Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ

From: Andy Lutomirski
Date: Wed Jul 30 2014 - 13:24:04 EST

On Wed, Jul 30, 2014 at 9:43 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote:
>> On 07/28, Andy Lutomirski wrote:
>> >
>> > @@ -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.

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