Messages in this thread | | | Date | Fri, 18 Aug 2023 14:47:21 -0400 | Subject | Re: [PATCH v3] sched/core: Use empty mask to reset cpumasks in sched_setaffinity() | From | Waiman Long <> |
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On 8/3/23 22:32, Waiman Long wrote: > Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested > cpumask"), user provided CPU affinity via sched_setaffinity(2) is > perserved even if the task is being moved to a different cpuset. However, > that affinity is also being inherited by any subsequently created child > processes which may not want or be aware of that affinity. > > One way to solve this problem is to provide a way to back off from > that user provided CPU affinity. This patch implements such a scheme > by using an empty cpumask to signal a reset of the cpumasks to the > default as allowed by the current cpuset. > > Before this patch, passing in an empty cpumask to sched_setaffinity(2) > will always return an -EINVAL error. With this patch, an alternative > error of -ENODEV will be returned returned if sched_setaffinity(2) > has been called before to set up user_cpus_ptr. In this case, the > user_cpus_ptr that stores the user provided affinity will be cleared and > the task's CPU affinity will be reset to that of the current cpuset. This > alternative error code of -ENODEV signals that the no CPU is specified > and, at the same time, a side effect of resetting cpu affinity to the > cpuset default. > > If sched_setaffinity(2) has not been called previously, an EINVAL error > will be returned with an empty cpumask just like before. Tests or > tools that rely on the behavior that an empty cpumask will return an > error code will not be affected. > > We will have to update the sched_setaffinity(2) manpage to document > this possible side effect of passing in an empty cpumask. > > Signed-off-by: Waiman Long <longman@redhat.com>
Ping.
Are there other concerns about this patch? I haven't seen any error report from kernel test robot so far.
Cheers, Longman
> --- > kernel/sched/core.c | 42 +++++++++++++++++++++++++++++++++--------- > 1 file changed, 33 insertions(+), 9 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index c52c2eba7c73..3ef7397f2a61 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -8317,7 +8317,12 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx) > } > > cpuset_cpus_allowed(p, cpus_allowed); > - cpumask_and(new_mask, ctx->new_mask, cpus_allowed); > + > + /* Default to cpus_allowed with NULL new_mask */ > + if (ctx->new_mask) > + cpumask_and(new_mask, ctx->new_mask, cpus_allowed); > + else > + cpumask_copy(new_mask, cpus_allowed); > > ctx->new_mask = new_mask; > ctx->flags |= SCA_CHECK; > @@ -8366,6 +8371,7 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx) > > long sched_setaffinity(pid_t pid, const struct cpumask *in_mask) > { > + bool reset_cpumasks = cpumask_empty(in_mask); > struct affinity_context ac; > struct cpumask *user_mask; > struct task_struct *p; > @@ -8403,15 +8409,26 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask) > goto out_put_task; > > /* > - * With non-SMP configs, user_cpus_ptr/user_mask isn't used and > - * alloc_user_cpus_ptr() returns NULL. > + * If an empty cpumask is passed in and user_cpus_ptr is set, > + * clear user_cpus_ptr and reset the current cpu affinity to the > + * default for the current cpuset. If user_cpus_ptr isn't set, > + * -EINVAL will be returned as before. > */ > - user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE); > - if (user_mask) { > - cpumask_copy(user_mask, in_mask); > - } else if (IS_ENABLED(CONFIG_SMP)) { > - retval = -ENOMEM; > - goto out_put_task; > + if (reset_cpumasks && p->user_cpus_ptr) { > + in_mask = NULL; /* To be updated in __sched_setaffinity */ > + user_mask = NULL; > + } else { > + /* > + * With non-SMP configs, user_cpus_ptr/user_mask isn't used > + * and alloc_user_cpus_ptr() returns NULL. > + */ > + user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE); > + if (user_mask) { > + cpumask_copy(user_mask, in_mask); > + } else if (IS_ENABLED(CONFIG_SMP)) { > + retval = -ENOMEM; > + goto out_put_task; > + } > } > > ac = (struct affinity_context){ > @@ -8423,6 +8440,13 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask) > retval = __sched_setaffinity(p, &ac); > kfree(ac.user_mask); > > + /* > + * Force an error return (-ENODEV), if no error yet, for the empty > + * cpumask case to avoid breaking existing tests. > + */ > + if (reset_cpumasks && !retval) > + retval = -ENODEV; > + > out_put_task: > put_task_struct(p); > return retval;
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