Messages in this thread | | | Subject | Re: [PATCH v5 1/3] sched: Stop nohz stats when decayed | From | Valentin Schneider <> | Date | Fri, 16 Feb 2018 12:53:36 +0000 |
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On 02/14/2018 03:26 PM, Vincent Guittot wrote: > Stopped the periodic update of blocked load when all idle CPUs have fully > decayed. We introduce a new nohz.has_blocked that reflect if some idle > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked > is set everytime that a Idle CPU can have blocked load and it is then clear > when no more blocked load has been detected during an update. We don't need > atomic operation but only to make cure of the right ordering when updating > nohz.idle_cpus_mask and nohz.has_blocked. > > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- > kernel/sched/sched.h | 1 + > 2 files changed, 102 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7af1fa9..5a6835e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > > [...] > > -static void update_nohz_stats(struct rq *rq) > +static bool update_nohz_stats(struct rq *rq) > { > #ifdef CONFIG_NO_HZ_COMMON > unsigned int cpu = rq->cpu; > > + if (!rq->has_blocked_load) > + return false; > + > if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) > - return; > + return false; > > if (!time_after(jiffies, rq->last_blocked_load_update_tick)) > - return; > + return true; > > update_blocked_averages(cpu); > + > + return rq->has_blocked_load; > +#else > + return false; > #endif > } >
(Wrongly thought that this bit was in a different patch, comment should have been squashed in previous reply...)
I've been thinking about this, and it's a messy one if we want to skip CPUs in idle_balance() / clear the nohz.has_blocked_flag.
AFAICT, the rq->has_blocked_load flag can be wrongly cleared: if we're calling update_nohz_stats() for CPU A, but CPU A got out/in of idle really quickly in that same timeframe, I'm not sure you can guarantee the clearing of rq->has_blocked_load done in update_blocked_averages() will always end up in memory before the setting of the flag in nohz_balance_enter_idle().
I was going to say we don't have this problem in _nohz_idle_balance() but actually I think we do. We have the checking of nohz.idle_cpus_mask after the smp_mb(), which makes sure the clear of nohz.has_blocked will never overwrite the set in nohz_balance_enter_idle(), but it doesn't guarantee the same for the rq flag. So we can have nohz CPUs with blocked load but with rq->has_blocked_load set to false. Which isn't a problem now but it is if we want to use the flag to skip CPUs.
Am I correct or am I going crazy ? There's a comment about this in nohz_balance_enter_idle() but I'm confused now:
/* * Can be set safely without rq->lock held * If a clear happens, it will have evaluated last additions because * rq->lock is held during the check and the clear */ rq->has_blocked_load = 1;
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