sched/topology: Allow multiple domains to claim sched_domain_shared

Recent optimizations of sd->shared assignment moved to allocating a
single instance of per-CPU sched_domain_shared objects per s_data.

Recent optimizations to select_idle_capacity() moved the sd->shared
assignments to "sd_asym" domain when ASYM_CPUCAPACITY is detected but
cache-aware scheduling mandates the presence of "sd_llc_shared" to
compute and cache per-LLC statistics.

Use an "alloc_flags" union in sched_domain_shared to claim a
sched_domain_shared object per sched_domain. Allocation starts searching
for an available / matching sched_domain_shared instance from the first
CPU of sched_domain_span(sd) (sd can be sd_llc, or sd_asym). If the
shared object is claimed by another domain, the instance corresponding
to next CPU in the domain span is explored until a matching / available
instance is found.

In case of a single CPU in sched_domain_span(), the domain will be
degenerated and a temporary overlap of ->shared objects across different
domains is acceptable.

"alloc_flags" forms a union with "nr_idle_scan" and the stale flags are
left as is when the sd->shared is published. The expectation is for the
first load balancing instance to correct the value just like the current
behavior, except the initial value is no longer 0.

Originally-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Andrea Righi <arighi@nvidia.com>

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fe09d3268bc9ce449b1e3f1349a74200d8cd11ea..b5d9d7c2b8add7068ee66ef2c50aad1b55dc015d 100644 (file)
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -67,7 +67,21 @@ struct sched_domain_shared {
        atomic_t        ref;
        atomic_t        nr_busy_cpus;
        int             has_idle_cores;
-       int             nr_idle_scan;
+       union {
+               int     nr_idle_scan;
+               /*
+                * Used during allocation to claim the sched_domain_shared
+                * object at multiple levels.
+                *
+                * Note: between build and the first periodic LB tick, which
+                * rewrites the union via update_idle_cpu_scan(), readers of
+                * nr_idle_scan may observe the transient SD_* flag value as
+                * the scan bound. The flag bits are small positive integers,
+                * so the effect is just a slightly relaxed scan bound for one
+                * window and self-heals on the first tick.
+                */
+               int     alloc_flags;
+       };
 #ifdef CONFIG_SCHED_CACHE
        unsigned long   util_avg;
        unsigned long   capacity;

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index dbfd9657f8974a4705388efd4365134b43f2f43b..df2ceb54c970e97b100df2bcdedcd6b7154fa596 100644 (file)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -623,6 +623,12 @@ static void free_sched_groups(struct sched_group *sg, int free_sgc)
        } while (sg != first);
 }
 
+static void free_sched_domain_shared(struct sched_domain_shared *sds)
+{
+       if (sds && atomic_dec_and_test(&sds->ref))
+               kfree(sds);
+}
+
 static void destroy_sched_domain(struct sched_domain *sd)
 {
        /*
@@ -631,9 +637,7 @@ static void destroy_sched_domain(struct sched_domain *sd)
         * dropping group/capacity references, freeing where none remain.
         */
        free_sched_groups(sd->groups, 1);
-
-       if (sd->shared && atomic_dec_and_test(&sd->shared->ref))
-               kfree(sd->shared);
+       free_sched_domain_shared(sd->shared);
 
 #ifdef CONFIG_SCHED_CACHE
        /* only the bottom sd has llc_counts array */
@@ -755,7 +759,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 
                        /* Pick reference to parent->shared. */
                        if (parent->shared) {
-                               WARN_ON_ONCE(tmp->shared);
+                               /*
+                                * It is safe to free a sd->shared that
+                                * has not been published yet. If a
+                                * sd->shared was published, the refcount
+                                * will end up being non-zero and it will
+                                * not be freed here.
+                                */
+                               free_sched_domain_shared(tmp->shared);
                                tmp->shared = parent->shared;
                                parent->shared = NULL;
                        }
@@ -2916,11 +2927,45 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
        }
 }
 
-static void init_sched_domain_shared(struct s_data *d, struct sched_domain *sd)
+static void
+init_sched_domain_shared(struct s_data *d, struct sched_domain *sd, int flags)
 {
-       int sd_id = cpumask_first(sched_domain_span(sd));
+       struct sched_domain_shared *sds = NULL;
+       int cpu;
+
+       /*
+        * Multiple domains can try to claim a shared object like
+        * SD_ASYM_CPUCAPACITY and SD_SHARE_LLC which can alias to
+        * same cpumask_first(sched_domain_span(sd)) CPU and can
+        * cause "nr_idle_scan" to be populated incorrectly during
+        * load balancing.
+        *
+        * Find the first CPU in sched_domain_span(sd) with an
+        * unclaimed domain (!alloc_flags) or where the alloc_flag
+        * matches the requested flag (SD_* flag)
+        *
+        * If the domain only has single CPU, allow temporary overlap
+        * in allocation since the domains will be degenerated later.
+        */
+       for_each_cpu(cpu, sched_domain_span(sd)) {
+               sds = *per_cpu_ptr(d->sds, cpu);
+
+               if (!sds->alloc_flags ||
+                   sd->span_weight == 1 ||
+                   sds->alloc_flags == flags) {
+                       sds->alloc_flags = flags;
+                       sd->shared = sds;
+                       break;
+               }
+       }
+
+       /*
+        * Use the sd_shared corresponding to the last
+        * CPU in the span if none are avaialable.
+        */
+       if (WARN_ON_ONCE(!sd->shared))
+               sd->shared = sds;
 
-       sd->shared = *per_cpu_ptr(d->sds, sd_id);
        /*
         * nr_busy_cpus is consumed only by the NOHZ kick path via
         * sd_balance_shared; on the asym-capacity path it is initialized but
@@ -2960,7 +3005,7 @@ static bool claim_asym_sched_domain_shared(struct s_data *d, int cpu)
        if (!sd_asym || (sd_asym->flags & SD_NUMA))
                return false;
 
-       init_sched_domain_shared(d, sd_asym);
+       init_sched_domain_shared(d, sd_asym, SD_ASYM_CPUCAPACITY);
        return true;
 }
 
@@ -3115,7 +3160,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
                        sd = sd->parent;
 
                if (sd->flags & SD_SHARE_LLC) {
-                       init_sched_domain_shared(&d, sd);
+                       init_sched_domain_shared(&d, sd, SD_SHARE_LLC);
 
                        /*
                         * In presence of higher domains, adjust the