On Thu, Dec 13, 2018 at 11:01:49AM +0100, Peter Zijlstra wrote: > On Wed, Dec 12, 2018 at 08:26:39PM -0500, Steven Rostedt wrote: > > On Thu, 13 Dec 2018 03:39:38 +0300 > > "Dmitry V. Levin" <l...@altlinux.org> wrote: > > > > > btw, I didn't ask for the implementation to be ugly. > > > You don't have to introduce polling into the kernel if you don't want to, > > > userspace is perfectly capable of invoking wait4(2) in a loop. > > > Just block the tracee, notify the tracer, and let it pick up the pieces. > > > > Note, there's been some discussion offlist to only have perf set a flag > > when it dropped an event and have the ptrace code do the heavy lifting > > of blocking the task and waking it back up. I think that would be a > > cleaner solution and wont muck with perf as badly. > > It's still really horrid -- the question is not if we can come up with > something, anything, to make strace work. The question is if we can > extend something in a sane and maintainable manner to allow this. > > So there's a whole bunch of problems I see with all this, in no > particular order: > > - we cannot block when writing to the actual buffer, and have to unroll > the callstack and bolt on the blocking manualy in a few specific > sites. This is ugly, inconsistent and maintenance heavy. > > - it only works for some 'magic' events that got the treatment, but not > for many other you might expect it to work for with no real > indication which and why. > > - the wakeups side is icky; the best I can come up with is making the > data page R/O and single stepping on write fault, but that isn't > multi-threading safe. > > Another alternative would be keeping the whole page R/O and > using write(2) or an ioctl() to update the head pointer. > > Again, if we're going to do this; it needs to be done well and > consistent and not as a special hack to enable strace-like > functionality. And without clean and sane solutions to the above I just > don't see it happening. > > Note that the first 2 points are equally true for ftrace; so I don't see > how we could sanely add it there either. > > > One, very big maybe, would be to add a new tracepoint type that includes > a might_sleep() and we very carefully undo all the preempt_disable and > go sleep where we should. That also gives the tracepoint crud the > information it needs to publish the capability to userspace.
nice, I like this one.. seems like the most clean solution jirka