From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dario Faggioli Subject: [PATCH v3 2/2] xen: credit1: avoid boosting vCPUs being "just" migrated Date: Fri, 12 Feb 2016 17:29:27 +0100 Message-ID: <20160212162926.25796.58870.stgit@Solace.station> References: <20160212162338.25796.17618.stgit@Solace.station> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1aUGb5-0007HY-TI for xen-devel@lists.xenproject.org; Fri, 12 Feb 2016 16:29:32 +0000 Received: by mail-wm0-f66.google.com with SMTP id p63so3596208wmp.1 for ; Fri, 12 Feb 2016 08:29:30 -0800 (PST) In-Reply-To: <20160212162338.25796.17618.stgit@Solace.station> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: xen-devel@lists.xenproject.org Cc: George Dunlap , Jan Beulich List-Id: xen-devel@lists.xenproject.org Moving a vCPU to a different pCPU means offlining it and then waking it up, on the new pCPU. Credit1 grants BOOST priority to vCPUs that wakes up, with the aim of improving I/O latency. The net effect of this all is that vCPUs get boosted when migrating, which shouldn't happen. For instance, this causes scheduling anomalies and, potentially, performance problems, as reported here: http://lists.xen.org/archives/html/xen-devel/2015-10/msg02851.html This patch fixes this by noting down (by means of a flag) the fact that the vCPU is about to undergo a migration. This way we can tell, later, during a wakeup, whether the vCPU is migrating or unblocking, and decide whether or not to apply the boosting. Note that it is important that atomic-safe bit operations are used when manipulating vCPUs' flags. Take the chance and add a comment about this. Signed-off-by: Dario Faggioli --- Cc: George Dunlap Cc: Jan Beulich --- Changes from v2: * test_and_clear() is necessary when accessing svc->flags; * added a comment about such need at the top, where the flags are defined. Changes from v1: * rewritten, following suggestion got during review: there are no wakeup flags any longer, and all is done in sched_credit.c by setting a flag in csched_cpu_pick() and testing (and cleating) it in csched_vcpu_wake(). --- xen/common/sched_credit.c | 30 ++++++++++++++++++++++++++---- 1 file changed, 26 insertions(+), 4 deletions(-) diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c index 5708701..597a784 100644 --- a/xen/common/sched_credit.c +++ b/xen/common/sched_credit.c @@ -63,9 +63,14 @@ /* * Flags + * + * Note that svc->flags (where these flags live) is protected by an + * inconsistent set of locks. Therefore atomic-safe bit operations must + * be used for accessing it. */ #define CSCHED_FLAG_VCPU_PARKED 0x0 /* VCPU over capped credits */ #define CSCHED_FLAG_VCPU_YIELD 0x1 /* VCPU yielding */ +#define CSCHED_FLAG_VCPU_MIGRATING 0x2 /* VCPU may have moved to a new pcpu */ /* @@ -787,6 +792,16 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc, bool_t commit) static int csched_cpu_pick(const struct scheduler *ops, struct vcpu *vc) { + struct csched_vcpu *svc = CSCHED_VCPU(vc); + + /* + * We have been called by vcpu_migrate() (in schedule.c), as part + * of the process of seeing if vc can be migrated to another pcpu. + * We make a note about this in svc->flags so that later, in + * csched_vcpu_wake() (still called from vcpu_migrate()) we won't + * get boosted, which we don't deserve as we are "only" migrating. + */ + set_bit(CSCHED_FLAG_VCPU_MIGRATING, &svc->flags); return _csched_cpu_pick(ops, vc, 1); } @@ -1022,11 +1037,18 @@ csched_vcpu_wake(const struct scheduler *ops, struct vcpu *vc) * more CPU resource intensive VCPUs without impacting overall * system fairness. * - * The one exception is for VCPUs of capped domains unpausing - * after earning credits they had overspent. We don't boost - * those. + * There are two cases, when we don't want to boost: + * - VCPUs that are waking up after a migration, rather than + * after having block; + * - VCPUs of capped domains unpausing after earning credits + * they had overspent. + * + * Note that checking whether we are "only" migrating must be + * done up front, as we do not want the clearing of the bit we + * set in csched_cpu_pick() to be short-circuited away. */ - if ( svc->pri == CSCHED_PRI_TS_UNDER && + if ( !test_and_clear_bit(CSCHED_FLAG_VCPU_MIGRATING, &svc->flags) && + svc->pri == CSCHED_PRI_TS_UNDER && !test_bit(CSCHED_FLAG_VCPU_PARKED, &svc->flags) ) { TRACE_2D(TRC_CSCHED_BOOST_START, vc->domain->domain_id, vc->vcpu_id);