On Tue, 15 Jul 2014 11:07:35 +0200 Petr Mládek <[email protected]> wrote:
> Just to be sure. Should I remove RING_BUFFER_ALLOW_SWAP config > option completely? No. Just that one #if block. The per cpu swap is still a bit icky in my humble opinion, and should stay a config for now. > > > Although your above comment is wrong. We can most > > definitely be in the middle of a swap operation. You forget that > > tracing is NMI safe. > > > > But the swap does disable tracing, then if a NMI were to preempt the > > swap, it will either be outside the critical part where it is still safe > > to perform the trace or it will be where the swap is happening and the > > tracing will be disabled. > > I probably did not use precise enough sentence. I wanted to explain > why we do not check for "cpu_buffer->record_disabled" here. I added > this test in the 1st version of the patch but then I realized that it > was not needed. > > As you say, there are two possibilities when swap is interrupted by > NMI. Either it is outside of the critical section and then the write is > completed before we continue with swapping. Or the interrupt is inside > the critical section and then the nested write fails early in > ring_buffer_write() or ring_buffer_lock_reserve() before this > code is called. > > A better comment might be: > > * Note that we will never be in the middle of a swap critical > * section here. If the swapping is interrupted when recording > * is disabled, the nested write will fail earlier in > * ring_buffer_write() or ring_buffer_lock_reserve(). > */ > > > Note that I am still learning about the preferred practice with comments > inside > the kernel code. I have spent a lot of time trying to understand the > ring buffer. I think that more comments like this would have > helped me to get the picture faster. Another possibility would be to > create Documentation/trace/ring-buffer.txt. Well there is a Documentation/trace/ring-buffer-design.txt doc ;-) > > I have more notes here. Let me know if I should cook up some patches > that would extend the ring buffer documentation according to my > experience. I have no problems with adding documentation to the code. -- Steve -- 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/
