[lttng-dev] [PATCH] Fix: call_rcu_thread() affinity failure

[Paul E. McKenney]

On Mon, Jun 29, 2015 at 06:56:34PM -0400, Mathieu Desnoyers wrote:
> Make call_rcu_thread() affine itself more persistently
> 
> Currently, URCU simply fails if a call_rcu_thread() fails to affine
> itself. This is problematic when execution is constrained by cgroup
> and hotunplugged CPUs. This commit therefore makes call_rcu_thread()
> retry setting its affinity every 256 grace periods, but only if it
> detects that it migrated to a different CPU. Since sched_getcpu() is
> cheap on many architectures, this check is less costly than going
> through a system call.
> 
> Reported-by: Michael Jeanson <mjeanson at efficios.com>
> Suggested-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>

A couple of issues, but otherwise good.  (They might even be issues
with your code rather than my eyes, you never know!)

							Thanx, Paul

> ---
>  urcu-call-rcu-impl.h | 36 +++++++++++++++++++++++++++++++-----
>  1 file changed, 31 insertions(+), 5 deletions(-)
> 
> diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
> index 5cc02d9..b82a59b 100644
> --- a/urcu-call-rcu-impl.h
> +++ b/urcu-call-rcu-impl.h
> @@ -45,6 +45,9 @@
>  #include "urcu/ref.h"
>  #include "urcu-die.h"
> 
> +#define SET_AFFINITY_CHECK_PERIOD		(1U << 8)	/* 256 */
> +#define SET_AFFINITY_CHECK_PERIOD_MASK		(SET_AFFINITY_CHECK_PERIOD - 1)
> +
>  /* Data structure that identifies a call_rcu thread. */
> 
>  struct call_rcu_data {
> @@ -62,6 +65,7 @@ struct call_rcu_data {
>  	unsigned long qlen; /* maintained for debugging. */
>  	pthread_t tid;
>  	int cpu_affinity;
> +	unsigned long gp_count;
>  	struct cds_list_head list;
>  } __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
> 
> @@ -203,22 +207,42 @@ static void call_rcu_unlock(pthread_mutex_t *pmp)
>  		urcu_die(ret);
>  }
> 
> +/*
> + * Periodically retry setting CPU affinity if we migrate.
> + * Losing affinity can be caused by CPU hotunplug/hotplug, or by
> + * cpuset(7).
> + */
>  #if HAVE_SCHED_SETAFFINITY
>  static
>  int set_thread_cpu_affinity(struct call_rcu_data *crdp)
>  {
>  	cpu_set_t mask;
> +	int ret;
> 
>  	if (crdp->cpu_affinity < 0)
>  		return 0;
> +	if (++crdp->gp_count & SET_AFFINITY_CHECK_PERIOD_MASK)
> +		return 0;
> +	if (urcu_sched_getcpu() != crdp->cpu_affinity)

Don't we want "==" here instead of "!="?

> +		return 0;
> 
>  	CPU_ZERO(&mask);
>  	CPU_SET(crdp->cpu_affinity, &mask);
>  #if SCHED_SETAFFINITY_ARGS == 2
> -	return sched_setaffinity(0, &mask);
> +	ret = sched_setaffinity(0, &mask);
>  #else
> -	return sched_setaffinity(0, sizeof(mask), &mask);
> +	ret = sched_setaffinity(0, sizeof(mask), &mask);
>  #endif
> +	/*
> +	 * EINVAL is fine: can be caused by hotunplugged CPUs, or by
> +	 * cpuset(7). This is why we should always retry is we detect

s/is we detect/if we detect/

> +	 * migration.
> +	 */
> +	if (ret && errno == EINVAL) {
> +		ret = 0;
> +		errno = 0;
> +	}
> +	return ret;
>  }
>  #else
>  static
> @@ -275,10 +299,8 @@ static void *call_rcu_thread(void *arg)
>  	unsigned long cbcount;
>  	struct call_rcu_data *crdp = (struct call_rcu_data *) arg;
>  	int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
> -	int ret;
> 
> -	ret = set_thread_cpu_affinity(crdp);
> -	if (ret)
> +	if (set_thread_cpu_affinity(crdp))
>  		urcu_die(errno);
> 
>  	/*
> @@ -298,6 +320,9 @@ static void *call_rcu_thread(void *arg)
>  		struct cds_wfcq_node *cbs, *cbs_tmp_n;
>  		enum cds_wfcq_ret splice_ret;
> 
> +		if (set_thread_cpu_affinity(crdp))
> +			urcu_die(errno);
> +
>  		if (uatomic_read(&crdp->flags) & URCU_CALL_RCU_PAUSE) {
>  			/*
>  			 * Pause requested. Become quiescent: remove
> @@ -391,6 +416,7 @@ static void call_rcu_data_init(struct call_rcu_data **crdpp,
>  	crdp->flags = flags;
>  	cds_list_add(&crdp->list, &call_rcu_data_list);
>  	crdp->cpu_affinity = cpu_affinity;
> +	crdp->gp_count = 0;
>  	cmm_smp_mb();  /* Structure initialized before pointer is planted. */
>  	*crdpp = crdp;
>  	ret = pthread_create(&crdp->tid, NULL, call_rcu_thread, crdp);
> -- 
> 2.1.4
>