Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs
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From: Peter Zijlstra <peterz@infradead.org>
Date: 2019-04-04 08:42:55
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On Wed, Apr 03, 2019 at 11:08:09PM +0300, Alexey Dobriyan wrote:
Currently there is no easy way to get the number of CPUs on the system.
And this patch doesn't change that :-) Still, it does the right thing and I like it. The point is that nr_cpu_ids is the length of the bitmap, but does not contain information on how many CPUs are in the system. Consider the case where the bitmap is sparse.
Applications are divided into 2 groups: One group allocates buffer and call sched_getaffinity(2) once. It works but either underallocate or overallocates and in the future such application will become buggy as Linux will start working on even more SMP-ier systems. Glibc in particular shipped with 1024 CPUs support maximum at some point which is quite surprising as glibc maitainers should know better. Another group dynamically grow buffer until cpumask fits. This is inefficient as multiple system calls are done. Nobody seems to parse "/sys/devices/system/cpu/possible". Even if someone does, parsing sysfs is much slower than necessary.
True; but I suppose glibc already does lots of that anyway, right? It does contain the right information.
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).
This also needs a manpage update. And I'm missing the libc people on Cc.
quoted hunk
Signed-off-by: Alexey Dobriyan <redacted> --- kernel/sched/core.c | 3 +++ 1 file changed, 3 insertions(+)--- a/kernel/sched/core.c +++ b/kernel/sched/core.c@@ -4942,6 +4942,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len, int ret; cpumask_var_t mask; + if (len == 0) + return nr_cpu_ids; + if ((len * BITS_PER_BYTE) < nr_cpu_ids) return -EINVAL; if (len & (sizeof(unsigned long)-1))