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Date: Tue, 31 Mar 2015 15:46:36 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: Tejun Heo <tj@...nel.org>
CC: <linux-kernel@...r.kernel.org>, Christoph Lameter <cl@...ux.com>,
Kevin Hilman <khilman@...aro.org>,
Mike Galbraith <bitbucket@...ine.de>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Viresh Kumar <viresh.kumar@...aro.org>,
Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 4/4 V5] workqueue: Allow modifying low level unbound workqueue
cpumask
On 03/25/2015 01:31 AM, Tejun Heo wrote:
> On Wed, Mar 18, 2015 at 12:40:17PM +0800, Lai Jiangshan wrote:
>> The oreder-workquue is ignore from the low level unbound workqueue cpumask,
>> it will be handled in near future.
>
> Ugh, right, ordered workqueues are tricky. Maybe we should change how
> ordered workqueues are implemented. Just gate work items at the
> workqueue layer instead of fiddling with max_active and the number of
> pwqs.
>
>> static struct wq_unbound_install_ctx *
>> wq_unbound_install_ctx_prepare(struct workqueue_struct *wq,
>> - const struct workqueue_attrs *attrs)
>> + const struct workqueue_attrs *attrs,
>> + cpumask_var_t unbound_cpumask)
>> {
> ...
>> /* make a copy of @attrs and sanitize it */
>> copy_workqueue_attrs(new_attrs, attrs);
>> - cpumask_and(new_attrs->cpumask, new_attrs->cpumask, wq_unbound_cpumask);
>> + copy_workqueue_attrs(pwq_attrs, attrs);
>> + cpumask_and(new_attrs->cpumask, new_attrs->cpumask, cpu_possible_mask);
>> + cpumask_and(pwq_attrs->cpumask, pwq_attrs->cpumask, unbound_cpumask);
>
> Hmmm... we weren't checking whether the intersection becomes null
> before.
Di you refer to the unquoted following code "cpumask_empty(pwq_attrs->cpumask)"?
It is explained in the changelog and the comments.
> Why are we doing it now? Note that this doesn't really make
> things water-tight as cpu on/offlining can still leave the mask w/o
> any online cpus. Shouldn't we just let the scheduler handle it as
> before?
Did you refer to "cpumask_and(new_attrs->cpumask, new_attrs->cpumask, cpu_possible_mask);"?
new_attrs will be copied to wq->unbound_attrs, so we hope it is sanity.
the same code before this patchset did the same work.
And it maybe be used for default pwq, and it can reduce the pool creation:
cpu_possible_mask = 0-7
wq_unbound_cpumask = 0-3
user1 try to set wq1: attrs->cpumask = 4-9
user2 try to set wq2: attrs->cpumask = 4-11
thus both wq1 and wq2's default pwq's pool is the same pool. (pool's cpumask = 4-7)
>
>> @@ -3712,6 +3726,9 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
>> * wq's, the default pwq should be used.
>> */
>> if (wq_calc_node_cpumask(wq->unbound_attrs, node, cpu_off, cpumask)) {
>> + cpumask_and(cpumask, cpumask, wq_unbound_cpumask);
>> + if (cpumask_empty(cpumask))
>> + goto use_dfl_pwq;
>
> So, this special handling is necessary only because we did special in
> the above for dfl_pwq. Why do we need these?
wq->unbound_attrs is user setting attrs, its cpumask is not controlled by
wq_unbound_cpumask. so we need these cpumask_and().
Another question:
Why wq->unbound_attrs' cpumask is not controlled by wq_unbound_cpumask?
I hope the wq->unbound_attrs is always as the same as the user's last setting,
regardless how much times the wq_unbound_cpumask is changed.
>
>> +static int unbounds_cpumask_apply(cpumask_var_t cpumask)
>> +{
> ..
>> + list_for_each_entry_safe(ctx, n, &ctxs, list) {
>> + if (ret >= 0)
>
> Let's do !ret.
>
>> + wq_unbound_install_ctx_commit(ctx);
>> + wq_unbound_install_ctx_free(ctx);
>> + }
> ...
>> +/**
>> + * workqueue_unbounds_cpumask_set - Set the low-level unbound cpumask
>> + * @cpumask: the cpumask to set
>> + *
>> + * The low-level workqueues cpumask is a global cpumask that limits
>> + * the affinity of all unbound workqueues. This function check the @cpumask
>> + * and apply it to all unbound workqueues and updates all pwqs of them.
>> + * When all succeed, it saves @cpumask to the global low-level unbound
>> + * cpumask.
>> + *
>> + * Retun: 0 - Success
>> + * -EINVAL - No online cpu in the @cpumask
>> + * -ENOMEM - Failed to allocate memory for attrs or pwqs.
>> + */
>> +int workqueue_unbounds_cpumask_set(cpumask_var_t cpumask)
>> +{
>> + int ret = -EINVAL;
>> +
>> + get_online_cpus();
>> + cpumask_and(cpumask, cpumask, cpu_possible_mask);
>> + if (cpumask_intersects(cpumask, cpu_online_mask)) {
>
> Does this make sense? We can't prevent cpus going down right after
> the mask is set. What's the point of preventing empty config if we
> can't prevent transitions into it and have to handle it anyway?
Like set_cpus_allowed_ptr(). The cpumask must be valid when setting,
although it can be transited into non-intersection later.
This code is originated from Frederic. Maybe he has some stronger reason.
>
>> +static ssize_t unbounds_cpumask_store(struct device *dev,
>> + struct device_attribute *attr,
>> + const char *buf, size_t count)
>
> Naming is too confusing. Please pick a name which clearly
> distinguishes per-wq and global masking.
What about these names?
wq_unbound_cpumask ==> wq_unbound_global_cpumask
workqueue_unbounds_cpumask_set() ==> workqueue_set_unbound_global_cpumask(). (public API)
unbounds_cpumask_store() ==> wq_store_unbound_global_cpumask() (static function for sysfs)
>
> Thanks.
>
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