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SubjectRe: [PATCH 08/21] x86/intel_rdt/cqm: Add RMID(Resource monitoring ID) management
On Mon, Jul 03, 2017 at 11:55:37AM +0200, Thomas Gleixner wrote:

>
> if (static_branch_likely(&rdt_mon_enable_key)) {
> if (unlikely(current->rmid)) {
> newstate.rmid = current->rmid;
> __set_bit(newstate.rmid, this_cpu_ptr(rmid_bitmap));

Non atomic op

> }
> }
>
> Now in rmid_free() we can collect that information:
>
> cpumask_clear(&tmpmask);
> cpumask_clear(rmid_entry->mask);
>
> cpus_read_lock();
> for_each_online_cpu(cpu) {
> if (test_and_clear_bit(rmid, per_cpu_ptr(cpu, rmid_bitmap)))

atomic op

> cpumask_set(cpu, tmpmask);
> }
>
> for_each_domain(d, resource) {
> cpu = cpumask_any_and(d->cpu_mask, tmpmask);
> if (cpu < nr_cpu_ids)
> cpumask_set(cpu, rmid_entry->mask);
> }
>
> list_add(&rmid_entry->list, &limbo_list);
>
> for_each_cpu(cpu, rmid_entry->mask)
> schedule_delayed_work_on(cpu, rmid_work);
> cpus_read_unlock();
>
> The work function:
>
> boot resched = false;
>
> list_for_each_entry(rme, limbo_list,...) {
> if (!cpumask_test_cpu(cpu, rme->mask))
> continue;
>
> if (!rmid_is_reusable(rme)) {
> resched = true;
> continue;
> }
>
> cpumask_clear_cpu(cpu, rme->mask);
> if (!cpumask_empty(rme->mask))
> continue;
>
> /* Ready for reuse */
> list_del(rme->list);
> list_add(&rme->list, &free_list);
> }
>
> The alloc function then becomes:
>
> if (list_empty(&free_list))
> return list_empty(&limbo_list) ? -ENOSPC : -EBUSY;
>
> The switch_to() covers the task rmids. The per cpu default rmids can be
> marked at the point where they are installed on a CPU in the per cpu
> rmid_bitmap. The free path is the same for per task and per cpu.
>
> Another thing which needs some thought it the CPU hotplug code. We need to
> make sure that pending work which is scheduled on an outgoing CPU is moved
> in the offline callback to a still online CPU of the same domain and not
> moved to some random CPU by the workqueue hotplug code.

just flush the workqueue for that CPU? That's what the workqueue core
_should_ do in any case. And that also covers the case where @cpu is the
last in the set of CPUs we could run on.

> There is another subtle issue. Assume a RMID is freed. The limbo stuff is
> scheduled on all domains which have online CPUs.
>
> Now the last CPU of a domain goes offline before the threshold for clearing
> the domain CPU bit in the rme->mask is reached.
>
> So we have two options here:
>
> 1) Clear the bit unconditionally when the last CPU of a domain goes
> offline.

Arguably this. This is cache level stuff, that means this is the last
CPU of a cache, so just explicitly kill the _entire_ cache and insta
mark everything good again; WBINVD ftw.

> 2) Arm a timer which clears the bit after a grace period
>
> #1 The RMID might become available for reuse right away because all other
> domains have not used it or have cleared their bits already.
>
> If one of the CPUs of that domain comes online again and is associated
> to that reused RMID again, then the counter content might still contain
> leftovers from the previous usage.

Not if we kill the cache on offline -- also, if all CPUs have been
offline, its not too weird to expect something like a package idle state
to have happened and shot down the caches anyway.

> #2 Prevents #1 but has it's own issues vs. serialization and coordination
> with CPU hotplug.
>
> I'd say we go for #1 as the simplest solution, document it and if really
> the need arises revisit it later.
>
> Thanks,
>
> tglx

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