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

[Mathieu Desnoyers]

----- On Jun 29, 2015, at 7:01 PM, Paul E. McKenney paulmck at linux.vnet.ibm.com wrote:

> 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!)

Your eyes are very good indeed. Fixing those 2 nits, and adding
your Acked-by.

Thanks!

Mathieu

> 
>							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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com