| Message ID | 20230306200849.376804-3-longman@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | cgroup/cpuset: Miscellaneous updates | expand |
Hello Waiman. On Mon, Mar 06, 2023 at 03:08:46PM -0500, Waiman Long <longman@redhat.com> wrote: > - /* > - * Percpu kthreads in top_cpuset are ignored > - */ > - if (top_cs && (task->flags & PF_KTHREAD) && > - kthread_is_per_cpu(task)) > - continue; > + const struct cpumask *possible_mask = task_cpu_possible_mask(task); > > - cpumask_and(new_cpus, cs->effective_cpus, > - task_cpu_possible_mask(task)); > + if (top_cs) { > + /* > + * Percpu kthreads in top_cpuset are ignored > + */ > + if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task)) > + continue; > + cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus); > + } else { > + cpumask_and(new_cpus, cs->effective_cpus, possible_mask); > + } I'm wrapping my head around this slightly. 1) I'd suggest swapping args in of cpumask_and() to have possible_mask consistently first. 2) Then I'm wondering whether two branches are truly different when effective_cpus := cpus_allowed - subparts_cpus top_cpuset.cpus_allowed == possible_mask (1) IOW, can you see a difference in what affinities are set to eligible top_cpuset tasks before and after this patch upon CPU hotplug? (Hm, (1) holds only in v2. So is this a fix for v1 only?) Thanks, Michal
On 3/14/23 13:34, Michal Koutný wrote: > Hello Waiman. > > On Mon, Mar 06, 2023 at 03:08:46PM -0500, Waiman Long <longman@redhat.com> wrote: >> - /* >> - * Percpu kthreads in top_cpuset are ignored >> - */ >> - if (top_cs && (task->flags & PF_KTHREAD) && >> - kthread_is_per_cpu(task)) >> - continue; >> + const struct cpumask *possible_mask = task_cpu_possible_mask(task); >> >> - cpumask_and(new_cpus, cs->effective_cpus, >> - task_cpu_possible_mask(task)); >> + if (top_cs) { >> + /* >> + * Percpu kthreads in top_cpuset are ignored >> + */ >> + if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task)) >> + continue; >> + cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus); >> + } else { >> + cpumask_and(new_cpus, cs->effective_cpus, possible_mask); >> + } > I'm wrapping my head around this slightly. > 1) I'd suggest swapping args in of cpumask_and() to have possible_mask > consistently first. I don't quite understand what you meant by "swapping args". It is effective new_cpus = cs->effective_cpus ∩ possible_mask. What is the point of swapping cs->effective_cpus and possible_mask. > 2) Then I'm wondering whether two branches are truly different when > effective_cpus := cpus_allowed - subparts_cpus > top_cpuset.cpus_allowed == possible_mask (1) effective_cpus may not be equal "cpus_allowed - subparts_cpus" if some of the CPUs are offline as effective_cpus contains only online CPUs. subparts_cpu can include offline cpus too. That is why I choose that expression. I will add a comment to clarify that. > > IOW, can you see a difference in what affinities are set to eligible > top_cpuset tasks before and after this patch upon CPU hotplug? > (Hm, (1) holds only in v2. So is this a fix for v1 only?) This is due to the fact that cpu hotplug code currently doesn't update the cpu affinity of tasks in the top cpuset. Tasks not in the top cpuset can rely on the hotplug code to update the cpu affinity appropriately. For the tasks in the top cpuset, we have to make sure that all the offline CPUs are included. Cheers, Longman
On Tue, Mar 14, 2023 at 03:02:53PM -0400, Waiman Long <longman@redhat.com> wrote: > > > + cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus); > > > + } else { > > > + cpumask_and(new_cpus, cs->effective_cpus, possible_mask); > > > + } > > I'm wrapping my head around this slightly. > > 1) I'd suggest swapping args in of cpumask_and() to have possible_mask > > consistently first. > I don't quite understand what you meant by "swapping args". It is effective > new_cpus = cs->effective_cpus ∩ possible_mask. What is the point of swapping > cs->effective_cpus and possible_mask. No effect except better readability (possible_mask comes first in the other branch above too, left hand arg as the "base" that's modified). > > 2) Then I'm wondering whether two branches are truly different when > > effective_cpus := cpus_allowed - subparts_cpus > > top_cpuset.cpus_allowed == possible_mask (1) > effective_cpus may not be equal "cpus_allowed - subparts_cpus" if some of > the CPUs are offline as effective_cpus contains only online CPUs. > subparts_cpu can include offline cpus too. That is why I choose that > expression. I will add a comment to clarify that. I see now that it returns offlined cpus to top cpuset's tasks. > > > > IOW, can you see a difference in what affinities are set to eligible > > top_cpuset tasks before and after this patch upon CPU hotplug? > > (Hm, (1) holds only in v2. So is this a fix for v1 only?) > > This is due to the fact that cpu hotplug code currently doesn't update the > cpu affinity of tasks in the top cpuset. Tasks not in the top cpuset can > rely on the hotplug code to update the cpu affinity appropriately. Oh, I mistook this for hotplug changing behavior but it's actually for updating top_cpuset when its children becomes a partition root. IIUC, top cpuset + hotplug has been treated specially because hotplug must have taken care of affinity regardless of cpuset. v1 allowed modification of top cpuset's mask (not sure if that worked), v2 won't allow direct top cpuset's mask modificiation but indirectly via partition root children. So this is a continuation for 3fb906e7fabb ("cgroup/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks") to ensure hotplug offline/online cycle won't overwrite top_cpuset tasks' affinities (when partition change during offlined period). This looks correct in this regard then. (I wish it were simpler but that's for a different/broader top cpuset+hotplug approach.) Thanks, Michal
On 3/15/23 06:06, Michal Koutný wrote: > On Tue, Mar 14, 2023 at 03:02:53PM -0400, Waiman Long <longman@redhat.com> wrote: >>>> + cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus); >>>> + } else { >>>> + cpumask_and(new_cpus, cs->effective_cpus, possible_mask); >>>> + } >>> I'm wrapping my head around this slightly. >>> 1) I'd suggest swapping args in of cpumask_and() to have possible_mask >>> consistently first. >> I don't quite understand what you meant by "swapping args". It is effective >> new_cpus = cs->effective_cpus ∩ possible_mask. What is the point of swapping >> cs->effective_cpus and possible_mask. > No effect except better readability (possible_mask comes first in the > other branch above too, left hand arg as the "base" that's modified). OK, I got it now. I will swap it as suggested. > >>> 2) Then I'm wondering whether two branches are truly different when >>> effective_cpus := cpus_allowed - subparts_cpus >>> top_cpuset.cpus_allowed == possible_mask (1) >> effective_cpus may not be equal "cpus_allowed - subparts_cpus" if some of >> the CPUs are offline as effective_cpus contains only online CPUs. >> subparts_cpu can include offline cpus too. That is why I choose that >> expression. I will add a comment to clarify that. > I see now that it returns offlined cpus to top cpuset's tasks. > >>> IOW, can you see a difference in what affinities are set to eligible >>> top_cpuset tasks before and after this patch upon CPU hotplug? >>> (Hm, (1) holds only in v2. So is this a fix for v1 only?) >> This is due to the fact that cpu hotplug code currently doesn't update the >> cpu affinity of tasks in the top cpuset. Tasks not in the top cpuset can >> rely on the hotplug code to update the cpu affinity appropriately. > Oh, I mistook this for hotplug changing behavior but it's actually for > updating top_cpuset when its children becomes a partition root. > > IIUC, top cpuset + hotplug has been treated specially because > hotplug must have taken care of affinity regardless of cpuset. > v1 allowed modification of top cpuset's mask (not sure if that > worked), v2 won't allow direct top cpuset's mask modificiation > but indirectly via partition root children. > > So this is a continuation for 3fb906e7fabb ("cgroup/cpuset: Don't filter > offline CPUs in cpuset_cpus_allowed() for top cpuset tasks") to ensure > hotplug offline/online cycle won't overwrite top_cpuset tasks' > affinities (when partition change during offlined period). > This looks correct in this regard then. > (I wish it were simpler but that's for a different/broader top > cpuset+hotplug approach.) You can't change the content of "cpuset.cpus" in the top cpuset (both v1 & v2). I believe the CPU hotplug does not attempt to update the cpumask of tasks in the top cpuset mainly due to potential locking issue as hotplug is triggered by hardware event. Partition change, however, is a userspace event. So there shouldn't be any locking implication other than the fact per-cpu kthreads should not have their cpumasks changed. To be consistent with commit 3fb906e7fabb ("cgroup/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks"), similar logic needs to be applied in the later case. Cheers, Longman
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index a801abad3bac..bbf57dcb2f68 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -1209,7 +1209,8 @@ void rebuild_sched_domains(void) * * Iterate through each task of @cs updating its cpus_allowed to the * effective cpuset's. As this function is called with cpuset_rwsem held, - * cpuset membership stays stable. + * cpuset membership stays stable. For top_cpuset, task_cpu_possible_mask() + * is used instead of effective_cpus. */ static void update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus) { @@ -1219,15 +1220,18 @@ static void update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus) css_task_iter_start(&cs->css, 0, &it); while ((task = css_task_iter_next(&it))) { - /* - * Percpu kthreads in top_cpuset are ignored - */ - if (top_cs && (task->flags & PF_KTHREAD) && - kthread_is_per_cpu(task)) - continue; + const struct cpumask *possible_mask = task_cpu_possible_mask(task); - cpumask_and(new_cpus, cs->effective_cpus, - task_cpu_possible_mask(task)); + if (top_cs) { + /* + * Percpu kthreads in top_cpuset are ignored + */ + if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task)) + continue; + cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus); + } else { + cpumask_and(new_cpus, cs->effective_cpus, possible_mask); + } set_cpus_allowed_ptr(task, new_cpus); } css_task_iter_end(&it);
Similar to commit 3fb906e7fabb ("group/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks"), the whole set of possible CPUs including offline ones should be used for setting cpumasks for tasks in the top cpuset when a cpuset partition is modified. Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/cgroup/cpuset.c | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-)