From: Vincent Guittot <vincent.guittot@linaro.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Ziljstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
Aubrey Li <aubrey.li@linux.intel.com>,
Barry Song <song.bao.hua@hisilicon.com>,
Juri Lelli <juri.lelli@redhat.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Linux-ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 4/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core
Date: Tue, 8 Dec 2020 17:15:01 +0100 [thread overview]
Message-ID: <CAKfTPtCUJcT42+wBmnUmr7s3Z0BFf5WUknKc22eyEe+vcNQ3qw@mail.gmail.com> (raw)
In-Reply-To: <20201208153501.1467-5-mgorman@techsingularity.net>
On Tue, 8 Dec 2020 at 16:35, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> select_idle_core is called when SMT is active and there is likely a free
> core available. It may find idle CPUs but this information is simply
> discarded and the scan starts over again with select_idle_cpu.
>
> This patch caches information on idle CPUs found during the search for
> a core and uses one if no core is found. This is a tradeoff. There may
> be a slight impact when utilisation is low and an idle core can be
> found quickly. It provides improvements as the number of busy CPUs
> approaches 50% of the domain size when SMT is enabled.
>
> With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled
>
> 5.10.0-rc6 5.10.0-rc6
> schedstat idlecandidate
> Hmean 1 500.06 ( 0.00%) 505.67 * 1.12%*
> Hmean 2 975.90 ( 0.00%) 974.06 * -0.19%*
> Hmean 4 1902.95 ( 0.00%) 1904.43 * 0.08%*
> Hmean 8 3761.73 ( 0.00%) 3721.02 * -1.08%*
> Hmean 16 6713.93 ( 0.00%) 6769.17 * 0.82%*
> Hmean 32 10435.31 ( 0.00%) 10312.58 * -1.18%*
> Hmean 64 12325.51 ( 0.00%) 13792.01 * 11.90%*
> Hmean 128 21225.21 ( 0.00%) 20963.44 * -1.23%*
> Hmean 256 20532.83 ( 0.00%) 20335.62 * -0.96%*
> Hmean 320 20334.81 ( 0.00%) 20147.25 * -0.92%*
>
> Note that there is a significant corner case. As the SMT scan may be
> terminated early, not all CPUs have been visited and select_idle_cpu()
> is still called for a full scan. This case is handled in the next
> patch.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/fair.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 413d895bbbf8..ed6f45832d97 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6066,6 +6066,7 @@ void __update_idle_core(struct rq *rq)
> */
> static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
> {
> + int idle_candidate = -1;
> struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> int core, cpu;
>
> @@ -6085,6 +6086,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> idle = false;
> break;
> }
> +
> + if (idle_candidate == -1 &&
> + cpumask_test_cpu(cpu, p->cpus_ptr)) {
> + idle_candidate = cpu;
> + }
> }
>
> if (idle)
> @@ -6098,7 +6104,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
> */
> set_idle_cores(target, 0);
>
> - return -1;
> + return idle_candidate;
> }
>
> /*
> --
> 2.26.2
>
next prev parent reply other threads:[~2020-12-08 16:16 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-08 15:34 [PATCH 0/4] Reduce scanning of runqueues in select_idle_sibling Mel Gorman
2020-12-08 15:34 ` [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2020-12-08 16:13 ` Vincent Guittot
2020-12-08 15:34 ` [PATCH 2/4] sched/fair: Move avg_scan_cost calculations under SIS_PROP Mel Gorman
2020-12-08 16:03 ` Vincent Guittot
2020-12-08 16:30 ` Mel Gorman
2020-12-09 5:28 ` Li, Aubrey
2020-12-09 9:05 ` Mel Gorman
2020-12-09 11:07 ` Li, Aubrey
2020-12-09 11:33 ` Mel Gorman
2020-12-10 5:18 ` Li, Aubrey
2020-12-10 9:32 ` Mel Gorman
2020-12-08 15:35 ` [PATCH 3/4] sched/fair: Do not replace recent_used_cpu with the new target Mel Gorman
2020-12-08 16:14 ` Vincent Guittot
2020-12-10 9:40 ` Vincent Guittot
2020-12-08 15:35 ` [PATCH 4/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core Mel Gorman
2020-12-08 16:15 ` Vincent Guittot [this message]
2020-12-09 14:37 ` [PATCH 0/4] Reduce scanning of runqueues in select_idle_sibling Mel Gorman
2020-12-10 8:00 ` Vincent Guittot
2020-12-10 9:38 ` Vincent Guittot
2020-12-10 11:04 ` Mel Gorman
2020-12-11 9:51 ` Vincent Guittot
2020-12-11 10:23 ` Mel Gorman
2020-12-12 10:02 ` Vincent Guittot
[not found] ` <20201211062542.3082-1-hdanton@sina.com>
2020-12-11 9:02 ` [PATCH 3/4] sched/fair: Do not replace recent_used_cpu with the new target Mel Gorman
[not found] ` <20201211093443.3259-1-hdanton@sina.com>
2020-12-11 9:45 ` Mel Gorman
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