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Message-ID: <Z9mxc4lzDTHSpLq4@gpd3>
Date: Tue, 18 Mar 2025 18:46:27 +0100
From: Andrea Righi <arighi@...dia.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
David Vernet <void@...ifault.com>,
Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine
without WF_SYNC
Hi Joel,
On Tue, Mar 18, 2025 at 01:00:47PM -0400, Joel Fernandes wrote:
...
> From: Joel Fernandes <joelagnelf@...dia.com>
> Subject: [PATCH] sched/ext: Make default idle CPU selection better
>
> Currently, sched_ext's default CPU selection is roughly something like
> this:
>
> 1. Look for FULLY IDLE CORES:
> 1.1. Select prev CPU (wakee) if its CORE is fully idle.
> 1.2. Or, pick any CPU from fully idle CORE in the L3, then NUMA.
> 1.3. Or, any idle CPU from fully idle CORE usable by task.
> 2. Or, use PREV CPU if it is idle.
> 3. Or any idle CPU in the LLC, NUMA.
> 4. Or finally any CPU usable by the task.
>
> This can end up select any idle core in the system even if that means
> jumping across NUMA nodes (basically 1.3 happens before 3.).
>
> Improve this by moving 1.3 to after 3 (so that skipping over NUMA
> happens only later) and also add selection of fully idle target (waker)
> core before looking for fully-idle cores in the LLC/NUMA. This is similar to
> what FAIR scheduler does.
>
> The new sequence is as follows:
>
> 1. Look for FULLY IDLE CORES:
> 1.1. Select prev CPU (wakee) if its CORE is fully idle.
> 1.2. Select target CPU (waker) if its CORE is fully idle and shares cache
> with prev. <- Added this.
> 1.3. Or, pick any CPU from fully idle CORE in the L3, then NUMA.
> 2. Or, use PREV CPU if it is idle.
> 3. Or any idle CPU in the LLC, NUMA.
> 4. Or, any idle CPU from fully idle CORE usable by task. <- Moved down.
> 5. Or finally any CPU usable by the task.
>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
> kernel/sched/ext.c | 26 +++++++++++++++++++-------
> 1 file changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5a81d9a1e31f..324e442319c7 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3558,6 +3558,16 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
> goto cpu_found;
> }
>
> + /*
> + * If the waker's CPU shares cache with @prev_cpu and is part
> + * of a fully idle core, select it.
> + */
> + if (cpus_share_cache(cpu, prev_cpu) &&
> + cpumask_test_cpu(cpu, idle_masks.smt) &&
> + test_and_clear_cpu_idle(cpu)) {
I think this is always false, because cpu is still in use by the waker and
its state hasn't been updated to idle yet.
> + goto cpu_found;
> + }
> +
-Andrea
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