Messages in this thread Patch in this message | | | Date | Tue, 20 Aug 2013 18:33:12 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock |
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On 08/20, Peter Zijlstra wrote: > > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -453,7 +453,8 @@ struct rq { > u64 clock; > u64 clock_task; > > - atomic_t nr_iowait; > + int nr_iowait_local; > + atomic_t nr_iowait_remote;
I am wondering how the extra lock(rq)/unlock(rq) in schedule() is bad compared to atomic_dec.
IOW, what if we simply make rq->nr_iowait "int" and change schedule() to update it? Something like below. Just curious.
As for nr_iowait_local + nr_iowait_remote, this doesn't look safe... in theory nr_iowait_cpu() or even nr_iowait() can return a negative number.
Oleg.
--- x/kernel/sched/core.c +++ x/kernel/sched/core.c @@ -2435,6 +2435,9 @@ need_resched: rq->curr = next; ++*switch_count; + if (unlikely(prev->in_iowait)) + rq->nr_iowait++; + context_switch(rq, prev, next); /* unlocks the rq */ /* * The context switch have flipped the stack from under us @@ -2442,6 +2445,12 @@ need_resched: * this task called schedule() in the past. prev == current * is still correct, but it can be moved to another cpu/rq. */ + if (unlikely(prev->in_iowait)) { + raw_spin_lock_irq(&rq->lock); + rq->nr_iowait--; + raw_spin_unlock_irq(&rq->lock); + } + cpu = smp_processor_id(); rq = cpu_rq(cpu); } else @@ -3939,31 +3948,24 @@ EXPORT_SYMBOL_GPL(yield_to); */ void __sched io_schedule(void) { - struct rq *rq = raw_rq(); - delayacct_blkio_start(); - atomic_inc(&rq->nr_iowait); blk_flush_plug(current); current->in_iowait = 1; schedule(); current->in_iowait = 0; - atomic_dec(&rq->nr_iowait); delayacct_blkio_end(); } EXPORT_SYMBOL(io_schedule); long __sched io_schedule_timeout(long timeout) { - struct rq *rq = raw_rq(); long ret; delayacct_blkio_start(); - atomic_inc(&rq->nr_iowait); blk_flush_plug(current); current->in_iowait = 1; ret = schedule_timeout(timeout); current->in_iowait = 0; - atomic_dec(&rq->nr_iowait); delayacct_blkio_end(); return ret; }
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