On Thu, Jul 31, 2014 at 06:54:23PM +0200, Oleg Nesterov wrote:

On 07/31, Frederic Weisbecker wrote:

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On Thu, Jul 31, 2014 at 06:42:46PM +0200, Oleg Nesterov wrote:

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On 07/31, Frederic Weisbecker wrote:

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On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote:

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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.

Yeah that's a point. I don't mind much the TIF_NOHZ test if you like.

And in my opinion

	if (work & TIF_XYZ)
		user_exit();

looks even more confusing. Because, once again, TIF_XYZ is not the
reason to call user_exit().

Not to mention this adds a minor performance penalty.

That's a point too! You guys both convinced me! ;-)

Very nice, now I know that you can agree with 2 opposite opinions at
the same time ;)

That's what an Acked-by from Schroedinger would look like!

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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.

That's what we do anyway. I haven't found a safe way to enabled context tracking
without tracking all CPUs.

And if we change this, then the code above becomes racy. The state of
TIF_XYZ can be changed right after the check. OK, it is racy anyway ;)
but still this adds more confusion.

No because all running tasks have this flag set when context tracking is
enabled. And context tracking can't be disabled on runtime.

Yes, yes, please note that I said "if we change this".

Yeah but the NO_HZ test wouldn't change much the situation I think.