linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Qais Yousef <qais.yousef@arm.com>
To: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	linux-kernel@vger.kernel.org, Qais Yousef <qais.yousef@arm.com>
Subject: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU
Date: Fri, 14 Feb 2020 16:39:49 +0000	[thread overview]
Message-ID: <20200214163949.27850-4-qais.yousef@arm.com> (raw)
In-Reply-To: <20200214163949.27850-1-qais.yousef@arm.com>

If a task was running on unfit CPU we could ignore migrating if the
priority level of the new fitting CPU is the *same* as the unfit one.

Add an extra check to select_task_rq_rt() to allow the push in case:

	* old_cpu.highest_priority == new_cpu.highest_priority
	* task_fits(p, new_cpu)

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---

I was seeing some delays in migrating a task to a big CPU sometimes although it
was free, and I think this fixes it.

TBH, I fail to see how the check of

	p->prio < cpu_rq(target)->rt.highest_prio.curr

is necessary as find_lowest_rq() surely implies the above condition by
definition?

Unless we're fighting a race condition here where the rt_rq priority has
changed between the time we selected the lowest_rq and taking the decision to
migrate, then this makes sense.


 kernel/sched/rt.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 0c8bac134d3a..5ea235f2cfe8 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1430,7 +1430,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 {
 	struct task_struct *curr;
 	struct rq *rq;
-	bool test;
+	bool test, fit;
 
 	/* For anything but wake ups, just return the task_cpu */
 	if (sd_flag != SD_BALANCE_WAKE && sd_flag != SD_BALANCE_FORK)
@@ -1471,16 +1471,32 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	       unlikely(rt_task(curr)) &&
 	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
 
-	if (test || !rt_task_fits_capacity(p, cpu)) {
+	fit = rt_task_fits_capacity(p, cpu);
+
+	if (test || !fit) {
 		int target = find_lowest_rq(p);
 
-		/*
-		 * Don't bother moving it if the destination CPU is
-		 * not running a lower priority task.
-		 */
-		if (target != -1 &&
-		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
-			cpu = target;
+		if (target != -1) {
+			/*
+			 * Don't bother moving it if the destination CPU is
+			 * not running a lower priority task.
+			 */
+			if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
+
+				cpu = target;
+
+			} else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
+
+				/*
+				 * If the priority is the same and the new CPU
+				 * is a better fit, then move, otherwise don't
+				 * bother here either.
+				 */
+				fit = rt_task_fits_capacity(p, target);
+				if (fit)
+					cpu = target;
+			}
+		}
 	}
 	rcu_read_unlock();
 
-- 
2.17.1


  parent reply	other threads:[~2020-02-14 16:40 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 16:39 [PATCH 0/3] RT Capacity Awareness Improvements Qais Yousef
2020-02-14 16:39 ` [PATCH 1/3] sched/rt: cpupri_find: implement fallback mechanism for !fit case Qais Yousef
2020-02-17 17:07   ` Valentin Schneider
2020-02-17 23:34     ` Qais Yousef
2020-02-18 10:01       ` Valentin Schneider
2020-02-17 19:09   ` Dietmar Eggemann
2020-02-17 23:45     ` Qais Yousef
2020-02-18  9:53       ` Dietmar Eggemann
2020-02-18 17:28         ` Qais Yousef
2020-02-18 16:46       ` Steven Rostedt
2020-02-18 17:27         ` Qais Yousef
2020-02-18 18:03           ` Steven Rostedt
2020-02-18 18:52             ` Qais Yousef
2020-02-14 16:39 ` [PATCH 2/3] sched/rt: allow pulling unfitting task Qais Yousef
2020-02-17  9:10   ` Pavan Kondeti
2020-02-17 11:20     ` Qais Yousef
2020-02-19 13:43     ` Qais Yousef
2020-02-21  8:07       ` Pavan Kondeti
2020-02-21 11:08         ` Qais Yousef
2020-02-14 16:39 ` Qais Yousef [this message]
2020-02-17  9:23   ` [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU Pavan Kondeti
2020-02-17 13:53     ` Qais Yousef
2020-02-18  4:16       ` Pavan Kondeti
2020-02-18 17:47         ` Qais Yousef
2020-02-19  2:46           ` Pavan Kondeti
2020-02-19 10:46             ` Qais Yousef
2020-02-19 14:02       ` Qais Yousef
2020-02-21  8:15         ` Pavan Kondeti
2020-02-21 11:12           ` Qais Yousef

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200214163949.27850-4-qais.yousef@arm.com \
    --to=qais.yousef@arm.com \
    --cc=bsegall@google.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=juri.lelli@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mgorman@suse.de \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=pkondeti@codeaurora.org \
    --cc=rostedt@goodmis.org \
    --cc=vincent.guittot@linaro.org \
    --subject='Re: [PATCH 3/3] sched/rt: fix pushing unfit tasks to a better CPU' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).