From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161925AbbKEP4g (ORCPT ); Thu, 5 Nov 2015 10:56:36 -0500 Received: from shelob.surriel.com ([74.92.59.67]:56825 "EHLO shelob.surriel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1032971AbbKEP4f (ORCPT ); Thu, 5 Nov 2015 10:56:35 -0500 Subject: Re: [PATCH] sched,numa cap pte scanning overhead to 3% of run time To: Peter Zijlstra References: <20151104132515.07e41b75@annuminas.surriel.com> <20151105153402.GR17308@twins.programming.kicks-ass.net> Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, mgorman@suse.de, jstancek@redhat.com From: Rik van Riel Message-ID: <563B7C2D.90008@surriel.com> Date: Thu, 5 Nov 2015 10:56:29 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <20151105153402.GR17308@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/05/2015 10:34 AM, Peter Zijlstra wrote: > On Wed, Nov 04, 2015 at 01:25:15PM -0500, Rik van Riel wrote: >> +++ b/kernel/sched/fair.c >> @@ -2155,6 +2155,7 @@ void task_numa_work(struct callback_head *work) >> unsigned long migrate, next_scan, now = jiffies; >> struct task_struct *p = current; >> struct mm_struct *mm = p->mm; >> + u64 runtime = p->se.sum_exec_runtime; >> struct vm_area_struct *vma; >> unsigned long start, end; >> unsigned long nr_pte_updates = 0; >> @@ -2277,6 +2278,20 @@ void task_numa_work(struct callback_head *work) >> else >> reset_ptenuma_scan(p); >> up_read(&mm->mmap_sem); >> + >> + /* >> + * There is a fundamental mismatch between the runtime based >> + * NUMA scanning at the task level, and the wall clock time >> + * NUMA scanning at the mm level. On a severely overloaded >> + * system, with very large processes, this mismatch can cause >> + * the system to spend all of its time in change_prot_numa(). >> + * Limit NUMA PTE scanning to 3% of the task's run time, if >> + * we spent so much time scanning we got rescheduled. >> + */ >> + if (unlikely(p->se.sum_exec_runtime != runtime)) { >> + u64 diff = p->se.sum_exec_runtime - runtime; >> + p->node_stamp += 32 * diff; >> + } > > I don't actually see how this does what it says it does If we got rescheduled during the assigning of runtime above, and this point, the scheduler should have updated the p->se.sum_exec_runtime statistic, given that update_curr is called from both dequeue_entity and enqueue_entity in fair.c Advancing the node_stamp by 32x the amount of time the task consumed between entering task_numa_work and this point should ensure task_numa_work does not get queued again until we have used 32x as much time doing something else. That should limit the CPU time used by task_numa_work. What am I missing? >> @@ -2302,7 +2317,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr) >> now = curr->se.sum_exec_runtime; >> period = (u64)curr->numa_scan_period * NSEC_PER_MSEC; >> >> - if (now - curr->node_stamp > period) { >> + if (now > curr->node_stamp + period) { >> if (!curr->node_stamp) >> curr->numa_scan_period = task_scan_min(curr); >> curr->node_stamp += period; > > And this really should be an independent patch. Although the fix I had > in mind looked like: > > if ((s64)(now - curr->node_stamp) > period) > > But I suppose this works too. I can resend this as a separate patch if you prefer.