On Thu, Jul 31, 2014 at 06:42:46PM +0200, Oleg Nesterov wrote: > On 07/31, Frederic Weisbecker wrote: > > > > On Wed, Jul 30, 2014 at 10:23:34AM -0700, Andy Lutomirski wrote: > > > > > > 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! ;-) > > > > 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.
