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While fixing some bugs in pahole (built-in.o files were not being
processed due to relocation problems) I found out about these packable
structures:
$ pahole --packable kernel/trace/ring_buffer.o | grep ring
ring_buffer 72 64 8
ring_buffer_per_cpu 112 104 8
If we take a look at the current layout of struct ring_buffer we can see
that we have two 4 bytes holes.
$ pahole -C ring_buffer kernel/trace/ring_buffer.o
struct ring_buffer {
unsigned int pages; /* 0 4 */
unsigned int flags; /* 4 4 */
int cpus; /* 8 4 */
/* XXX 4 bytes hole, try to pack */
cpumask_var_t cpumask; /* 16 8 */
atomic_t record_disabled; /* 24 4 */
/* XXX 4 bytes hole, try to pack */
struct mutex mutex; /* 32 32 */
/* --- cacheline 1 boundary (64 bytes) --- */
struct ring_buffer_per_cpu * * buffers; /* 64 8 */
/* size: 72, cachelines: 2, members: 7 */
/* sum members: 64, holes: 2, sum holes: 8 */
/* last cacheline: 8 bytes */
};
So, if I ask pahole to reorganize it:
$ pahole -C ring_buffer --reorganize kernel/trace/ring_buffer.o
struct ring_buffer {
unsigned int pages; /* 0 4 */
unsigned int flags; /* 4 4 */
int cpus; /* 8 4 */
atomic_t record_disabled; /* 12 4 */
cpumask_var_t cpumask; /* 16 8 */
struct mutex mutex; /* 24 32 */
struct ring_buffer_per_cpu * * buffers; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
/* size: 64, cachelines: 1, members: 7 */
}; /* saved 8 bytes and 1 cacheline! */
We get it using just one 64 bytes cacheline.
To see what it did:
$ pahole -C ring_buffer --reorganize --show_reorg_steps \
kernel/trace/ring_buffer.o | grep \/
/* Moving 'record_disabled' from after 'cpumask' to after 'cpus' */
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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|---|---|---|
| .. | ||
| blktrace.c | ||
| ftrace.c | ||
| Kconfig | ||
| kmemtrace.c | ||
| Makefile | ||
| ring_buffer.c | ||
| trace_boot.c | ||
| trace_branch.c | ||
| trace_functions_graph.c | ||
| trace_functions.c | ||
| trace_hw_branches.c | ||
| trace_irqsoff.c | ||
| trace_mmiotrace.c | ||
| trace_nop.c | ||
| trace_output.c | ||
| trace_output.h | ||
| trace_power.c | ||
| trace_sched_switch.c | ||
| trace_sched_wakeup.c | ||
| trace_selftest_dynamic.c | ||
| trace_selftest.c | ||
| trace_stack.c | ||
| trace_stat.c | ||
| trace_stat.h | ||
| trace_sysprof.c | ||
| trace_workqueue.c | ||
| trace.c | ||
| trace.h | ||