Messages in this thread | | | Date | Mon, 14 Mar 2022 16:38:31 +0530 | From | Srikar Dronamraju <> | Subject | Re: [PATCH v6] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest_group |
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* K Prateek Nayak <kprateek.nayak@amd.com> [2022-03-08 12:07:49]:
> In the case of systems containing multiple LLCs per socket, like > AMD Zen systems, users want to spread bandwidth hungry applications > across multiple LLCs. Stream is one such representative workload where > the best performance is obtained by limiting one stream thread per LLC. > To ensure this, users are known to pin the tasks to a specify a subset > of the CPUs consisting of one CPU per LLC while running such bandwidth > hungry tasks. > > Suppose we kickstart a multi-threaded task like stream with 8 threads > using taskset or numactl to run on a subset of CPUs on a 2 socket Zen3 > server where each socket contains 128 CPUs > (0-63,128-191 in one socket, 64-127,192-255 in another socket) > > Eg: numactl -C 0,16,32,48,64,80,96,112 ./stream8 > > Here each CPU in the list is from a different LLC and 4 of those LLCs > are on one socket, while the other 4 are on another socket. > > Ideally we would prefer that each stream thread runs on a different > CPU from the allowed list of CPUs. However, the current heuristics in > find_idlest_group() do not allow this during the initial placement. > > Suppose the first socket (0-63,128-191) is our local group from which > we are kickstarting the stream tasks. The first four stream threads > will be placed in this socket. When it comes to placing the 5th > thread, all the allowed CPUs are from the local group (0,16,32,48) > would have been taken. > > However, the current scheduler code simply checks if the number of > tasks in the local group is fewer than the allowed numa-imbalance > threshold. This threshold was previously 25% of the NUMA domain span > (in this case threshold = 32) but after the v6 of Mel's patchset > "Adjust NUMA imbalance for multiple LLCs", got merged in sched-tip, > Commit: e496132ebedd ("sched/fair: Adjust the allowed NUMA imbalance > when SD_NUMA spans multiple LLCs") it is now equal to number of LLCs > in the NUMA domain, for processors with multiple LLCs. > (in this case threshold = 8). > > For this example, the number of tasks will always be within threshold > and thus all the 8 stream threads will be woken up on the first socket > thereby resulting in sub-optimal performance. > > The following sched_wakeup_new tracepoint output shows the initial > placement of tasks in the current tip/sched/core on the Zen3 machine: > > stream-5045 [032] d..2. 167.914699: sched_wakeup_new: comm=stream pid=5047 prio=120 target_cpu=048 > stream-5045 [032] d..2. 167.914746: sched_wakeup_new: comm=stream pid=5048 prio=120 target_cpu=000 > stream-5045 [032] d..2. 167.914846: sched_wakeup_new: comm=stream pid=5049 prio=120 target_cpu=016 > stream-5045 [032] d..2. 167.914891: sched_wakeup_new: comm=stream pid=5050 prio=120 target_cpu=032 > stream-5045 [032] d..2. 167.914928: sched_wakeup_new: comm=stream pid=5051 prio=120 target_cpu=032 > stream-5045 [032] d..2. 167.914976: sched_wakeup_new: comm=stream pid=5052 prio=120 target_cpu=032 > stream-5045 [032] d..2. 167.915011: sched_wakeup_new: comm=stream pid=5053 prio=120 target_cpu=032 > > Once the first four threads are distributed among the allowed CPUs of > socket one, the rest of the treads start piling on these same CPUs > when clearly there are CPUs on the second socket that can be used. > > Following the initial pile up on a small number of CPUs, though the > load-balancer eventually kicks in, it takes a while to get to {4}{4} > and even {4}{4} isn't stable as we observe a bunch of ping ponging > between {4}{4} to {5}{3} and back before a stable state is reached > much later (1 Stream thread per allowed CPU) and no more migration is > required. > > We can detect this piling and avoid it by checking if the number of > allowed CPUs in the local group are fewer than the number of tasks > running in the local group and use this information to spread the > 5th task out into the next socket (after all, the goal in this > slowpath is to find the idlest group and the idlest CPU during the > initial placement!). > > The following sched_wakeup_new tracepoint output shows the initial > placement of tasks after adding this fix on the Zen3 machine: > > stream-4733 [032] d..2. 116.017980: sched_wakeup_new: comm=stream pid=4735 prio=120 target_cpu=048 > stream-4733 [032] d..2. 116.018032: sched_wakeup_new: comm=stream pid=4736 prio=120 target_cpu=000 > stream-4733 [032] d..2. 116.018127: sched_wakeup_new: comm=stream pid=4737 prio=120 target_cpu=064 > stream-4733 [032] d..2. 116.018185: sched_wakeup_new: comm=stream pid=4738 prio=120 target_cpu=112 > stream-4733 [032] d..2. 116.018235: sched_wakeup_new: comm=stream pid=4739 prio=120 target_cpu=096 > stream-4733 [032] d..2. 116.018289: sched_wakeup_new: comm=stream pid=4740 prio=120 target_cpu=016 > stream-4733 [032] d..2. 116.018334: sched_wakeup_new: comm=stream pid=4741 prio=120 target_cpu=080 > > We see that threads are using all of the allowed CPUs and there is > no pileup. > > No output is generated for tracepoint sched_migrate_task with this > patch due to a perfect initial placement which removes the need > for balancing later on - both across NUMA boundaries and within > NUMA boundaries for stream. > > Following are the results from running 8 Stream threads with and > without pinning on a dual socket Zen3 Machine (2 x 64C/128T): > > Pinning is done using: numactl -C 0,16,32,48,64,80,96,112 ./stream8 > > 5.17.0-rc1 5.17.0-rc1 5.17.0-rc1 > tip sched/core tip sched/core tip sched/core > (no pinning) +pinning + this-patch > + pinning > > Copy: 97699.28 (0.00 pct) 95933.60 (-1.80 pct) 156578.91 (60.26 pct) > Scale: 107754.15 (0.00 pct) 91869.88 (-14.74 pct) 149783.25 (39.00 pct) > Add: 126383.29 (0.00 pct) 105730.86 (-16.34 pct) 186493.09 (47.56 pct) > Triad: 124896.78 (0.00 pct) 106394.38 (-14.81 pct) 184733.48 (47.90 pct) > > Pinning currently hurts the performance compared to unbound case on > tip/sched/core. With the addition of this patch, we are able to > outperform tip/sched/core by a good margin with pinning. > > Following are the results from running 16 Stream threads with and > without pinning on a dual socket Skylake Machine (2 x 24C/48T): > > Pinning is done using: numactl -C 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 ./stream16 > > 5.17.0-rc1 5.17.0-rc1 5.17.0-rc1 > tip sched/core tip sched/core tip sched/core > (no pinning) +pinning + this-patch > + pinning > > Copy: 126620.67 (0.00 pct) 141062.10 (11.40 pct) 147615.44 (16.58 pct) > Scale: 91313.51 (0.00 pct) 112879.61 (23.61 pct) 122591.28 (34.25 pct) > Add: 102035.43 (0.00 pct) 125889.98 (23.37 pct) 138179.01 (35.42 pct) > Triad: 102281.91 (0.00 pct) 123743.48 (20.98 pct) 138940.41 (35.84 pct) > > In case of Skylake machine, with single LLC per socket, we see good > improvement brought about by pinning which is further benefited by > this patch. > > Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com> > Acked-by: Mel Gorman <mgorman@techsingularity.net>
Looks good to me.
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> --- > Changelog v5-->v6: > - Move the cpumask variable declaration to the block it is > used in. > - Collect tags from v5. > Changelog v4-->v5: > - Only perform cpumask operations if nr_cpus_allowed is not > equal to num_online_cpus based on Mel's suggestion. > --- > kernel/sched/fair.c | 16 +++++++++++++--- > 1 file changed, 13 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 16874e112fe6..6cc90d76250f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -9183,6 +9183,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) > > case group_has_spare: > if (sd->flags & SD_NUMA) { > + int imb; > #ifdef CONFIG_NUMA_BALANCING > int idlest_cpu; > /* > @@ -9200,10 +9201,19 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) > * Otherwise, keep the task close to the wakeup source > * and improve locality if the number of running tasks > * would remain below threshold where an imbalance is > - * allowed. If there is a real need of migration, > - * periodic load balance will take care of it. > + * allowed while accounting for the possibility the > + * task is pinned to a subset of CPUs. If there is a > + * real need of migration, periodic load balance will > + * take care of it. > */ > - if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, sd->imb_numa_nr)) > + imb = sd->imb_numa_nr; > + if (p->nr_cpus_allowed != num_online_cpus()) { > + struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); > + > + cpumask_and(cpus, sched_group_span(local), p->cpus_ptr); > + imb = min(cpumask_weight(cpus), sd->imb_numa_nr); > + } > + if (allow_numa_imbalance(local_sgs.sum_nr_running + 1, imb)) > return NULL; > } > > -- > 2.25.1 >
-- Thanks and Regards Srikar Dronamraju
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