From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932599AbcIQB3J (ORCPT ); Fri, 16 Sep 2016 21:29:09 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:56498 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752117AbcIQB3C (ORCPT ); Fri, 16 Sep 2016 21:29:02 -0400 DMARC-Filter: OpenDMARC Filter v1.3.1 smtp.codeaurora.org AC91B61784 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=pass smtp.mailfrom=joonwoop@codeaurora.org From: Joonwoo Park To: Peter Zijlstra Cc: Srivatsa Vaddagiri , Ingo Molnar , linux-kernel@vger.kernel.org, Joonwoo Park Subject: [PATCH] sched: Fix SCHED_HRTICK bug leading to late preemption of tasks Date: Fri, 16 Sep 2016 18:28:51 -0700 Message-Id: <1474075731-11550-1-git-send-email-joonwoop@codeaurora.org> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Srivatsa Vaddagiri SCHED_HRTICK feature is useful to preempt SCHED_FAIR tasks on-the-dot (just when they would have exceeded their ideal_runtime). It makes use of a per-cpu hrtimer resource and hence alarming that hrtimer should be based on total SCHED_FAIR tasks a cpu has across its various cfs_rqs, rather than being based on number of tasks in a particular cfs_rq (as implemented currently). As a result, with current code, its possible for a running task (which is the sole task in its cfs_rq) to be preempted much after its ideal_runtime has elapsed, resulting in increased latency for tasks in other cfs_rq on same cpu. Fix this by alarming sched hrtimer based on total number of SCHED_FAIR tasks a CPU has across its various cfs_rqs. Cc: Ingo Molnar Cc: Peter Zijlstra Cc: linux-kernel@vger.kernel.org Signed-off-by: Srivatsa Vaddagiri Signed-off-by: Joonwoo Park --- joonwoop: Do we also need to update or remove if-statement inside hrtick_update()? I guess not because hrtick_update() doesn't want to start hrtick when cfs_rq has large number of nr_running where slice is longer than sched_latency. kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4088eed..c55c566 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4458,7 +4458,7 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p) WARN_ON(task_rq(p) != rq); - if (cfs_rq->nr_running > 1) { + if (rq->cfs.h_nr_running > 1) { u64 slice = sched_slice(cfs_rq, se); u64 ran = se->sum_exec_runtime - se->prev_sum_exec_runtime; s64 delta = slice - ran; -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation