Hello,
On Fri, Oct 13, 2023 at 02:11:19PM -0400, Waiman Long wrote:
When the "isolcpus" boot command line option is used to add a set of isolated CPUs, those CPUs will be excluded automatically from wq_unbound_cpumask to avoid running work functions from unbound workqueues.
Recently cpuset has been extended to allow the creation of partitions of isolated CPUs dynamically. To make it closer to the "isolcpus" in functionality, the CPUs in those isolated cpuset partitions should be excluded from wq_unbound_cpumask as well. This can be done currently by explicitly writing to the workqueue's cpumask sysfs file after creating the isolated partitions. However, this process can be error prone. Ideally, the cpuset code should be allowed to request the workqueue code to exclude those isolated CPUs from wq_unbound_cpumask so that this operation can be done automatically and the isolated CPUs will be returned back to wq_unbound_cpumask after the destructions of the isolated cpuset partitions.
This patch adds a new workqueue_unbound_exclude_cpumask() to enable that. This new function will exclude the specified isolated CPUs from wq_unbound_cpumask. To be able to restore those isolated CPUs back after the destruction of isolated cpuset partitions, a new wq_user_unbound_cpumask is added to store the user provided unbound cpumask either from the boot command line options or from writing to the cpumask sysfs file. This new cpumask provides the basis for CPU exclusion.
The behaviors around wq_unbound_cpumask is getting pretty inconsistent:
1. Housekeeping excludes isolated CPUs on boot but allows user to override it to include isolated CPUs afterwards.
2. If an unbound wq's cpumask doesn't have any intersection with wq_unbound_cpumask we ignore the per-wq cpumask and falls back to wq_unbound_cpumask.
3. You're adding a masking layer on top with exclude which fails to set if the intersection is empty.
Can we do the followings for consistency?
1. User's requested_unbound_cpumask is stored separately (as in this patch).
2. The effect wq_unbound_cpumask is determined by requested_unbound_cpumask & housekeeping_cpumask & cpuset_allowed_cpumask. The operation order matters. When an & operation yields an cpumask, the cpumask from the previous step is the effective one.
3. Expose these cpumasks in sysfs so that what's happening is obvious.
+int workqueue_unbound_exclude_cpumask(cpumask_var_t exclude_cpumask) +{
- cpumask_var_t cpumask;
- int ret = 0;
- if (!zalloc_cpumask_var(&cpumask, GFP_KERNEL))
return -ENOMEM;- /*
* The caller of this function may have called cpus_read_lock(),* use cpus_read_trylock() to avoid potential deadlock.*/- if (!cpus_read_trylock())
return -EBUSY;
This means that a completely unrelated cpus_write_lock() can fail this operation and thus cpuset config writes. Let's please not do this. Can't we just make sure that the caller holds the lock?
- mutex_lock(&wq_pool_mutex);
- if (!cpumask_andnot(cpumask, wq_user_unbound_cpumask, exclude_cpumask))
ret = -EINVAL; /* The new cpumask can't be empty */
For better or worse, the usual mode-of-failure for "no usable CPU" is just falling back to something which works rather than failing the operation. Let's follow that.
Thanks.