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Date: Tue, 21 Nov 2006 12:03:17 -0500 From: Mathieu Desnoyers <compudj@...stal.dyndns.org> To: Sergei Shtylyov <sshtylyov@...mvista.com> Cc: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org, ltt-dev@...fik.org, mgreer@...sta.com, mlachwani@...sta.com Subject: Re: LTTng do_page_fault vs handle_mm_fault instrumentation * Sergei Shtylyov (sshtylyov@...mvista.com) wrote: > Hello. > > Mathieu Desnoyers wrote: > > >I would like to discuss your suggestion of moving the do_page_fault > >instrumentation to handle_mm_fault. On one side, it helps removing > >architecture > >dependant instrumentation, but on the other hand : > > >1- We cannot access the struct pt_regs in all cases (there may be an > >invalid > > current task struct). > >2- We cannot distinguish between calls to handle_mm_fault from the page > >fault > > handler or from get_user_pages. > >3- Some people complain about not having enough information about the > >cause of > > the page fault (see the forward below). > > > >So instead of staying between my users who ask for those feature and kernel > >developers who wish to reduce the intrusiveness of instrumentation (which > >is a > >nice goal : moving the syscall entry/exit instrumentation do > >do_syscall_trace > >has helped simplifying the instrumentation), I prefer to open the > >discussion > >about it. > > It seems I've missed the whole story behind this move. > For me, it was more a question of consistency: if we're trying to trace > all trap handlers, why not page fault one? So, I just wanted my old LTT > tracepoints back. :-) > This topic brings the question about how near must be the instrumentation from the hardware events. We have to take into account that the tracer uses the hardware to trace : mainly, it writes an event into vmalloc'd buffer, which will generate vmalloc faults on some architectures. Therefore, reentrancy with the "hardware" events becomes part of the problem. Because of this, we can't simply instrument the page fault handler at the beginning and end, as it would end up doing infinite recursive calls. I see two trends : on one side, the developers and people interested into measuring performance want instrumentation as close to the hardware events as possible. Some wants to know the exact execution flow, including the error conditions in the trap handlers. People doing performance measurements wants to account correctly the amount of time spent in those handlers. The other side is the kernel developer who does not want to clutter the code with too much instrumentation. Instrumentation around the handle_mm_fault handler call, inside do_page_fault, looked to me as a good compromise : it can access the struct pt_regs, it will never be called from either a vmalloc fault or an erroneous page fault caused by the tracer itself (which of course, never happens, but who knows...). It won't, however, give information about some error paths in the page fault handler, mainly related to kernel faults. It is also a little farther from the page fault handler "real" entry and exit points, but I consider it a minor impact compared to the cost of entering the trap on currently existing architectures. Note that I plan to create a "calibration" module someday so we could know how much time to add to the duration of the page faults, knowing the time required to enter in and return from the fault. Mathieu > >Ideas/comments are welcome. > > >Regards, > > >Mathieu > > WBR, Sergei > OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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