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

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Date:   Wed, 18 Oct 2023 09:41:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH-cgroup 1/4] workqueue: Add
 workqueue_unbound_exclude_cpumask() to exclude CPUs from wq_unbound_cpumask

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