From: Matt Fleming <matt.flem...@intel.com>
It's possible to run into issues with re-using unused monitoring IDs
because there may be stale cachelines associated with that ID from a
previous allocation. This can cause the LLC occupancy values to be
inaccurate.
To attempt to mitigate this problem we place the IDs on a least recently
used list, essentially a FIFO. The basic idea is that the longer the
time period between ID re-use the lower the probability that stale
cachelines exist in the cache.
Cc: Jiri Olsa <jo...@redhat.com>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Matt Fleming <matt.flem...@intel.com>
---
arch/x86/kernel/cpu/perf_event_intel_cqm.c | 100 ++++++++++++++++++++++++++---
1 file changed, 92 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
index b16458ff274e..60e0043ca922 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -25,7 +25,7 @@ struct intel_cqm_state {
static DEFINE_PER_CPU(struct intel_cqm_state, cqm_state);
/*
- * Protects cache_cgroups.
+ * Protects cache_cgroups and cqm_rmid_lru.
*/
static DEFINE_MUTEX(cache_mutex);
@@ -64,36 +64,120 @@ static u64 __rmid_read(unsigned long rmid)
return val;
}
-static unsigned long *cqm_rmid_bitmap;
+struct cqm_rmid_entry {
+ u64 rmid;
+ struct list_head list;
+};
+
+/*
+ * A least recently used list of RMIDs.
+ *
+ * Oldest entry at the head, newest (most recently used) entry at the
+ * tail. This list is never traversed, it's only used to keep track of
+ * the lru order. That is, we only pick entries of the head or insert
+ * them on the tail.
+ *
+ * All entries on the list are 'free', and their RMIDs are not currently
+ * in use. To mark an RMID as in use, remove its entry from the lru
+ * list.
+ *
+ * This list is protected by cache_mutex.
+ */
+static LIST_HEAD(cqm_rmid_lru);
+
+/*
+ * We use a simple array of pointers so that we can lookup a struct
+ * cqm_rmid_entry in O(1). This alleviates the callers of __get_rmid()
+ * and __put_rmid() from having to worry about dealing with struct
+ * cqm_rmid_entry - they just deal with rmids, i.e. integers.
+ *
+ * Once this array is initialized it is read-only. No locks are required
+ * to access it.
+ *
+ * All entries for all RMIDs can be looked up in the this array at all
+ * times.
+ */
+static struct cqm_rmid_entry **cqm_rmid_ptrs;
+
+static inline struct cqm_rmid_entry *__rmid_entry(int rmid)
+{
+ struct cqm_rmid_entry *entry;
+
+ entry = cqm_rmid_ptrs[rmid];
+ WARN_ON(entry->rmid != rmid);
+
+ return entry;
+}
/*
* Returns < 0 on fail.
+ *
+ * We expect to be called with cache_mutex held.
*/
static int __get_rmid(void)
{
- return bitmap_find_free_region(cqm_rmid_bitmap, cqm_max_rmid, 0);
+ struct cqm_rmid_entry *entry;
+
+ lockdep_assert_held(&cache_mutex);
+
+ if (list_empty(&cqm_rmid_lru))
+ return -EAGAIN;
+
+ entry = list_first_entry(&cqm_rmid_lru, struct cqm_rmid_entry, list);
+ list_del(&entry->list);
+
+ return entry->rmid;
}
static void __put_rmid(int rmid)
{
- bitmap_release_region(cqm_rmid_bitmap, rmid, 0);
+ struct cqm_rmid_entry *entry;
+
+ lockdep_assert_held(&cache_mutex);
+
+ entry = __rmid_entry(rmid);
+
+ list_add_tail(&entry->list, &cqm_rmid_lru);
}
static int intel_cqm_setup_rmid_cache(void)
{
- cqm_rmid_bitmap = kmalloc(sizeof(long) * BITS_TO_LONGS(cqm_max_rmid),
GFP_KERNEL);
- if (!cqm_rmid_bitmap)
+ struct cqm_rmid_entry *entry;
+ int r;
+
+ cqm_rmid_ptrs = kmalloc(sizeof(struct cqm_rmid_entry *) *
+ (cqm_max_rmid + 1), GFP_KERNEL);
+ if (!cqm_rmid_ptrs)
return -ENOMEM;
- bitmap_zero(cqm_rmid_bitmap, cqm_max_rmid);
+ for (r = 0; r <= cqm_max_rmid; r++) {
+ struct cqm_rmid_entry *entry;
+
+ entry = kmalloc(sizeof(*entry), GFP_KERNEL);
+ if (!entry)
+ goto fail;
+
+ INIT_LIST_HEAD(&entry->list);
+ entry->rmid = r;
+ cqm_rmid_ptrs[r] = entry;
+
+ list_add_tail(&entry->list, &cqm_rmid_lru);
+ }
/*
* RMID 0 is special and is always allocated. It's used for all
* tasks that are not monitored.
*/
- bitmap_allocate_region(cqm_rmid_bitmap, 0, 0);
+ entry = __rmid_entry(0);
+ list_del(&entry->list);
return 0;
+fail:
+ while (r--)
+ kfree(cqm_rmid_ptrs[r]);
+
+ kfree(cqm_rmid_ptrs);
+ return -ENOMEM;
}
/*
--
1.9.3
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