On Wed, May 28, 2014 at 04:55:39PM +0100, Kevin Hilman wrote: > Hi Will,
Hey Kevin, > Will Deacon <[email protected]> writes: > > Apologies if we've discussed this before (it rings a bell), but why are we > > penalising the fast syscall path with this? Shouldn't TIF_NOHZ contribute to > > out _TIF_WORK_MASK, then we could do the tracking on the syscall slow path? > > I'll answer here since Larry inherited this design decision from me. > > I considered (and even implemented) forcing the slow syscall path > based on TIF_NOHZ but decided (perhaps wrongly) not to. I guess the > choice is between: > > - forcing the overhead of syscall tracing path on all > TIF_NOHZ processes > > - forcing the (much smaller) ct_user_exit overhead on all syscalls, > (including the fast syscall path) > > I had decided that the former was better, but as I write this, I'm > thinking that the NOHZ tasks should probably eat the extra overhead > since we expect their interactions with the kernel to be minimal anyways > (part of the goal of full NOHZ.) > > Ultimately, I'm OK with either way and have the other version ready. I was just going by the comment in kernel/context_tracking.c: * The context tracking uses the syscall slow path to implement its user-kernel * boundaries probes on syscalls. This way it doesn't impact the syscall fast * path on CPUs that don't do context tracking. which doesn't match what the current patch does. It also makes it sounds like context tracking is really a per-CPU thing, but I've never knowingly used it before. I think putting this on the slowpath is inline with the expectations in the core code. > > I think that would tidy up your mov into x19 too. > > That's correct. If we force the syscall_trace path, the ct_user_enter > wouldn't have to do any context save/restore. That would be nice. > > Also -- how do you track ret_from_fork in the child with these patches? > > Not sure I follow the question, but ret_from_fork calls > ret_to_user, which calls kernel_exit, which calls ct_user_enter. Sorry, I got myself in a muddle. I noticed that x19 is live in ret_from_fork so made a mental note to check that is ok (I think it is) but then concluded incorrectly that you don't trace there. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
