Re: [Patch] Revised locking for taskstats interface

[Jay Lan <jlan@engr.sgi.com>]

Shailabh Nagar wrote:
>Convert locking used within taskstats interface and delay accounting
>code to be more fine-grained.
>  

I ran tests without locking and with per-TG processing disabled.

The tests completed without a panic without the lock.
So, the protection we need is not on per-task delayacct, but when
thread group processing needs to access other tasks.
A lock is needed for TG, not for delayacct.

However, the tests data showed no impact on performance due
to  the lock itself though. I would not argue for a cleaner design
on this if 'locking only on TG' is difficult to implement.

Regards,
 - jay

>Dynamic allocation of the delays data structure led to exit race
>conditions that were being fixed through use of a global mutex at the
>taskstats interface level. The same mutex was also being used for
>protecting per-task delay structure allocation. Together, these were
>causing higher contention and unnecessary serialization.
>
>This patch switches to a per-task locking for protecting tsk->delays
>and eliminates global locking within the taskstats interface.
>
>Results collected by Jay Lan (jlan@sgi.com) from running a test
>with rapid forks/exits shows substantial improvement in system time.
>For a benchmark that only forks+exits 1000 threads and runs for
>5000 iterations, the reduction seen are as follows:
>
>	base	+patch   %improvement
>user	0.06	0.07	-16%		
>system	1.34	0.86	35%
>elapsed	622	470	24%
>
>
>Signed-off-by: Shailabh Nagar <nagar@watson.ibm.com>
>---
>
> include/linux/sched.h |    1 +
> kernel/delayacct.c    |   18 ++++++++++++++++--
> kernel/taskstats.c    |    7 -------
> 3 files changed, 17 insertions(+), 9 deletions(-)
>
>Index: linux-2.6.17/include/linux/sched.h
>===================================================================
>--- linux-2.6.17.orig/include/linux/sched.h	2006-06-22 02:56:44.000000000 -0400
>+++ linux-2.6.17/include/linux/sched.h	2006-06-22 15:18:09.000000000 -0400
>@@ -933,6 +933,7 @@ struct task_struct {
> 	 */
> 	struct pipe_inode_info *splice_pipe;
> #ifdef	CONFIG_TASK_DELAY_ACCT
>+	spinlock_t delays_lock;
> 	struct task_delay_info *delays;
> #endif
> };
>Index: linux-2.6.17/kernel/delayacct.c
>===================================================================
>--- linux-2.6.17.orig/kernel/delayacct.c	2006-06-22 02:56:44.000000000 -0400
>+++ linux-2.6.17/kernel/delayacct.c	2006-06-22 15:18:09.000000000 -0400
>@@ -41,6 +41,10 @@ void delayacct_init(void)
>
> void __delayacct_tsk_init(struct task_struct *tsk)
> {
>+	spin_lock_init(&tsk->delays_lock);
>+	/* No need to acquire tsk->delays_lock for allocation here unless
>+	   __delayacct_tsk_init called after tsk is attached to tasklist
>+	*/
> 	tsk->delays = kmem_cache_zalloc(delayacct_cache, SLAB_KERNEL);
> 	if (tsk->delays)
> 		spin_lock_init(&tsk->delays->lock);
>@@ -49,9 +53,9 @@ void __delayacct_tsk_init(struct task_st
> void __delayacct_tsk_exit(struct task_struct *tsk)
> {
> 	struct task_delay_info *delays = tsk->delays;
>-	mutex_lock(&taskstats_exit_mutex);
>+	spin_lock(&tsk->delays_lock);
> 	tsk->delays = NULL;
>-	mutex_unlock(&taskstats_exit_mutex);
>+	spin_unlock(&tsk->delays_lock);
> 	kmem_cache_free(delayacct_cache, delays);
> }
>
>@@ -114,6 +118,14 @@ int __delayacct_add_tsk(struct taskstats
> 	struct timespec ts;
> 	unsigned long t1,t2,t3;
>
>+	spin_lock(&tsk->delays_lock);
>+
>+	/* Though tsk->delays accessed later, early exit avoids
>+	 * unnecessary returning of other data
>+	 */
>+	if (!tsk->delays)
>+		goto done;
>+
> 	tmp = (s64)d->cpu_run_real_total;
> 	cputime_to_timespec(tsk->utime + tsk->stime, &ts);
> 	tmp += timespec_to_ns(&ts);
>@@ -148,6 +160,8 @@ int __delayacct_add_tsk(struct taskstats
> 	d->swapin_count += tsk->delays->swapin_count;
> 	spin_unlock(&tsk->delays->lock);
>
>+done:
>+	spin_unlock(&tsk->delays_lock);
> 	return 0;
> }
>
>Index: linux-2.6.17/kernel/taskstats.c
>===================================================================
>--- linux-2.6.17.orig/kernel/taskstats.c	2006-06-22 02:56:44.000000000 -0400
>+++ linux-2.6.17/kernel/taskstats.c	2006-06-22 15:27:09.000000000 -0400
>@@ -25,7 +25,6 @@
> static DEFINE_PER_CPU(__u32, taskstats_seqnum) = { 0 };
> static int family_registered = 0;
> kmem_cache_t *taskstats_cache;
>-DEFINE_MUTEX(taskstats_exit_mutex);
>
> static struct genl_family family = {
> 	.id		= GENL_ID_GENERATE,
>@@ -193,7 +192,6 @@ static int taskstats_send_stats(struct s
> 	if (rc < 0)
> 		return rc;
>
>-	mutex_lock(&taskstats_exit_mutex);
> 	if (info->attrs[TASKSTATS_CMD_ATTR_PID]) {
> 		u32 pid = nla_get_u32(info->attrs[TASKSTATS_CMD_ATTR_PID]);
> 		rc = fill_pid(pid, NULL, &stats);
>@@ -219,7 +217,6 @@ static int taskstats_send_stats(struct s
> 		goto err;
> 	}
>
>-	mutex_unlock(&taskstats_exit_mutex);
> 	nla_nest_end(rep_skb, na);
>
> 	return send_reply(rep_skb, info->snd_pid, TASKSTATS_MSG_UNICAST);
>@@ -228,7 +225,6 @@ nla_put_failure:
> 	return genlmsg_cancel(rep_skb, reply);
> err:
> 	nlmsg_free(rep_skb);
>-	mutex_unlock(&taskstats_exit_mutex);
> 	return rc;
> }
>
>@@ -246,8 +242,6 @@ void taskstats_exit_send(struct task_str
> 	if (!family_registered || !tidstats)
> 		return;
>
>-	mutex_lock(&taskstats_exit_mutex);
>-
> 	is_thread_group = !thread_group_empty(tsk);
> 	rc = 0;
>
>@@ -305,7 +299,6 @@ nla_put_failure:
> err_skb:
> 	nlmsg_free(rep_skb);
> ret:
>-	mutex_unlock(&taskstats_exit_mutex);
> 	return;
> }
>
>