[DTrace-devel] [PATCH v2 2/5] Implement a 'aggs' per-CPU BPF map to store aggregation data
Kris Van Hees
kris.van.hees at oracle.com
Tue Nov 24 13:50:54 PST 2020
Aggregation data consists of data items (all uint64_t) that are updated
when an aggregation function is executed. This is quite different from
other data recording actions that append data to an output buffer.
We need support for concurrency control to allow reading the data during
non-atomic updates. We use a latch mechanism, i.e. a multiversion
concurrency control mechanism, to satify these requirements.
Simply put, the per-CPU aggregation data area stores a sequence id that
tracks the generation of the data updates. Since only one aggregation
can be updated at a time on a single CPU, one latch is sufficient for
all aggregations (per CPU-buffer). We also allocate two copies of each
aggregation, which we will call A and B.
When we update the aggregation data, we first modify A and then we modify
B, while ensuring that while we modify one, the reader is directed to
read from the other.
Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
libdtrace/dt_bpf.c | 20 ++++++++++++++++-
libdtrace/dt_dctx.h | 16 ++++++++------
libdtrace/dt_dlibs.c | 1 +
test/unittest/codegen/tst.stack_layout.r | 28 ++++++++++++------------
4 files changed, 43 insertions(+), 22 deletions(-)
diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
index eb295b00..f7028b90 100644
--- a/libdtrace/dt_bpf.c
+++ b/libdtrace/dt_bpf.c
@@ -144,6 +144,11 @@ set_task_offsets(dtrace_hdl_t *dtp)
* - state: DTrace session state, used to communicate state between BPF
* programs and userspace. The content of the map is defined in
* dt_state.h.
+ * - aggs: Aggregation data buffer map, associated with each CPU. The
+ * map is implemented as a global per-CPU map with a singleton
+ * element (key 0). Every aggregation is stored with two copies
+ * of its data to provide a lockless latch-based mechanism for
+ * atomic reading and writing.
* - buffers: Perf event output buffer map, associating a perf event output
* buffer with each CPU. The map is indexed by CPU id.
* - cpuinfo: CPU information map, associating a cpuinfo_t structure with
@@ -191,7 +196,7 @@ set_task_offsets(dtrace_hdl_t *dtp)
int
dt_bpf_gmap_create(dtrace_hdl_t *dtp)
{
- int gvarc, tvarc;
+ int gvarc, tvarc, aggsz;
int ci_mapfd;
uint32_t key = 0;
@@ -202,6 +207,9 @@ dt_bpf_gmap_create(dtrace_hdl_t *dtp)
/* Mark global maps creation as completed. */
dt_gmap_done = 1;
+ /* Determine the aggregation buffer size. */
+ aggsz = dt_idhash_nextoff(dtp->dt_aggs, 1, 0);
+
/* Determine the number of global and TLS variables. */
gvarc = dt_idhash_peekid(dtp->dt_globals) - DIF_VAR_OTHER_UBASE;
tvarc = dt_idhash_peekid(dtp->dt_tls) - DIF_VAR_OTHER_UBASE;
@@ -214,6 +222,16 @@ dt_bpf_gmap_create(dtrace_hdl_t *dtp)
if (dtp->dt_stmap_fd == -1)
return -1; /* dt_errno is set for us */
+ /*
+ * If there is aggregation data to be collected, we need to add a
+ * uint64_t to the map value size to hold a latch sequence number (seq)
+ * for concurrent access to the data.
+ */
+ if (aggsz > 0 &&
+ create_gmap(dtp, "aggs", BPF_MAP_TYPE_PERCPU_ARRAY,
+ sizeof(uint32_t), sizeof(uint64_t) + aggsz, 1) == -1)
+ return -1; /* dt_errno is set for us */
+
if (create_gmap(dtp, "buffers", BPF_MAP_TYPE_PERF_EVENT_ARRAY,
sizeof(uint32_t), sizeof(uint32_t),
dtp->dt_conf.num_online_cpus) == -1)
diff --git a/libdtrace/dt_dctx.h b/libdtrace/dt_dctx.h
index 3be4dbc4..7ebf4aea 100644
--- a/libdtrace/dt_dctx.h
+++ b/libdtrace/dt_dctx.h
@@ -37,12 +37,14 @@ typedef struct dt_dctx {
dt_activity_t *act; /* pointer to activity state */
dt_mstate_t *mst; /* DTrace machine state */
char *buf; /* Output buffer scratch memory */
+ char *agg; /* Aggregation data */
} dt_dctx_t;
#define DCTX_CTX offsetof(dt_dctx_t, ctx)
#define DCTX_ACT offsetof(dt_dctx_t, act)
#define DCTX_MST offsetof(dt_dctx_t, mst)
#define DCTX_BUF offsetof(dt_dctx_t, buf)
+#define DCTX_AGG offsetof(dt_dctx_t, agg)
#define DCTX_SIZE ((int16_t)sizeof(dt_dctx_t))
/*
@@ -92,23 +94,23 @@ typedef struct dt_dctx {
* +----------------+
* SCRATCH_BASE = -512 | Scratch Memory |
* +----------------+
- * LVAR_END = LVAR(n) = -256 | LVAR n | (n = DT_LVAR_MAX = 18)
+ * LVAR_END = LVAR(n) = -256 | LVAR n | (n = DT_LVAR_MAX = 17)
* +----------------+
* | ... |
* +----------------+
- * LVAR(1) = -120 | LVAR 1 |
+ * LVAR(1) = -128 | LVAR 1 |
* +----------------+
- * LVAR_BASE = LVAR(0) = -112 | LVAR 0 |
+ * LVAR_BASE = LVAR(0) = -120 | LVAR 0 |
* +----------------+
- * SPILL(n) = -104 | %r8 | (n = DT_STK_NREGS - 1 = 8)
+ * SPILL(n) = -112 | %r8 | (n = DT_STK_NREGS - 1 = 8)
* +----------------+
* | ... |
* +----------------+
- * SPILL(1) = -48 | %r1 |
+ * SPILL(1) = -56 | %r1 |
* +----------------+
- * SPILL_BASE = SPILL(0) = -40 | %r0 |
+ * SPILL_BASE = SPILL(0) = -48 | %r0 |
* +----------------+
- * DCTX = -32 | DTrace Context | -1
+ * DCTX = -40 | DTrace Context | -1
* +----------------+
*/
#define DT_STK_BASE ((int16_t)0)
diff --git a/libdtrace/dt_dlibs.c b/libdtrace/dt_dlibs.c
index 1e3701e0..607ebbe0 100644
--- a/libdtrace/dt_dlibs.c
+++ b/libdtrace/dt_dlibs.c
@@ -61,6 +61,7 @@ static const dt_ident_t dt_bpf_symbols[] = {
DT_BPF_SYMBOL(dt_set_tvar, DT_IDENT_SYMBOL),
DT_BPF_SYMBOL(dt_strnlen, DT_IDENT_SYMBOL),
/* BPF maps */
+ DT_BPF_SYMBOL(aggs, DT_IDENT_PTR),
DT_BPF_SYMBOL(buffers, DT_IDENT_PTR),
DT_BPF_SYMBOL(cpuinfo, DT_IDENT_PTR),
DT_BPF_SYMBOL(gvars, DT_IDENT_PTR),
diff --git a/test/unittest/codegen/tst.stack_layout.r b/test/unittest/codegen/tst.stack_layout.r
index 2333cf95..78d41c82 100644
--- a/test/unittest/codegen/tst.stack_layout.r
+++ b/test/unittest/codegen/tst.stack_layout.r
@@ -1,17 +1,17 @@
Base: 0
-dctx: -32
-%r0: -40
-%r1: -48
-%r2: -56
-%r3: -64
-%r4: -72
-%r5: -80
-%r6: -88
-%r7: -96
-%r8: -104
-lvar[ -1]: -111 (ID -1)
-lvar[ 0]: -112 (ID 0)
-lvar[ 1]: -120 (ID 1)
-lvar[ 18]: -256 (ID 18)
+dctx: -40
+%r0: -48
+%r1: -56
+%r2: -64
+%r3: -72
+%r4: -80
+%r5: -88
+%r6: -96
+%r7: -104
+%r8: -112
+lvar[ -1]: -119 (ID -1)
+lvar[ 0]: -120 (ID 0)
+lvar[ 1]: -128 (ID 1)
+lvar[ 17]: -256 (ID 17)
lvar[ -1]: -257 (ID -1)
scratch: -257 .. -512
--
2.28.0
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