Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751186AbbHWFkj (ORCPT ); Sun, 23 Aug 2015 01:40:39 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:38175 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750831AbbHWFkh (ORCPT ); Sun, 23 Aug 2015 01:40:37 -0400 Date: Sun, 23 Aug 2015 07:40:32 +0200 From: Ingo Molnar To: Frederic Weisbecker Cc: Peter Zijlstra , LKML , Vatika Harlalka , Chris Metcalf , Thomas Gleixner , Preeti U Murthy , Christoph Lameter , "Paul E . McKenney" Subject: Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers Message-ID: <20150823054032.GA28133@gmail.com> References: <1439516774-4614-1-git-send-email-fweisbec@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1439516774-4614-1-git-send-email-fweisbec@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4108 Lines: 114 * Frederic Weisbecker wrote: > From: Vatika Harlalka > > The problem addressed in this patch is about affining unpinned timers. > Adaptive or Full Dynticks CPUs are currently disturbed by unnecessary > jitter due to firing of such timers on them. > > This patch will affine timers to online CPUs which are not full dynticks > in NOHZ_FULL configured systems. It should not introduce overhead in > nohz full off case due to static keys. > > Reviewed-by: Preeti U Murthy > Signed-off by: Vatika Harlalka > Cc: Ingo Molnar > Cc: Peter Zijlstra > Cc: Christoph Lameter > Cc: Thomas Gleixner > Cc: Paul E. McKenney > Cc: Chris Metcalf > Signed-off-by: Frederic Weisbecker > --- > include/linux/tick.h | 9 ++++++++- > kernel/sched/core.c | 7 +++++-- > 2 files changed, 13 insertions(+), 3 deletions(-) > > diff --git a/include/linux/tick.h b/include/linux/tick.h > index 3741ba1..51e6493 100644 > --- a/include/linux/tick.h > +++ b/include/linux/tick.h > @@ -143,13 +143,20 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) > if (tick_nohz_full_enabled()) > cpumask_or(mask, mask, tick_nohz_full_mask); > } > - > +static inline int housekeeping_any_cpu(void) > +{ > + return cpumask_any_and(housekeeping_mask, cpu_online_mask); > +} > extern void __tick_nohz_full_check(void); > extern void tick_nohz_full_kick(void); > extern void tick_nohz_full_kick_cpu(int cpu); > extern void tick_nohz_full_kick_all(void); > extern void __tick_nohz_task_switch(struct task_struct *tsk); > #else > +static inline int housekeeping_any_cpu(void) > +{ > + return smp_processor_id(); > +} > static inline bool tick_nohz_full_enabled(void) { return false; } > static inline bool tick_nohz_full_cpu(int cpu) { return false; } > static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { } > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 9917c96..4fd42e4 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -623,18 +623,21 @@ int get_nohz_timer_target(void) > int i, cpu = smp_processor_id(); > struct sched_domain *sd; > > - if (!idle_cpu(cpu)) > + if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu)) > return cpu; > > rcu_read_lock(); > for_each_domain(cpu, sd) { > for_each_cpu(i, sched_domain_span(sd)) { > - if (!idle_cpu(i)) { > + if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) { > cpu = i; > goto unlock; > } > } > } > + > + if (!is_housekeeping_cpu(cpu)) > + cpu = housekeeping_any_cpu(); > unlock: > rcu_read_unlock(); > return cpu; So I almost applied this yesterday, but had the following question: what ensures that housekeeping_mask isn't empty? If it's empty then housekeeping_any_cpu() returns cpumask_any_and() of an empty cpumask - which returns an out of range index AFAICS - which will crash and burn in: kernel/time/hrtimer.c: return &per_cpu(hrtimer_bases, get_nohz_timer_target()); kernel/time/timer.c: return per_cpu_ptr(&tvec_bases, get_nohz_timer_target()); housekeeping_mask itself is derived from tick_nohz_full_mask (it's the inverse of it in essence), and tick_nohz_full_mask is set via two methods, either via a boot parameter: if (cpulist_parse(str, tick_nohz_full_mask) < 0) { in tick_nohz_full_setup(). What ensures here that tick_nohz_full_mask is not completely full - making housekeeping_mask empty? The other method is via CONFIG_NO_HZ_FULL_ALL: cpumask_setall(tick_nohz_full_mask); here it's fully set - triggering the bug I'm worried about. So what am I missing, what prevents CONFIG_NO_HZ_FULL_ALL from crashing? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/