Date: Tue, 3 Mar 2009 08:42:18 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: paulmck@linux.vnet.ibm.com, Bharata B Rao <bharata.rao@gmail.com>,
       Li Zefan <lizf@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       Paul Menage <menage@google.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] remove rq->lock from cpuacct cgroup (Was Re:
 [PATCH] cpuacct: add a branch prediction
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On Mon, 02 Mar 2009 15:56:10 +0100
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Fri, 2009-02-27 at 12:22 +0900, KAMEZAWA Hiroyuki wrote:
> 
> Comments below..
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > cgroup/cpuacct subsystem counts cpu usage by 64bit coutnter in
> > per-cpu object. In read-side (via cpuacct.usage file), for reading 64bit
> > value in safe manner, it takes rq->lock of (other) cpus.
> > 
> > In general, taking rq->lock of other cpus from codes not for scheduler
> > is not good. This patch tries to remove rq->lock used in read-side.
> > 
> > To read 64bit value in safe, this patch uses seqcounter.
> > 
> > Pros.
> >   - rq->lock is not necessary.
> > Cons.
> >   - When updating counter, sequence number must be updated.
> >     (I hope this per-cpu sequence number is on cache...)
> >   - not simple.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  kernel/sched.c |  141 ++++++++++++++++++++++++++++++++++++++++++---------------
> >  1 file changed, 105 insertions(+), 36 deletions(-)
> > 
> > Index: mmotm-2.6.29-Feb24/kernel/sched.c
> > ===================================================================
> > --- mmotm-2.6.29-Feb24.orig/kernel/sched.c
> > +++ mmotm-2.6.29-Feb24/kernel/sched.c
> > @@ -9581,6 +9581,67 @@ struct cgroup_subsys cpu_cgroup_subsys =
> >  
> >  #ifdef CONFIG_CGROUP_CPUACCT
> >  
> > +#ifndef CONFIG_64BIT
> > +DEFINE_PER_CPU(struct seqcount, cpuacct_cgroup_seq);
> > +
> > +static inline void cpuacct_start_counter_update(void)
> > +{
> > +	/* This is called under rq->lock and IRQ is off */
> > +	struct seqcount *s = &get_cpu_var(cpuacct_cgroup_seq);
> > +
> > +	write_seqcount_begin(s);
> > +	put_cpu_var(cpuacct_cgroup_seq);
> > +}
> > +
> > +static inline void cpuacct_end_counter_update(void)
> > +{
> > +	struct seqcount *s = &get_cpu_var(cpuacct_cgroup_seq);
> > +
> > +	write_seqcount_end(s);
> > +	put_cpu_var(cpuacct_cgroup_seq);
> > +}
> 
> It seems odd we disable/enable preemption in both, I would expect for
> start to disable preemption, and have end enable it again, or use
> __get_cpu_var() and assume preemption is already disabled (callsites are
> under rq->lock, right?)
> 
yes. calles are under rq->lock...ok, you're right.
I'll remove preepmtp disabling codes.


> > +static inline u64
> > +cpuacct_read_counter(u64 *val, int cpu)
> > +{
> > +	struct seqcount *s = &per_cpu(cpuacct_cgroup_seq, cpu);
> > +	unsigned int seq;
> > +	u64 data;
> > +
> > +	do {
> > +		seq = read_seqcount_begin(s);
> > +		data = *val;
> > +	} while (read_seqcount_retry(s, seq));
> > +	return data;
> > +}
> > +/* This is a special funtion called against "offline" cpus. */
> > +static inline void cpuacct_reset_offline_counter(u64 *val, int cpu)
> > +{
> > +	struct seqcount *s = &per_cpu(cpuacct_cgroup_seq, cpu);
> > +
> > +	preempt_disable();
> > +	write_seqcount_begin(s);
> > +	*val = 0;
> > +	write_seqcount_end(s);
> > +	preempt_enable();
> > +}
> 
> And here you double disable preemption, quite useless if you take a
> remote cpu's per-cpu data.
> 
Ok, maybe this "reset" from a user should be under rq->lock.
(But reading will not be under rq->lock.)


> > +#else
> > +static inline void cpuacct_start_counter_update(void)
> > +{
> > +}
> > +static inline void cpuacct_end_counter_update(void)
> > +{
> > +}
> > +static inline u64 cpuacct_read_counter(u64 *val, int cpu)
> > +{
> > +	return *val;
> > +}
> > +static inline void cpuacct_reset_offline_counter(u64 *val, int cpu)
> > +{
> > +	*val = 0;
> > +}
> > +#endif
> > +
> >  /*
> >   * CPU accounting code for task groups.
> >   *
> > @@ -9596,6 +9657,11 @@ struct cpuacct {
> >  	struct cpuacct *parent;
> >  };
> >  
> > +struct cpuacct_work {
> > +	struct work_struct work;
> > +	struct cpuacct *cpuacct;
> > +};
> > +
> >  struct cgroup_subsys cpuacct_subsys;
> >  
> >  /* return cpu accounting group corresponding to this container */
> > @@ -9643,39 +9709,29 @@ cpuacct_destroy(struct cgroup_subsys *ss
> >  	kfree(ca);
> >  }
> >  
> > +/* In 32bit enviroment, seqcounter is used for reading 64bit in safe way */
> >  static u64 cpuacct_cpuusage_read(struct cpuacct *ca, int cpu)
> >  {
> >  	u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
> >  	u64 data;
> >  
> > -#ifndef CONFIG_64BIT
> > -	/*
> > -	 * Take rq->lock to make 64-bit read safe on 32-bit platforms.
> > -	 */
> > -	spin_lock_irq(&cpu_rq(cpu)->lock);
> > -	data = *cpuusage;
> > -	spin_unlock_irq(&cpu_rq(cpu)->lock);
> > -#else
> > -	data = *cpuusage;
> > -#endif
> > +	data = cpuacct_read_counter(cpuusage, cpu);
> >  
> >  	return data;
> >  }
> >  
> > -static void cpuacct_cpuusage_write(struct cpuacct *ca, int cpu, u64 val)
> > +/* called by per-cpu workqueue */
> > +static void cpuacct_cpuusage_reset_cpu(struct work_struct *work)
> >  {
> > +	struct cpuacct_work *cw = container_of(work, struct cpuacct_work, work);
> > +	struct cpuacct *ca = cw->cpuacct;
> > +	int cpu = get_cpu();
> >  	u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
> >  
> > -#ifndef CONFIG_64BIT
> > -	/*
> > -	 * Take rq->lock to make 64-bit write safe on 32-bit platforms.
> > -	 */
> > -	spin_lock_irq(&cpu_rq(cpu)->lock);
> > -	*cpuusage = val;
> > -	spin_unlock_irq(&cpu_rq(cpu)->lock);
> > -#else
> > -	*cpuusage = val;
> > -#endif
> > +	cpuacct_start_counter_update();
> > +	*cpuusage = 0;
> > +	cpuacct_end_counter_update();
> > +	put_cpu();
> >  }
> >  
> >  /* return total cpu usage (in nanoseconds) of a group */
> > @@ -9691,23 +9747,34 @@ static u64 cpuusage_read(struct cgroup *
> >  	return totalcpuusage;
> >  }
> >  
> > -static int cpuusage_write(struct cgroup *cgrp, struct cftype *cftype,
> > -								u64 reset)
> > +static int cpuacct_cpuusage_reset(struct cgroup *cgrp, unsigned int event)
> >  {
> >  	struct cpuacct *ca = cgroup_ca(cgrp);
> > -	int err = 0;
> > -	int i;
> > -
> > -	if (reset) {
> > -		err = -EINVAL;
> > -		goto out;
> > +	int cpu;
> > +	/*
> > +	 * Reset All counters....doesn't need to be fast.
> > +	 * "ca" will be stable while doing this. We are in write() syscall.
> > +	 */
> > +	get_online_cpus();
> > +	/*
> > +	 * Because we use alloc_percpu() for allocating counter, we have
> > +	 * a counter per a possible cpu. Reset all online's by workqueue and
> > +	 * reset offline cpu's directly.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		if (cpu_online(cpu)) {
> > +			struct cpuacct_work cw;
> > +			INIT_WORK(&cw.work, cpuacct_cpuusage_reset_cpu);
> > +			cw.cpuacct = ca;
> > +			schedule_work_on(cpu, &cw.work);
> > +			flush_work(&cw.work);
> > +		} else {
> > +			u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
> > +			cpuacct_reset_offline_counter(cpuusage, cpu);
> > +		}
> 
> I'm not particularly convinced this is the right way, schedule_work_on()
> sounds way expensive for setting a variable to 0.
> 
yes, yes..

> Furthermore, if you want something like schedule_work_on() for each cpu,
> there's schedule_on_each_cpu().
> 
It can't pass arguments...Maybe I should use rq->lock here to reset
other cpu's value.

Thank you for review.
-Kame

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