Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 31 Aug 2023 14:11:03 -0500 | From | David Vernet <> | Subject | Re: [RFC PATCH 3/3] sched/fair: Add a per-shard overload flag |
| |
On Thu, Aug 31, 2023 at 04:15:08PM +0530, K Prateek Nayak wrote:
Hi Prateek,
> Even with the two patches, I still observe the following lock > contention when profiling the tbench 128-clients run with IBS: > > - 12.61% swapper [kernel.vmlinux] [k] native_queued_spin_lock_slowpath > - 10.94% native_queued_spin_lock_slowpath > - 10.73% _raw_spin_lock > - 9.57% __schedule > schedule_idle > do_idle > + cpu_startup_entry > - 0.82% task_rq_lock > newidle_balance > pick_next_task_fair > __schedule > schedule_idle > do_idle > + cpu_startup_entry > > Since David mentioned rq->avg_idle check is probably not the right step > towards the solution, this experiment introduces a per-shard > "overload" flag. Similar to "rq->rd->overload", per-shard overload flag > notifies of the possibility of one or more rq covered in the shard's > domain having a queued task. shard's overload flag is set at the same > time as "rq->rd->overload", and is cleared when shard's list is found > to be empty.
I think this is an interesting idea, but I feel that it's still working against the core proposition of SHARED_RUNQ, which is to enable work conservation.
> With these changes, following are the results for tbench 128-clients:
Just to make sure I understand, this is to address the contention we're observing on tbench with 64 - 256 clients, right? That's my understanding from Gautham's reply in [0].
[0]: https://lore.kernel.org/all/ZOc7i7wM0x4hF4vL@BLR-5CG11610CF.amd.com/
If so, are we sure this change won't regress other workloads that would have benefited from the work conservation?
Also, I assume that you don't see the improved contention without this, even if you include your fix to the newidle_balance() that has us skip over the <= LLC domain?
Thanks, David
P.S. Taking off on vacation now, so any replies will be very delayed. Thanks again for working on this!
> > tip : 1.00 (var: 1.00%) > tip + v3 + series till patch 2 : 0.41 (var: 1.15%) (diff: -58.81%) > tip + v3 + full series : 1.01 (var: 0.36%) (diff: +00.92%) > > Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com> > --- > kernel/sched/fair.c | 13 +++++++++++-- > kernel/sched/sched.h | 17 +++++++++++++++++ > 2 files changed, 28 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 446ffdad49e1..31fe109fdaf0 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -186,6 +186,7 @@ static void shared_runq_reassign_domains(void) > rq->cfs.shared_runq = shared_runq; > rq->cfs.shard = &shared_runq->shards[shard_idx]; > rq_unlock(rq, &rf); > + WRITE_ONCE(rq->cfs.shard->overload, 0); > } > } > > @@ -202,6 +203,7 @@ static void __shared_runq_drain(struct shared_runq *shared_runq) > list_for_each_entry_safe(p, tmp, &shard->list, shared_runq_node) > list_del_init(&p->shared_runq_node); > raw_spin_unlock(&shard->lock); > + WRITE_ONCE(shard->overload, 0); > } > } > > @@ -258,13 +260,20 @@ shared_runq_pop_task(struct shared_runq_shard *shard, int target) > { > struct task_struct *p; > > - if (list_empty(&shard->list)) > + if (!READ_ONCE(shard->overload)) > return NULL; > > + if (list_empty(&shard->list)) { > + WRITE_ONCE(shard->overload, 0); > + return NULL; > + } > + > raw_spin_lock(&shard->lock); > p = list_first_entry_or_null(&shard->list, struct task_struct, > shared_runq_node); > - if (p && is_cpu_allowed(p, target)) > + if (!p) > + WRITE_ONCE(shard->overload, 0); > + else if (is_cpu_allowed(p, target)) > list_del_init(&p->shared_runq_node); > else > p = NULL; > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index f50176f720b1..e8d4d948f742 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -601,6 +601,20 @@ do { \ > struct shared_runq_shard { > struct list_head list; > raw_spinlock_t lock; > + /* > + * shared_runq_shard can contain running tasks. > + * In such cases where all the tasks are running, > + * it is futile to attempt to pull tasks from the > + * list. Overload flag is used to indicate case > + * where one or more rq in the shard domain may > + * have a queued task. If the flag is 0, it is > + * very likely that all tasks in the shard are > + * running and cannot be migrated. This is not > + * guarded by the shard lock, and since it may > + * be updated often, it is placed into its own > + * cacheline. > + */ > + int overload ____cacheline_aligned; > } ____cacheline_aligned; > > /* This would likely work better as a configurable knob via debugfs */ > @@ -2585,6 +2599,9 @@ static inline void add_nr_running(struct rq *rq, unsigned count) > if (prev_nr < 2 && rq->nr_running >= 2) { > if (!READ_ONCE(rq->rd->overload)) > WRITE_ONCE(rq->rd->overload, 1); > + > + if (rq->cfs.shard && !READ_ONCE(rq->cfs.shard->overload)) > + WRITE_ONCE(rq->cfs.shard->overload, 1); > } > #endif > > -- > 2.34.1 >
| ![\](/images/icornerr.gif) |