* Alexey Dobriyan: >> >> Patch overloads sched_getaffinity(len=0) to simply return "nr_cpu_ids". >> >> This will make gettting CPU mask require at most 2 system calls >> >> and will eliminate unnecessary code. >> >> >> >> len=0 is chosen so that >> >> * passing zeroes is the simplest thing >> >> >> >> syscall(__NR_sched_getaffinity, 0, 0, NULL) >> >> >> >> will simply do the right thing, >> >> >> >> * old kernels returned -EINVAL unconditionally. >> >> >> >> Note: glibc segfaults upon exiting from system call because it tries to >> >> clear the rest of the buffer if return value is positive, so >> >> applications will have to use syscall(3). >> >> Good news is that it proves noone uses sched_getaffinity(pid, 0, NULL). >> >> Given that old kernels fail with EINVAL, that evidence is fairly >> restricted. >> >> I'm not sure if it's a good idea to overload this interface. I expect >> that users will want to call sched_getaffinity (the system call wrapper) >> with cpusetsize == 0 to query the value, so there will be pressure on >> glibc to remove the memset. At that point we have an API that obscurely >> fails with old glibc versions, but suceeds with newer ones, which isn't >> great. > > I can do "if (len == 536870912)" so that bit count overflows on old > kernels into EINVAL and is unlikely to be used ever. I don't see how this solves this particular issue. It will still result in a mysterious crash if programs use an updated system call wrapper. Thanks, Florian
