From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: Re: [PATCH] xen: Always ask the scheduler to re-place the vcpu when the affinity changes Date: Mon, 4 Mar 2013 13:45:20 +0000 Message-ID: <5134A570.7050604@eu.citrix.com> References: <1362399557-20214-1-git-send-email-george.dunlap@eu.citrix.com> <5134A33402000078000C2CF1@nat28.tlf.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5134A33402000078000C2CF1@nat28.tlf.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich Cc: "Keir (Xen.org)" , "xen-devel@lists.xen.org" List-Id: xen-devel@lists.xenproject.org On 04/03/13 12:35, Jan Beulich wrote: >>>> On 04.03.13 at 13:19, George Dunlap wrote: >> It's probably a good idea to re-evaluate placement whenever the >> affinity changes. >> >> This additionally has the benefit of removing scheduler-specific >> exceptions introduced in git c/s e6a6fd63. >> >> Signed-off-by: George Dunlap >> --- >> xen/common/schedule.c | 7 ++++--- >> 1 file changed, 4 insertions(+), 3 deletions(-) >> >> diff --git a/xen/common/schedule.c b/xen/common/schedule.c >> index de11110..dbef5af 100644 >> --- a/xen/common/schedule.c >> +++ b/xen/common/schedule.c >> @@ -613,9 +613,10 @@ int vcpu_set_affinity(struct vcpu *v, const cpumask_t >> *affinity) >> vcpu_schedule_lock_irq(v); >> >> cpumask_copy(v->cpu_affinity, affinity); >> - if ( VCPU2OP(v)->sched_id == XEN_SCHEDULER_SEDF || >> - !cpumask_test_cpu(v->processor, v->cpu_affinity) ) >> - set_bit(_VPF_migrating, &v->pause_flags); >> + >> + /* Always ask the scheduler to re-evaluate placement >> + * when changing the affinity */ >> + set_bit(_VPF_migrating, &v->pause_flags); >> >> vcpu_schedule_unlock_irq(v); >> > The code right below the context visible here is > > if ( test_bit(_VPF_migrating, &v->pause_flags) ) > { > vcpu_sleep_nosync(v); > vcpu_migrate(v); > } > > and I think the conditional could (and should) now be removed. Yeah, I wasn't sure what to make of that one -- it looked as though it was coded such that someone else might be able to clear _VPF_migrating between releasing the lock and this test. But if that can happen, it's racy anyway. vcpu_force_reschedule() has this "set, unlock, re-check" pattern. It looks like there actually is a way that it could conceivably be cleared: if the vcpu is running on another pcpu, if after we release the lock the vcpu is de-scheduled and context_saved() is called, it will check for _VPF_migrating and call vcpu_migrate(), which can clear the flag. But that's probably a rare enough occurrence that it's better overall to take the occasional double-migrate. -George