On 8/15/19 9:38 AM, Dmitry Safonov wrote:
As it has been discussed on timens RFC, adding a new conditional branch `if (inside_time_ns)` on VDSO for all processes is undesirable. It will add a penalty for everybody as branch predictor may mispredict the jump. Also there are instruction cache lines wasted on cmp/jmp. Those effects of introducing time namespace are very much unwanted having in mind how much work have been spent on micro-optimisation vdso code. The propose is to allocate a second vdso code with dynamically patched out (disabled by static_branch) timens code on boot time. Allocate another vdso and copy original code.
I'm unconvinced that any of this magic is wise. I think you should make a special timens vvar page that causes the normal fastpath to fail (using a special vclock mode, a special seq value, or a special "last" value) and then make the failure path detect that timens is in use and use the timens path.
--Andy
