[RFC, PATCH] CLONE_NEWIPC and exit_group()

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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Hi,

Patch to move kern_unmount() out of exit_group() code path is below.
Dmitry, could you check if it's beneficial for your use-case?

Results are not that impressive. Microbenchmark:

#define _GNU_SOURCE
#include <unistd.h>
#include <sched.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
        int i;

        for (i = 0; i < 1000; i++) {
                if (fork())
                        continue;

                unshare(CLONE_NEWIPC);
                exit(0);
        }

        while (wait(NULL) > 0)
                ;

        return 0;
}

Before:

 Performance counter stats for './test' (10 runs):

       2645.849247 task-clock                #    3.203 CPUs utilized            ( +-  3.43% )
             2,375 context-switches          #    0.001 M/sec                    ( +-  0.35% )
             1,579 CPU-migrations            #    0.001 M/sec                    ( +-  0.90% )
            37,516 page-faults               #    0.014 M/sec                    ( +-  0.44% )
     5,739,887,800 cycles                    #    2.169 GHz                      ( +-  3.50% ) [84.21%]
     5,126,092,712 stalled-cycles-frontend   #   89.31% frontend cycles idle     ( +-  3.78% ) [84.47%]
     3,779,607,146 stalled-cycles-backend    #   65.85% backend  cycles idle     ( +-  4.06% ) [68.26%]
     1,210,768,660 instructions              #    0.21  insns per cycle
                                             #    4.23  stalled cycles per insn  ( +-  1.01% ) [86.28%]
       213,318,802 branches                  #   80.624 M/sec                    ( +-  1.16% ) [84.49%]
         2,417,038 branch-misses             #    1.13% of all branches          ( +-  0.70% ) [84.55%]

       0.826165497 seconds time elapsed                                          ( +-  1.26% )

After:

 Performance counter stats for './test' (10 runs):

       4248.846649 task-clock                #    6.370 CPUs utilized            ( +- 13.50% )
             2,343 context-switches          #    0.001 M/sec                    ( +-  1.51% )
             1,624 CPU-migrations            #    0.000 M/sec                    ( +-  2.53% )
            37,416 page-faults               #    0.009 M/sec                    ( +-  0.41% )
     9,314,096,247 cycles                    #    2.192 GHz                      ( +- 13.64% ) [83.75%]
     8,482,679,429 stalled-cycles-frontend   #   91.07% frontend cycles idle     ( +- 14.46% ) [83.79%]
     5,807,497,239 stalled-cycles-backend    #   62.35% backend  cycles idle     ( +- 14.79% ) [67.65%]
     1,556,594,531 instructions              #    0.17  insns per cycle
                                             #    5.45  stalled cycles per insn  ( +-  5.41% ) [85.00%]
       282,682,358 branches                  #   66.532 M/sec                    ( +-  5.56% ) [84.32%]
         2,610,583 branch-misses             #    0.92% of all branches          ( +-  4.42% ) [83.90%]

       0.667023551 seconds time elapsed                                          ( +- 12.10% )

Any thoughts if it makes sense?

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 5499c92..1a4cfd8 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -67,6 +67,8 @@ struct ipc_namespace {
 
 	/* user_ns which owns the ipc ns */
 	struct user_namespace *user_ns;
+
+	struct work_struct free_ns_work;
 };
 
 extern struct ipc_namespace init_ipc_ns;
diff --git a/ipc/namespace.c b/ipc/namespace.c
index f362298c..edbf885 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -16,6 +16,8 @@
 
 #include "util.h"
 
+static void free_ns(struct work_struct *work);
+
 static struct ipc_namespace *create_ipc_ns(struct task_struct *tsk,
 					   struct ipc_namespace *old_ns)
 {
@@ -27,6 +29,7 @@ static struct ipc_namespace *create_ipc_ns(struct task_struct *tsk,
 		return ERR_PTR(-ENOMEM);
 
 	atomic_set(&ns->count, 1);
+	INIT_WORK(&ns->free_ns_work, free_ns);
 	err = mq_init_ns(ns);
 	if (err) {
 		kfree(ns);
@@ -116,6 +119,15 @@ static void free_ipc_ns(struct ipc_namespace *ns)
 	kfree(ns);
 }
 
+static void free_ns(struct work_struct *work)
+{
+	struct ipc_namespace *ns = container_of(work, struct ipc_namespace,
+			free_ns_work);
+
+	mq_put_mnt(ns);
+	free_ipc_ns(ns);
+}
+
 /*
  * put_ipc_ns - drop a reference to an ipc namespace.
  * @ns: the namespace to put
@@ -137,8 +149,7 @@ void put_ipc_ns(struct ipc_namespace *ns)
 	if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
 		mq_clear_sbinfo(ns);
 		spin_unlock(&mq_lock);
-		mq_put_mnt(ns);
-		free_ipc_ns(ns);
+		schedule_work(&ns->free_ns_work);
 	}
 }
 
-- 
 Kirill A. Shutemov

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