Re: [GIT PULL] cputime: Full dynticks task/cputime accounting v7

[Mats Liljegren]

Kevin Hilman wrote:
> Frederic Weisbecker <fweisbec@xxxxxxxxx> writes:
> 
> > Ingo,
> >
> > Please pull the new full dynticks cputime accounting code that
> > can be found at:
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > 	tags/full-dynticks-cputime-for-mingo
> >
> > My last concern is the dependency on CONFIG_64BIT. We rely on cputime_t
> > being u64 for reasonable nanosec granularity implementation. And therefore
> > we need a single instruction fetch to read kernel cpustat for atomicity
> > requirement against concurrent incrementation, which only 64 bit archs
> > can provide.
> 
> Actually, moderately recent 32-bit ARMs can do atomic 64-bit load/stores
> too.   
> 
> Also, is it just kernel_cpustat increments that need protection? or do
> the various reads of the task_struct's cputime fields also need
> protection (hmm, thinking twice, maybe those are already sufficiently
> protected by the vtime_seqlock?)
> 
> > It's probably no big deal to solve this issue. What we need is simply some
> > atomic accessors.
> 
> What about using the atomic64_* accessors?  Those would just use the
> native loads/stores on arches that have them, otherwise
> CONFIG_GENERIC_ATOMIC64 provides some fallbacks.
> 
> To give it a try, below is a quick patch to convert kernel_cpustat to
> atomic64.  I only got as far as compile testing and basic boot testing
> on a 32-bit ARM platform, but let me know if this is the right
> direction.
> 
> > There is just no emergency though as this new option depends on the context
> > tracking subsystem that only x86-64 (and soon ppc64) implements yet. And
> > this set is complex enough already. I think we can deal with that later.
> 
> I've started working on the ARM version of the context_tracker, so
> "later" is coming quickly and I will do what I can to help this along.
> 
> Kevin
> 
> 
> 
> >From 9881fc4b86a63244018a4bdb5a383f3b862a0b8b Mon Sep 17 00:00:00 2001
> From: Kevin Hilman <khilman@xxxxxxxxxx>
> Date: Thu, 14 Feb 2013 17:46:08 -0800
> Subject: [PATCH] kernel_cpustat: convert to atomic 64-bit accessors
> 
> WIP: only compile tested and basic boot tested on 32-bit ARM platform
> ---
>  fs/proc/stat.c              | 36 ++++++++++++++++++------------------
>  fs/proc/uptime.c            |  2 +-
>  include/linux/kernel_stat.h |  2 +-
>  kernel/sched/cputime.c      | 22 +++++++++++-----------
>  4 files changed, 31 insertions(+), 31 deletions(-)
> 
> diff --git a/fs/proc/stat.c b/fs/proc/stat.c
> index e296572..2a4c723 100644
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -52,7 +52,7 @@ static u64 get_idle_time(int cpu)
>  
>  	if (idle_time == -1ULL)
>  		/* !NO_HZ or cpu offline so we can rely on cpustat.idle */
> -		idle = kcpustat_cpu(cpu).cpustat[CPUTIME_IDLE];
> +		idle = atomic64_read(&kcpustat_cpu(cpu).cpustat[CPUTIME_IDLE]);
>  	else
>  		idle = usecs_to_cputime64(idle_time);
>  
> @@ -68,7 +68,7 @@ static u64 get_iowait_time(int cpu)
>  
>  	if (iowait_time == -1ULL)
>  		/* !NO_HZ or cpu offline so we can rely on cpustat.iowait */
> -		iowait = kcpustat_cpu(cpu).cpustat[CPUTIME_IOWAIT];
> +		iowait = atomic64_read(&kcpustat_cpu(cpu).cpustat[CPUTIME_IOWAIT]);
>  	else
>  		iowait = usecs_to_cputime64(iowait_time);
>  
> @@ -95,16 +95,16 @@ static int show_stat(struct seq_file *p, void *v)
>  	jif = boottime.tv_sec;
>  
>  	for_each_possible_cpu(i) {
> -		user += kcpustat_cpu(i).cpustat[CPUTIME_USER];
> -		nice += kcpustat_cpu(i).cpustat[CPUTIME_NICE];
> -		system += kcpustat_cpu(i).cpustat[CPUTIME_SYSTEM];
> +		user += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_USER]);
> +		nice += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_NICE]);
> +		system += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_SYSTEM]);
>  		idle += get_idle_time(i);
>  		iowait += get_iowait_time(i);
> -		irq += kcpustat_cpu(i).cpustat[CPUTIME_IRQ];
> -		softirq += kcpustat_cpu(i).cpustat[CPUTIME_SOFTIRQ];
> -		steal += kcpustat_cpu(i).cpustat[CPUTIME_STEAL];
> -		guest += kcpustat_cpu(i).cpustat[CPUTIME_GUEST];
> -		guest_nice += kcpustat_cpu(i).cpustat[CPUTIME_GUEST_NICE];
> +		irq += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_IRQ]);
> +		softirq += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_SOFTIRQ]);
> +		steal += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_STEAL]);
> +		guest += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_GUEST]);
> +		guest_nice += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_GUEST_NICE]);
>  		sum += kstat_cpu_irqs_sum(i);
>  		sum += arch_irq_stat_cpu(i);
>  
> @@ -132,16 +132,16 @@ static int show_stat(struct seq_file *p, void *v)
>  
>  	for_each_online_cpu(i) {
>  		/* Copy values here to work around gcc-2.95.3, gcc-2.96 */
> -		user = kcpustat_cpu(i).cpustat[CPUTIME_USER];
> -		nice = kcpustat_cpu(i).cpustat[CPUTIME_NICE];
> -		system = kcpustat_cpu(i).cpustat[CPUTIME_SYSTEM];
> +		user = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_USER]);
> +		nice = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_NICE]);
> +		system = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_SYSTEM]);
>  		idle = get_idle_time(i);
>  		iowait = get_iowait_time(i);
> -		irq = kcpustat_cpu(i).cpustat[CPUTIME_IRQ];
> -		softirq = kcpustat_cpu(i).cpustat[CPUTIME_SOFTIRQ];
> -		steal = kcpustat_cpu(i).cpustat[CPUTIME_STEAL];
> -		guest = kcpustat_cpu(i).cpustat[CPUTIME_GUEST];
> -		guest_nice = kcpustat_cpu(i).cpustat[CPUTIME_GUEST_NICE];
> +		irq = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_IRQ]);
> +		softirq = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_SOFTIRQ]);
> +		steal = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_STEAL]);
> +		guest = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_GUEST]);
> +		guest_nice = atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_GUEST_NICE]);
>  		seq_printf(p, "cpu%d", i);
>  		seq_put_decimal_ull(p, ' ', cputime64_to_clock_t(user));
>  		seq_put_decimal_ull(p, ' ', cputime64_to_clock_t(nice));
> diff --git a/fs/proc/uptime.c b/fs/proc/uptime.c
> index 9610ac7..10c0f6e 100644
> --- a/fs/proc/uptime.c
> +++ b/fs/proc/uptime.c
> @@ -18,7 +18,7 @@ static int uptime_proc_show(struct seq_file *m, void *v)
>  
>  	idletime = 0;
>  	for_each_possible_cpu(i)
> -		idletime += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE];
> +		idletime += atomic64_read(&kcpustat_cpu(i).cpustat[CPUTIME_IDLE]);
>  
>  	do_posix_clock_monotonic_gettime(&uptime);
>  	monotonic_to_bootbased(&uptime);
> diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
> index ed5f6ed..45b9f71 100644
> --- a/include/linux/kernel_stat.h
> +++ b/include/linux/kernel_stat.h
> @@ -32,7 +32,7 @@ enum cpu_usage_stat {
>  };
>  
>  struct kernel_cpustat {
> -	u64 cpustat[NR_STATS];
> +	atomic64_t cpustat[NR_STATS];
>  };
>  
>  struct kernel_stat {
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index ccff275..c1ac2b2 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -125,7 +125,7 @@ static inline void task_group_account_field(struct task_struct *p, int index,
>  	 * is the only cgroup, then nothing else should be necessary.
>  	 *
>  	 */
> -	__get_cpu_var(kernel_cpustat).cpustat[index] += tmp;
> +	atomic64_add(tmp, &__get_cpu_var(kernel_cpustat).cpustat[index]);
>  
>  #ifdef CONFIG_CGROUP_CPUACCT
>  	if (unlikely(!cpuacct_subsys.active))
> @@ -176,7 +176,7 @@ void account_user_time(struct task_struct *p, cputime_t cputime,
>  static void account_guest_time(struct task_struct *p, cputime_t cputime,
>  			       cputime_t cputime_scaled)
>  {
> -	u64 *cpustat = kcpustat_this_cpu->cpustat;
> +	atomic64_t *cpustat = kcpustat_this_cpu->cpustat;
>  
>  	/* Add guest time to process. */
>  	p->utime += cputime;
> @@ -186,11 +186,11 @@ static void account_guest_time(struct task_struct *p, cputime_t cputime,
>  
>  	/* Add guest time to cpustat. */
>  	if (TASK_NICE(p) > 0) {
> -		cpustat[CPUTIME_NICE] += (__force u64) cputime;
> -		cpustat[CPUTIME_GUEST_NICE] += (__force u64) cputime;
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_NICE]);
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_GUEST_NICE]);
>  	} else {
> -		cpustat[CPUTIME_USER] += (__force u64) cputime;
> -		cpustat[CPUTIME_GUEST] += (__force u64) cputime;
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_USER]);
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_GUEST]);
>  	}
>  }
>  
> @@ -250,9 +250,9 @@ void account_system_time(struct task_struct *p, int hardirq_offset,
>   */
>  void account_steal_time(cputime_t cputime)
>  {
> -	u64 *cpustat = kcpustat_this_cpu->cpustat;
> +	atomic64_t *cpustat = kcpustat_this_cpu->cpustat;
>  
> -	cpustat[CPUTIME_STEAL] += (__force u64) cputime;
> +	atomic64_add((__force u64) cputime, &cpustat[CPUTIME_STEAL]);
>  }
>  
>  /*
> @@ -261,13 +261,13 @@ void account_steal_time(cputime_t cputime)
>   */
>  void account_idle_time(cputime_t cputime)
>  {
> -	u64 *cpustat = kcpustat_this_cpu->cpustat;
> +	atomic64_t *cpustat = kcpustat_this_cpu->cpustat;
>  	struct rq *rq = this_rq();
>  
>  	if (atomic_read(&rq->nr_iowait) > 0)
> -		cpustat[CPUTIME_IOWAIT] += (__force u64) cputime;
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_IOWAIT]);
>  	else
> -		cpustat[CPUTIME_IDLE] += (__force u64) cputime;
> +		atomic64_add((__force u64) cputime, &cpustat[CPUTIME_IDLE]);
>  }
>  
>  static __always_inline bool steal_account_process_tick(void)
> -- 
> 1.8.1.2
> 
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I had to do some changes to get it to compile. You can check my patch below
and see whether I got it all wrong...

/Mats