On 07/15/2015 09:27 AM, Ken Moffat wrote:
New title, I originally posted this last night but I've now made a little progress in identifying what changed. Previous thread was labelled for AMD Phenom, but it is more general. CC'ing Jeff because he replied to the original, I guess he probably won't be interested after this.Yesterday was the first day I had done any real compiling with 4.2-rc. I started out using -rc1, and make -j4 on a recent LFS/BLFS system (gcc-5.1.0, make-4.1, etc). I wanted to start trying to build kde5, and I expected a lot of issues. What I did not expect was that qt5 would build as if I was using -j1. Examination eventually identified that with 4.2-rc1 and 4.2-rc2, make ran the number of jobs I had specified, but the total of the CPU percentages ('top' from procps-ng-3.3.10) maxed out at 100%. On 4.1 kernels the percentage with -j4 maxes out at 400% (my machine has 4 cores). I suspected either an unfortunate choice in 'make oldconfig', or something specific to an AMD Phenom / gcc-5.1. Today I have tried make -j4 on two other machines with 4.2-rc kernels [ building the current git stable release ]. On my i3 SandyBridge everything was fine, CPU usage approached 400%. On my AMD A10-7850K I have the same problem as on the phenom. Not surprising, I began by using the Phenom config when I got the A10, then adapted it to suit, whereas the i3 has much less memory so I haven't made many changes since I got it. Comparing the configs for the i3 and the A10, the first thing which looked as if it might be relevant was the _CPU_ACCOUNTING choices. Those on the A10 seem to be driven from CONFIG_NO_HZ_FULL so I began by changing that to CONFIG_NO_HZ_IDLE. Payday ;-) make -j4 now approaches 400% CPU usage. The config differences follow. Perhaps it is actually one of the subsequent choices that is the problem. And I guess it could still be a gcc-5.1 issue.
Before going nuts bisecting, it could be worth running perf record -a -g -e cycles (or perhaps -e task-clock instead of -e cycles). It could also be worth manually sampling /proc/PID/stack a few times for a process that isn't making as much progress as it should.
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