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SubjectRe: [tip:sched/urgent] sched: sched_getaffinity(): Allow less than NR_CPUS length
On Mon, Mar 15, 2010 at 07:43:02AM +0000, tip-bot for KOSAKI Motohiro wrote:
> Commit-ID: cd3d8031eb4311e516329aee03c79a08333141f1
> Gitweb: http://git.kernel.org/tip/cd3d8031eb4311e516329aee03c79a08333141f1
> Author: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> AuthorDate: Fri, 12 Mar 2010 16:15:36 +0900
> Committer: Ingo Molnar <mingo@elte.hu>
> CommitDate: Mon, 15 Mar 2010 08:28:44 +0100
...
> IOW we hope they are not annoyed by this issue ...

The change looks ok but I can't reproduce the problem.

I'm running on a distro kernel that has NR_CPUS=4096. Glibc has also has a
definition of __CPU_SETSIZE (I assume this change was made by the distro but
am not certain):

sched.c
...
#if defined _SCHED_H && !defined __cpu_set_t_defined
# define __cpu_set_t_defined
/* Size definition for CPU sets. */
# define __CPU_SETSIZE 4096
# define __NCPUBITS (8 * sizeof (__cpu_mask))
Your test program runs ok:

% strace t
...
sched_getaffinity(0, 512, { ffff, 0, 0, 0, 0, 0, 0, 0 }) = 64


Also note that we've run on IA64 systems with NR_CPUS=4096 for several years w/o
hitting any problems.

Bottom line. I don't think the change will affect us.


>
> sched: sched_getaffinity(): Allow less than NR_CPUS length
>
> [ Note, this commit changes the syscall ABI for > 1024 CPUs systems. ]
>
> Recently, some distro decided to use NR_CPUS=4096 for mysterious reasons.
> Unfortunately, glibc sched interface has the following definition:
>
> # define __CPU_SETSIZE 1024
> # define __NCPUBITS (8 * sizeof (__cpu_mask))
> typedef unsigned long int __cpu_mask;
> typedef struct
> {
> __cpu_mask __bits[__CPU_SETSIZE / __NCPUBITS];
> } cpu_set_t;
>
> It mean, if NR_CPUS is bigger than 1024, cpu_set_t makes an
> ABI issue ...
>
> More recently, Sharyathi Nagesh reported following test program makes
> misterious syscall failure:
>
> -----------------------------------------------------------------------
> #define _GNU_SOURCE
> #include<stdio.h>
> #include<errno.h>
> #include<sched.h>
>
> int main()
> {
> cpu_set_t set;
> if (sched_getaffinity(0, sizeof(cpu_set_t), &set) < 0)
> printf("\n Call is failing with:%d", errno);
> }
> -----------------------------------------------------------------------
>
> Because the kernel assumes len argument of sched_getaffinity() is bigger
> than NR_CPUS. But now it is not correct.
>
> Now we are faced with the following annoying dilemma, due to
> the limitations of the glibc interface built in years ago:
>
> (1) if we change glibc's __CPU_SETSIZE definition, we lost
> binary compatibility of _all_ application.
>
> (2) if we don't change it, we also lost binary compatibility of
> Sharyathi's use case.
>
> Then, I would propse to change the rule of the len argument of
> sched_getaffinity().
>
> Old:
> len should be bigger than NR_CPUS
> New:
> len should be bigger than maximum possible cpu id
>
> This creates the following behavior:
>
> (A) In the real 4096 cpus machine, the above test program still
> return -EINVAL.
>
> (B) NR_CPUS=4096 but the machine have less than 1024 cpus (almost
> all machines in the world), the above can run successfully.
>
> Fortunatelly, BIG SGI machine is mainly used for HPC use case. It means
> they can rebuild their programs.
>
> IOW we hope they are not annoyed by this issue ...
>
> Reported-by: Sharyathi Nagesh <sharyath@in.ibm.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Acked-by: Ulrich Drepper <drepper@redhat.com>
> Acked-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Jack Steiner <steiner@sgi.com>
> Cc: Russ Anderson <rja@sgi.com>
> Cc: Mike Travis <travis@sgi.com>
> LKML-Reference: <20100312161316.9520.A69D9226@jp.fujitsu.com>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
> kernel/sched.c | 10 +++++++---
> 1 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9ab3cd7..6eaef3d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4902,7 +4902,9 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
> int ret;
> cpumask_var_t mask;
>
> - if (len < cpumask_size())
> + if (len < nr_cpu_ids)
> + return -EINVAL;
> + if (len & (sizeof(unsigned long)-1))
> return -EINVAL;
>
> if (!alloc_cpumask_var(&mask, GFP_KERNEL))
> @@ -4910,10 +4912,12 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
>
> ret = sched_getaffinity(pid, mask);
> if (ret == 0) {
> - if (copy_to_user(user_mask_ptr, mask, cpumask_size()))
> + int retlen = min(len, cpumask_size());
> +
> + if (copy_to_user(user_mask_ptr, mask, retlen))
> ret = -EFAULT;
> else
> - ret = cpumask_size();
> + ret = retlen;
> }
> free_cpumask_var(mask);
>


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