linux-kernel - [PATCH 2/3] kmod: Add up-to-date explanations on the purpose of each asynchronous levels

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Message-Id: <1436196821-13962-3-git-send-email-fweisbec@gmail.com>
Date:	Mon,  6 Jul 2015 17:33:40 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 2/3] kmod: Add up-to-date explanations on the purpose of each asynchronous levels

There seem to be quite some confusions on the comments, likely due to
changes that came after them.

Now since it's very non obvious why we have 3 levels of asynchronous
code to implement usermodehelpers, it's important to comment in detail
the reason of this layout.

Cc: Rik van Riel <riel@...hat.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christoph Lameter <cl@...ux.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 kernel/kmod.c | 32 ++++++++++++++++++++++++++++----
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 4682e91..f940b21 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -269,7 +269,11 @@ out:
 	do_exit(0);
 }
 
-/* Keventd can't block, but this (a child) can. */
+/*
+ * We couldn't wait for usermodehelper completion from khelper without
+ * blocking other pending concurrent usermodehelper targets. This is why
+ * the UMH_WAIT_PROC flavour runs in its own thread.
+ */
 static int call_usermodehelper_exec_sync(void *data)
 {
 	struct subprocess_info *sub_info = data;
@@ -285,8 +289,8 @@ static int call_usermodehelper_exec_sync(void *data)
 		/*
 		 * Normally it is bogus to call wait4() from in-kernel because
 		 * wait4() wants to write the exit code to a userspace address.
-		 * But call_usermodehelper_exec_sync() always runs as keventd,
-		 * and put_user() to a kernel address works OK for kernel
+		 * But call_usermodehelper_exec_sync() always runs as kernel
+		 * thread and put_user() to a kernel address works OK for kernel
 		 * threads, due to their having an mm_segment_t which spans the
 		 * entire address space.
 		 *
@@ -307,7 +311,15 @@ static int call_usermodehelper_exec_sync(void *data)
 	do_exit(0);
 }
 
-/* This is run by khelper thread  */
+/*
+ * This function doesn't need to be called asynchronously. But we need to create
+ * the usermodehelper kernel threads from a task that is affine to all CPUs
+ * (or nohz housekeeping ones) such that they inherit a global affinity. Khelper
+ * workqueue simply provides that.
+ * call_usermodehelper() can be called from tasks with a reduced CPU
+ * affinity (eg: per-cpu workqueues) and we don't want usermodehelper targets to
+ * contend any busy CPU.
+ */
 static void call_usermodehelper_exec_work(struct work_struct *work)
 {
 	struct subprocess_info *sub_info =
@@ -693,6 +705,18 @@ struct ctl_table usermodehelper_table[] = {
 
 void __init usermodehelper_init(void)
 {
+	/*
+	 * The singlethread property here stands for the need of a workqueue
+	 * with wide CPUs affinity, in order to create usermodehelper kernel
+	 * threads inheriting this attribute irrespective of
+	 * call_usermodehelper() callers. Non-singlethread workqueues are
+	 * otherwise per-cpu and wouldn't produce the desired effect.
+	 *
+	 * The ordering guarantee as a side-effect isn't necessary but shouldn't
+	 * introduce performance issue. All we do is creating two kernel threads.
+	 * This should be fast enough not to block concurrent usermodehelper
+	 * callers.
+	 */
 	khelper_wq = create_singlethread_workqueue("khelper");
 	BUG_ON(!khelper_wq);
 }
-- 
2.1.4

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