Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer to improve scalability

[Paul E. McKenney]

On Thu, Mar 05, 2015 at 04:35:09PM +0100, Frederic Weisbecker wrote:
> On Mon, Mar 02, 2015 at 10:42:11AM -0800, Jason Low wrote:
> > v1->v2:
> > - Peter suggested that cputimer->running does not need to be atomic,
> >   so we can leave it as an integer.
> > - Address a race condition that could occur in update_gt_cputime().
> > - Add helper functions to avoid repeating code.
> > 
> > While running a database workload, we found a scalability issue
> > with itimers.
> > 
> > Much of the problem was caused by the thread_group_cputimer spinlock.
> > Each time we account for group system/user time, we need to obtain a
> > thread_group_cputimer's spinlock to update the timers. On larger
> > systems (such as a 16 socket machine), this caused more than 30% of
> > total time spent trying to obtain this kernel lock to update these
> > group timer stats.
> > 
> > This patch converts the timers to 64 bit atomic variables and use
> > atomic add to update them without a lock. With this patch, the percent
> > of total time spent updating thread group cputimer timers was reduced
> > from 30% down to less than 1%.
> > 
> > Signed-off-by: Jason Low <jason.low2@xxxxxx>
> > ---
> >  include/linux/init_task.h      |    7 +++--
> >  include/linux/sched.h          |   10 ++-----
> >  kernel/fork.c                  |    3 --
> >  kernel/sched/stats.h           |   12 ++------
> >  kernel/time/posix-cpu-timers.c |   55 +++++++++++++++++++++++----------------
> >  5 files changed, 42 insertions(+), 45 deletions(-)
> > 
> > diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> > index 3037fc0..c4cdec7 100644
> > --- a/include/linux/init_task.h
> > +++ b/include/linux/init_task.h
> > @@ -50,9 +50,10 @@ extern struct fs_struct init_fs;
> >  	.cpu_timers	= INIT_CPU_TIMERS(sig.cpu_timers),		\
> >  	.rlim		= INIT_RLIMITS,					\
> >  	.cputimer	= { 						\
> > -		.cputime = INIT_CPUTIME,				\
> > -		.running = 0,						\
> > -		.lock = __RAW_SPIN_LOCK_UNLOCKED(sig.cputimer.lock),	\
> > +		.utime = ATOMIC64_INIT(0),                              \
> > +		.stime = ATOMIC64_INIT(0),                              \
> > +		.sum_exec_runtime = ATOMIC64_INIT(0),                   \
> > +		.running = 0						\
> >  	},								\
> >  	.cred_guard_mutex =						\
> >  		 __MUTEX_INITIALIZER(sig.cred_guard_mutex),		\
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 8db31ef..d6b0f76 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -588,9 +588,10 @@ struct task_cputime {
> >   * used for thread group CPU timer calculations.
> >   */
> >  struct thread_group_cputimer {
> > -	struct task_cputime cputime;
> > +	atomic64_t utime;
> > +	atomic64_t stime;
> > +	atomic64_t sum_exec_runtime;
> >  	int running;
> > -	raw_spinlock_t lock;
> >  };
> >  
> >  #include <linux/rwsem.h>
> > @@ -2942,11 +2943,6 @@ static __always_inline bool need_resched(void)
> >  void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times);
> >  void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times);
> >  
> > -static inline void thread_group_cputime_init(struct signal_struct *sig)
> > -{
> > -	raw_spin_lock_init(&sig->cputimer.lock);
> > -}
> > -
> >  /*
> >   * Reevaluate whether the task has signals pending delivery.
> >   * Wake the task if so.
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 4dc2dda..df9dfe9 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1037,9 +1037,6 @@ static void posix_cpu_timers_init_group(struct signal_struct *sig)
> >  {
> >  	unsigned long cpu_limit;
> >  
> > -	/* Thread group counters. */
> > -	thread_group_cputime_init(sig);
> > -
> >  	cpu_limit = ACCESS_ONCE(sig->rlim[RLIMIT_CPU].rlim_cur);
> >  	if (cpu_limit != RLIM_INFINITY) {
> >  		sig->cputime_expires.prof_exp = secs_to_cputime(cpu_limit);
> > diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
> > index 4ab7043..adda94e 100644
> > --- a/kernel/sched/stats.h
> > +++ b/kernel/sched/stats.h
> > @@ -215,9 +215,7 @@ static inline void account_group_user_time(struct task_struct *tsk,
> >  	if (!cputimer_running(tsk))
> >  		return;
> >  
> > -	raw_spin_lock(&cputimer->lock);
> > -	cputimer->cputime.utime += cputime;
> > -	raw_spin_unlock(&cputimer->lock);
> > +	atomic64_add(cputime, &cputimer->utime);
> >  }
> >  
> >  /**
> > @@ -238,9 +236,7 @@ static inline void account_group_system_time(struct task_struct *tsk,
> >  	if (!cputimer_running(tsk))
> >  		return;
> >  
> > -	raw_spin_lock(&cputimer->lock);
> > -	cputimer->cputime.stime += cputime;
> > -	raw_spin_unlock(&cputimer->lock);
> > +	atomic64_add(cputime, &cputimer->stime);
> >  }
> >  
> >  /**
> > @@ -261,7 +257,5 @@ static inline void account_group_exec_runtime(struct task_struct *tsk,
> >  	if (!cputimer_running(tsk))
> >  		return;
> >  
> > -	raw_spin_lock(&cputimer->lock);
> > -	cputimer->cputime.sum_exec_runtime += ns;
> > -	raw_spin_unlock(&cputimer->lock);
> > +	atomic64_add(ns, &cputimer->sum_exec_runtime);
> >  }
> > diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
> > index a16b678..ba93c23 100644
> > --- a/kernel/time/posix-cpu-timers.c
> > +++ b/kernel/time/posix-cpu-timers.c
> > @@ -173,6 +173,14 @@ posix_cpu_clock_set(const clockid_t which_clock, const struct timespec *tp)
> >  	return error;
> >  }
> >  
> > +/* Sample thread_group_cputimer values in "cputimer", copy results to "times" */
> > +static inline void sample_group_cputimer(struct task_cputime *times,
> > +			                 struct thread_group_cputimer *cputimer)
> > +{
> > +        times->utime = atomic64_read(&cputimer->utime);
> > +        times->stime = atomic64_read(&cputimer->stime);
> > +        times->sum_exec_runtime = atomic64_read(&cputimer->sum_exec_runtime);
> 
> So, in the case we are calling that right after setting cputimer->running, I guess we are fine
> because we just updated cputimer with the freshest values.
> 
> But if we are reading this a while after, say several ticks further, there is a chance that
> we read stale values since we don't lock anymore.
> 
> I don't know if it matters or not, I guess it depends how stale it can be and how much precision
> we expect from posix cpu timers. It probably doesn't matter.
> 
> But just in case, atomic64_read_return(&cputimer->utime, 0) would make sure we get the freshest
> value because it performs a full barrier, at the cost of more overhead of course.

Well, if we are running within a guest OS, we might be delayed at any point
for quite some time.  Even with interrupts disabled.

							Thanx, Paul

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