From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265164AbUAWDeR (ORCPT ); Thu, 22 Jan 2004 22:34:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S266502AbUAWDeR (ORCPT ); Thu, 22 Jan 2004 22:34:17 -0500 Received: from mail-04.iinet.net.au ([203.59.3.36]:35458 "HELO mail.iinet.net.au") by vger.kernel.org with SMTP id S265164AbUAWDeI (ORCPT ); Thu, 22 Jan 2004 22:34:08 -0500 Message-ID: <4010954A.7090904@cyberone.com.au> Date: Fri, 23 Jan 2004 14:30:18 +1100 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030827 Debian/1.4-3 X-Accept-Language: en MIME-Version: 1.0 To: Rusty Russell CC: Andrew Morton , mingo@elte.hu, linux-kernel@vger.kernel.org, John Hawkes Subject: Re: [PATCH] Directed migration: Don't Change cpumask in sched_balance_exec() References: <20040122082303.333432C08A@lists.samba.org> In-Reply-To: <20040122082303.333432C08A@lists.samba.org> Content-Type: multipart/mixed; boundary="------------010404090108080104050703" Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org This is a multi-part message in MIME format. --------------010404090108080104050703 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Rusty Russell wrote: >In message <400F5D71.7010702@cyberone.com.au> you write: > >>Hi Rusty, >>Yes I do like it. It hass bothered me that the visible cpus_allowed >>mask is changed in order to do the balancing. Thanks Rusty. Ingo? >> > >Note that I should have used for_each_online_cpu, not for_each_cpu, >since we explicitly don't want to look at offline cpus. > >New patch with one-line change below... >Rusty. > I'm just reviewing this and porting it to -mm now. I have a few comments to start with. CPUs have (I think) always been int in sched.c, you're using unsigned int. No big deal, but I'll change them to int. My patches introduce a "move_tasks" function, so I'll rename yours to migrate_task and __migrate_task. Your version of move_task (now migrate_task) never actually used dest_cpu unless I am seeing things. Fixed. Hows that? --------------010404090108080104050703 Content-Type: text/plain; name="sched-directed-migration.patch" Content-Transfer-Encoding: 7bit Content-Disposition: inline; filename="sched-directed-migration.patch" Name: Directed Migration: Don't Change cpumask in sched_balance_exec() Author: Rusty Russell Status: Experimental D: The current sched_balance_exec() sets the task's cpus_allowed mask D: temporarily to move it to a different CPU. This has several D: issues, including the fact that a task will see its affinity at a D: bogus value. D: D: So we change the migration_req_t to explicitly specify a D: destination CPU, rather than the migration thread deriving it from D: cpus_allowed. If the requested CPU is no longer valid (racing with D: another set_cpus_allowed, say), it can be ignored: if the task is D: not allowed on this CPU, there will be another migration request D: pending. D: D: This change allows sched_balance_exec() to tell the migration D: thread what to do without changing the cpus_allowed mask. D: D: So we rename __set_cpus_allowed() to move_task(), as the D: cpus_allowed mask is now set by the caller. And move_task_away(), D: which the migration thread uses to actually perform the move, is D: renamed __move_task(). D: D: I also ignore offline CPUs in sched_best_cpu(), so sched_migrate_task() D: doesn't need to check for offline CPUs. D: D: Alterior motive: this approach also plays well with CPU Hotplug. D: Previously that patch might have seen a task with cpus_allowed only D: containing the dying CPU (temporarily due to sched_balance_exec) and D: forcibly reset it to all cpus, which might be wrong. The other approach D: is to hold the cpucontrol sem around sched_balance_exec(), which is D: too much of a bottleneck. linux-2.6-npiggin/kernel/sched.c | 65 +++++++++++++++++++-------------------- 1 files changed, 32 insertions(+), 33 deletions(-) diff -puN kernel/sched.c~sched-directed-migration kernel/sched.c --- linux-2.6/kernel/sched.c~sched-directed-migration 2004-01-23 14:09:22.000000000 +1100 +++ linux-2.6-npiggin/kernel/sched.c 2004-01-23 14:27:31.000000000 +1100 @@ -521,37 +521,30 @@ inline int task_curr(task_t *p) typedef struct { struct list_head list; task_t *task; + int dest_cpu; struct completion done; } migration_req_t; /* - * The task's runqueue lock must be held, and the new mask must be valid. + * The task's runqueue lock must be held. * Returns true if you have to wait for migration thread. */ -static int __set_cpus_allowed(task_t *p, cpumask_t new_mask, - migration_req_t *req) +static int migrate_task(task_t *p, int dest_cpu, migration_req_t *req) { runqueue_t *rq = task_rq(p); - p->cpus_allowed = new_mask; - /* - * Can the task run on the task's current CPU? If not then - * migrate the thread off to a proper CPU. - */ - if (cpu_isset(task_cpu(p), new_mask)) - return 0; - /* * If the task is not on a runqueue (and not running), then * it is sufficient to simply update the task's cpu field. */ if (!p->array && !task_running(rq, p)) { - set_task_cpu(p, any_online_cpu(p->cpus_allowed)); + set_task_cpu(p, dest_cpu); return 0; } init_completion(&req->done); req->task = p; + req->dest_cpu = dest_cpu; list_add(&req->list, &rq->migration_queue); return 1; } @@ -1041,7 +1034,7 @@ unsigned long nr_running(void) { unsigned long i, sum = 0; - for (i = 0; i < NR_CPUS; i++) + for_each_cpu(i) sum += cpu_rq(i)->nr_running; return sum; @@ -1131,28 +1124,18 @@ static void sched_migrate_task(task_t *p runqueue_t *rq; migration_req_t req; unsigned long flags; - cpumask_t old_mask, new_mask = cpumask_of_cpu(dest_cpu); rq = task_rq_lock(p, &flags); - old_mask = p->cpus_allowed; - if (!cpu_isset(dest_cpu, old_mask) || !cpu_online(dest_cpu)) + if (!cpu_isset(dest_cpu, p->cpus_allowed)) goto out; /* force the process onto the specified CPU */ - if (__set_cpus_allowed(p, new_mask, &req)) { + if (migrate_task(p, dest_cpu, &req)) { /* Need to wait for migration thread. */ task_rq_unlock(rq, &flags); wake_up_process(rq->migration_thread); wait_for_completion(&req.done); - - /* If we raced with sys_sched_setaffinity, don't - * restore mask. */ - rq = task_rq_lock(p, &flags); - if (likely(cpus_equal(p->cpus_allowed, new_mask))) { - /* Restore old mask: won't need migration - * thread, since current cpu is allowed. */ - BUG_ON(__set_cpus_allowed(p, old_mask, NULL)); - } + return; } out: task_rq_unlock(rq, &flags); @@ -1169,7 +1152,7 @@ static int sched_best_cpu(struct task_st best_cpu = this_cpu = task_cpu(p); min_load = INT_MAX; - for (i = 0; i < NR_CPUS; i++) { + for_each_online_cpu(i) { unsigned long load; if (!cpu_isset(i, domain->span)) continue; @@ -3011,7 +2994,12 @@ int set_cpus_allowed(task_t *p, cpumask_ goto out; } - if (__set_cpus_allowed(p, new_mask, &req)) { + p->cpus_allowed = new_mask; + /* Can the task run on the task's current CPU? If so, we're done */ + if (cpu_isset(task_cpu(p), new_mask)) + goto out; + + if (migrate_task(p, any_online_cpu(new_mask), &req)) { /* Need help from migration thread: drop lock and wait. */ task_rq_unlock(rq, &flags); wake_up_process(rq->migration_thread); @@ -3025,8 +3013,16 @@ out: EXPORT_SYMBOL_GPL(set_cpus_allowed); -/* Move (not current) task off this cpu, onto dest cpu. */ -static void move_task_away(struct task_struct *p, int dest_cpu) +/* + * Move (not current) task off this cpu, onto dest cpu. We're doing + * this because either it can't run here any more (set_cpus_allowed() + * away from this CPU, or CPU going down), or because we're + * attempting to rebalance this task on exec (sched_balance_exec). + * + * So we race with normal scheduler movements, but that's OK, as long + * as the task is no longer on this CPU. + */ +static void __migrate_task(struct task_struct *p, int dest_cpu) { runqueue_t *rq_dest; unsigned long flags; @@ -3035,8 +3031,12 @@ static void move_task_away(struct task_s local_irq_save(flags); double_rq_lock(this_rq(), rq_dest); + /* Already moved. */ if (task_cpu(p) != smp_processor_id()) - goto out; /* Already moved */ + goto out; + /* Affinity changed (again). */ + if (!cpu_isset(dest_cpu, p->cpus_allowed)) + goto out; set_task_cpu(p, dest_cpu); if (p->array) { @@ -3096,8 +3096,7 @@ static int migration_thread(void * data) list_del_init(head->next); spin_unlock_irq(&rq->lock); - move_task_away(req->task, - any_online_cpu(req->task->cpus_allowed)); + __migrate_task(req->task, req->dest_cpu); complete(&req->done); } return 0; _ --------------010404090108080104050703--