On Wed, Jul 30, 2014 at 9:43 AM, Frederic Weisbecker [off-list ref] wrote:

On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote:

quoted

On 07/28, Andy Lutomirski wrote:

quoted

@@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs)
 {
    long ret = 0;

-   user_exit();
+   /*
+    * If TIF_NOHZ is set, we are required to call user_exit() before
+    * doing anything that could touch RCU.
+    */
+   if (test_thread_flag(TIF_NOHZ))
+           user_exit();

Personally I still think this change just adds more confusion, but I leave
this to you and Frederic.

It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we
need to call user_exit() just because we enter the kernel. TIF_NOHZ is just
the implementation detail which triggers this slow path.

At least it should be correct, unless I am confused even more than I think.

Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests
this is only about tracing? syscall_slowpath_enter() could be an alternative.
But that's still tracing in a general sense so...

At the end of the day, the syscall slowpath code calls a bunch of
functions depending on what TIF_XYZ flags are set.  As long as it's
structured like "if (TIF_A) do_a(); if (TIF_B) do_b();" or something
like that, it's comprehensible.  But once random functions with no
explicit flag checks or comments start showing up, it gets confusing.

If it's indeed all-or-nothing, I could remove the check and add a
comment.  But please keep in mind that, currently, the slow path is
*slow*, and my patches only improve the entry case.  So enabling
context tracking on every task will hurt.

--Andy