* H. Peter Anvin (hpa@xxxxxxxxx) wrote: > Mathieu Desnoyers wrote: >> The point is to provide a way to dynamically enable code at runtime >> without noticeable performance impact on the system. It's principally >> useful to control the markers in the kernel, which can be placed in very >> frequently executed code paths. The original markers add a memory read, >> test and conditional branch at each marker site. By using the immediate >> values patchset, it goes down to a load immediate value, test and branch. >> However, Ingo was still unhappy with the conditional branch, so I cooked >> this jump patching optimization on top of the immediate values. It >> looks for an expected pattern which limits the liveliness of the %al and >> ZF registers to the 3 instructions and, if it finds it, patches a jump >> located just before the mov instruction to skip the whole pattern and >> behave exactly like the conditional branch. >> So basically we get code dynamically actvated by patching a single jump. > > Note that all these optimizations only make sense if the case where we > *take* the "marker" is frequent, *and* the marker itself is not too > expensive. > Yes, this is the case. Using breakpoints for markers quickly becomes noticeable for thing such as scheduler instrumentation, page fault handler instrumentation, etc. And yes, I have developed kernel tracer, LTTng, which takes care of writing the data to trace buffers efficiently. The last time I took performance measurements, it was performing locking and writing to the memory buffer in about 270ns on a 3GHz Pentium 4. It might be a tiny bit slower now that it parses the markers format strings dynamically, but nothing very significant. But there is another point that markers do which the breakpoint won't give you : they extract local variables from functions and they identify them with field names which separates the instrumentation from the actual kernel implementation details. In order to do that, I rely on gcc building a stack frame for a function call, which I don't want to build unnecessarity when the marker is disabled. This is why I use a jump to skip passing the arguments on the stack and the function call. Mathieu > If that is not the case, just put in a noop that is dynamically patched to > an INT3 or ICEBP instruction (one byte) or an INT instruction (two bytes), > take the exception, look up the source address and revector to the marker > code. > > -hpa -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
