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Date: Tue, 28 Mar 2023 12:03:58 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Ricardo Neri <ricardo.neri@...el.com>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Steven Rostedt <rostedt@...dmis.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Valentin Schneider <vschneid@...hat.com>,
Lukasz Luba <lukasz.luba@....com>,
Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
"Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 10/24] sched/fair: Use IPCC scores to select a busiest runqueue
On Tue, 7 Feb 2023 at 06:01, Ricardo Neri
<ricardo.neri-calderon@...ux.intel.com> wrote:
>
> For two runqueues of equal priority and equal number of running of tasks,
> select the one whose current task would have the highest IPC class score
> if placed on the destination CPU.
You failed to explain why it make sense to compare current task score
whereas we will most probably not pull this task at the end
>
> For now, use IPCC scores only for scheduling domains with the
> SD_ASYM_PACKING flag.
>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Ionela Voinescu <ionela.voinescu@....com>
> Cc: Joel Fernandes (Google) <joel@...lfernandes.org>
> Cc: Len Brown <len.brown@...el.com>
> Cc: Lukasz Luba <lukasz.luba@....com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Tim C. Chen <tim.c.chen@...el.com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: x86@...nel.org
> Cc: linux-pm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
> Changes since v2:
> * Only use IPCC scores to break ties if the sched domain uses
> asym_packing. (Ionela)
> * Handle errors of arch_get_ipcc_score(). (Ionela)
>
> Changes since v1:
> * Fixed a bug when selecting a busiest runqueue: when comparing two
> runqueues with equal nr_running, we must compute the IPCC score delta
> of both.
> * Renamed local variables to improve the layout of the code block.
> (PeterZ)
> * Used the new interface names.
> ---
> kernel/sched/fair.c | 64 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 64 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 72d88270b320..d3c22dc145f7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9399,6 +9399,37 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
> return sched_asym_ipcc_prefer(a_stats, b_stats);
> }
>
> +/**
> + * ipcc_score_delta - Get the IPCC score delta wrt the load balance's dst_cpu
> + * @p: A task
> + * @env: Load balancing environment
> + *
> + * Returns: The IPCC score delta that @p would get if placed in the destination
> + * CPU of @env. LONG_MIN to indicate that the delta should not be used.
> + */
> +static long ipcc_score_delta(struct task_struct *p, struct lb_env *env)
> +{
> + unsigned long score_src, score_dst;
> + unsigned short ipcc = p->ipcc;
> +
> + if (!sched_ipcc_enabled())
> + return LONG_MIN;
> +
> + /* Only asym_packing uses IPCC scores at the moment. */
> + if (!(env->sd->flags & SD_ASYM_PACKING))
> + return LONG_MIN;
> +
> + score_dst = arch_get_ipcc_score(ipcc, env->dst_cpu);
> + if (IS_ERR_VALUE(score_dst))
> + return LONG_MIN;
> +
> + score_src = arch_get_ipcc_score(ipcc, task_cpu(p));
> + if (IS_ERR_VALUE(score_src))
> + return LONG_MIN;
> +
> + return score_dst - score_src;
> +}
> +
> #else /* CONFIG_IPC_CLASSES */
> static void update_sg_lb_ipcc_stats(int dst_cpu, struct sg_lb_stats *sgs,
> struct rq *rq)
> @@ -9429,6 +9460,11 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
> return false;
> }
>
> +static long ipcc_score_delta(struct task_struct *p, struct lb_env *env)
> +{
> + return LONG_MIN;
> +}
> +
> #endif /* CONFIG_IPC_CLASSES */
>
> /**
> @@ -10589,6 +10625,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> {
> struct rq *busiest = NULL, *rq;
> unsigned long busiest_util = 0, busiest_load = 0, busiest_capacity = 1;
> + long busiest_ipcc_delta = LONG_MIN;
> unsigned int busiest_nr = 0;
> int i;
>
> @@ -10705,8 +10742,35 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>
> case migrate_task:
> if (busiest_nr < nr_running) {
> + struct task_struct *curr;
> +
> busiest_nr = nr_running;
> busiest = rq;
> +
> + /*
> + * Remember the IPCC score delta of busiest::curr.
> + * We may need it to break a tie with other queues
> + * with equal nr_running.
> + */
> + curr = rcu_dereference(busiest->curr);
> + busiest_ipcc_delta = ipcc_score_delta(curr, env);
Hmm, i don't like this at all
Also, curr is the least probable task to be pulled which means that
all this his useless
> + /*
> + * If rq and busiest have the same number of running
> + * tasks and IPC classes are supported, pick rq if doing
> + * so would give rq::curr a bigger IPC boost on dst_cpu.
> + */
> + } else if (busiest_nr == nr_running) {
> + struct task_struct *curr;
> + long delta;
> +
> + curr = rcu_dereference(rq->curr);
> + delta = ipcc_score_delta(curr, env);
> +
> + if (busiest_ipcc_delta < delta) {
> + busiest_ipcc_delta = delta;
> + busiest_nr = nr_running;
> + busiest = rq;
> + }
> }
> break;
>
> --
> 2.25.1
>
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