Re: [PATCH-cgroup 1/4] workqueue: Add workqueue_unbound_exclude_cpumask() to exclude CPUs from wq_unbound_cpumask

[Waiman Long]

On 10/18/23 05:24, Tejun Heo wrote:
> 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.
Sure. I will do that.
> 3. Expose these cpumasks in sysfs so that what's happening is obvious.

I can expose the requested_unbound_cpumask. As for the isolated CPUs, 
see my other reply.

> > +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?
This condition is actually triggered by a few hotplug tests in 
test_cpuset_prs.sh. I will make sure that either cpu read or write lock 
is held before calling this function and eliminate rcu read locking here.
> > +	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.

In this case, it is just leaving the current unbound cpumask unchanged. 
I will follow the precedence discussed above to make sure that there is 
a graceful fallback.

Cheers,
Longman