Avi Kivity wrote: > Jan Kiszka wrote: >> Avi Kivity wrote: >> >>> Liu, Eric E wrote: >>> >>>> Hi, >>>> The following patches port xentrace to kvm which is useful for >>>> performance tuning and debugging. >>>> >>>> It is designed to allow debugging traces of kvm to be generated on >>>> Up/Smp machines. Each trace entry is outputted in a trace ring buffer >>>> for per cpu which is mapped to userspace, and the userspace tools can >>>> analyze the data according to some formats definitions. >>>> >>>> Since we already have had debugfs_entries and some other kernel debug >>>> mechanism to use, does this kvmtrace make sense? Any comment is >>>> welcomed. >>> This looks very useful. While kvm_stat provides good data, this is >>> much more in depth. >>> >>> The kernel already contains a method of transferring percpu data to >>> userspace; see Documentation/filesystems/relay.txt. It supports >>> mmap() and read(). Please see if it is a good fit. >>> >> >> I would also suggest to use the new kernel standard for instrumentation: >> trace_mark(). >> >> This would also allow to reuse the trace points with other tracer and >> maybe even obsolete a separate transportation channel, e.g. when LTTng >> is once :-/ merged. >> > > Can one have markers automatically recorded? Or do you need to connect > the marker with a logging function?
Yes, of course you need a probe function. Either your own one, or one provided by other tracers. > > If the latter, it's too difficult to use. I don't think it is. Check linux/samples/markers/probe-example.c. BTW, RCU-Trace (not yet mainlined) make use of markers as well, see -rt. And in contrast to the suggested implementation, markers have less impact on the code if built-in but disabled. So there would be no reasons for not shipping kvm production binaries that are prepared to be traced. But if you prefer the inlined version, I bet you'll have to hide it from reviewers at LKML. ;) Jan
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