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PEBS always had the capability to log samples to its buffers without an interrupt. Traditionally perf has not used this but always set the PEBS threshold to one. For frequently occurring events (like cycles or branches or load/store) this in term requires using a relatively high sampling period to avoid overloading the system, by only processing PMIs. This in term increases sampling error. For the common cases we still need to use the PMI because the PEBS hardware has various limitations. The biggest one is that it can not supply a callgraph. It also requires setting a fixed period, as the hardware does not support adaptive period. Another issue is that it cannot supply a time stamp and some other options. To supply a TID it requires flushing on context switch. It can however supply the IP, the load/store address, TSX information, registers, and some other things. So we can make PEBS work for some specific cases, basically as long as you can do without a callgraph and can set the period you can use this new PEBS mode. The main benefit is the ability to support much lower sampling period (down to -c 1000) without extensive overhead. One use cases is for example to increase the resolution of the c2c tool. Another is double checking when you suspect the standard sampling has too much sampling error. Some numbers on the overhead, using cycle soak, comparing the elapsed time from "kernbench -M -H" between plain (threshold set to one) and multi (large threshold). The test command for plain: "perf record --time -e cycles:p -c $period -- kernbench -M -H" The test command for multi: "perf record --no-time -e cycles:p -c $period -- kernbench -M -H" ( The only difference of test command between multi and plain is time stamp options. Since time stamp is not supported by large PEBS threshold, it can be used as a flag to indicate if large threshold is enabled during the test. ) period plain(Sec) multi(Sec) Delta 10003 32.7 16.5 16.2 20003 30.2 16.2 14.0 40003 18.6 14.1 4.5 80003 16.8 14.6 2.2 100003 16.9 14.1 2.8 800003 15.4 15.7 -0.3 1000003 15.3 15.2 0.2 2000003 15.3 15.1 0.1 With periods below 100003, plain (threshold one) cause much more overhead. With 10003 sampling period, the Elapsed Time for multi is even 2X faster than plain. Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com> Signed-off-by: Kan Liang <kan.liang@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: acme@infradead.org Cc: eranian@google.com Link: http://lkml.kernel.org/r/1430940834-8964-5-git-send-email-kan.liang@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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