{ t_u, (1 - Uinact) } dt, which would suggest this is a sanity check on Utot, which I suspect can be over 1. Is this what is happening? #define BW_SHIFT 20 #define BW_UNIT (1 << BW_SHIFT) static inline u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity *dl_se) { u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot - Uact */ u64 u_act; /* * What we want to write is: * * max(BW_UNIT - u_inact, dl_se->dl_bw) * * but we cannot do that since Utot can be larger than 1, * which means u_inact can be larger than 1, which would * have the above result in negative values. */ if (u_inact > (BW_UNIT - dl_se->dl_bw)) u_act = dl_se->dl_bw; else u_act = BW_UNIT - u_inact; return (delta * u_act) >> BW_SHIFT; } Hmm?
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