Messages in this thread Patch in this message | | | From | Byungchul Park <> | Subject | [RESEND PATCH v12 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq() | Date | Mon, 18 Jun 2018 13:58:09 +0900 |
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Hello Steven,
I've changed the code a little bit to avoid a compile warning caused by 'const' args of find_cpu(). Can I keep your Reviewed-by?
BEFORE: static int find_cpu(const struct cpumask *mask, const struct sched_domain *sd, const struct sched_domain *prefer)
AFTER: static int find_cpu(const struct cpumask *mask, struct sched_domain *sd, struct sched_domain *prefer)
(I temporarily removed the Reviewed-by you gave me.) Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
-----8<----- From 205b197043085947ae30cd939bc12e436c328fe5 Mon Sep 17 00:00:00 2001 From: Byungchul Park <byungchul.park@lge.com> Date: Mon, 4 Jun 2018 16:47:45 +0900 Subject: [RESEND PATCH v12 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()
It would be better to try to check other siblings first if SD_PREFER_SIBLING is flaged when pushing tasks - migration.
Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Byungchul Park <byungchul.park@lge.com> --- kernel/sched/rt.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 75 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index ef3c4e6..b2aff1a 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1623,12 +1623,33 @@ static struct task_struct *pick_highest_pushable_task(struct rq *rq, int cpu) static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask); +/* + * Find the first CPU in: mask & sd & ~prefer + */ +static int find_cpu(const struct cpumask *mask, + struct sched_domain *sd, + struct sched_domain *prefer) +{ + int cpu; + + for_each_cpu(cpu, mask) { + if (!cpumask_test_cpu(cpu, sched_domain_span(sd))) + continue; + if (prefer && cpumask_test_cpu(cpu, sched_domain_span(prefer))) + continue; + break; + } + + return cpu; +} + static int find_lowest_rq(struct task_struct *task) { - struct sched_domain *sd; + struct sched_domain *sd, *prefer = NULL; struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask); int this_cpu = smp_processor_id(); int cpu = task_cpu(task); + int fallback_cpu = -1; /* Make sure the mask is initialized first */ if (unlikely(!lowest_mask)) @@ -1673,9 +1694,37 @@ static int find_lowest_rq(struct task_struct *task) return this_cpu; } - best_cpu = cpumask_first_and(lowest_mask, - sched_domain_span(sd)); + /* + * If a CPU exists that is in the lowest_mask and + * the current sd span, but not in the prefer sd + * span, then that becomes our choice. + * + * Of course, the lowest possible CPU is already + * under consideration through lowest_mask. + */ + best_cpu = find_cpu(lowest_mask, sd, prefer); + if (best_cpu < nr_cpu_ids) { + /* + * If current domain is SD_PREFER_SIBLING + * flaged, we have to try to check other + * siblings first. + */ + if (sd->flags & SD_PREFER_SIBLING) { + prefer = sd; + + /* + * fallback_cpu should be one + * in the closest domain among + * SD_PREFER_SIBLING domains, + * in case that more than one + * SD_PREFER_SIBLING domains + * exist in the hierachy. + */ + if (fallback_cpu == -1) + fallback_cpu = best_cpu; + continue; + } rcu_read_unlock(); return best_cpu; } @@ -1684,6 +1733,29 @@ static int find_lowest_rq(struct task_struct *task) rcu_read_unlock(); /* + * If fallback_cpu is valid, all our guesses failed *except* for + * SD_PREFER_SIBLING domain. Now, we can return the fallback CPU. + * + * XXX: Consider the following example, 4 cores SMT2 system: + * + * LLC [0 - 7] + * SMT [0 1][2 3][4 5][6 7] + * o x o x x x x x + * + * where 'o': occupied and 'x': empty. + * + * A wakeup on CPU0 will exclude CPU1 and choose CPU3, since + * CPU1 is in a SD_PREFER_SIBLING sd and CPU3 is not. However, + * in this case, CPU4 would have been a better choice, since + * CPU3 is a (SMT) thread of an already loaded core. + * + * Doing it 'right' is difficult and expensive. The current + * solution is an acceptable approximation. + */ + if (fallback_cpu != -1) + return fallback_cpu; + + /* * And finally, if there were no matches within the domains * just give the caller *something* to work with from the compatible * locations. -- 1.9.1
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