torvalds/linux
3
0
Fork 0
mirror of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git synced 2026-04-08 23:49:14 +08:00
Code Issues Packages Projects Releases Wiki Activity

perf tools: Update topdown documentation to permit rdpmc calls

Browse Source 
Update Topdown documentation to permit calls to rdpmc, and describe
interaction with system calls.

Signed-off-by: Ray Kinsella <mdr@ashroe.eu>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: http://lore.kernel.org/lkml/20210421091009.1711565-1-mdr@ashroe.eu
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
Ray Kinsella
2021-04-21 10:10:09 +01:00
committed by Arnaldo Carvalho de Melo
parent bb7db8699b
commit a4b0fccfbd
1 changed files with 18 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
tools/perf/Documentation/topdown.txt
View File

@@ -72,6 +72,7 @@ For example, the perf_event_attr structure can be initialized with
The Fixed counter 3 must be the leader of the group.
#include <linux/perf_event.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
@@ -95,6 +96,11 @@ int slots_fd = perf_event_open(&slots, 0, -1, -1, 0);
if (slots_fd < 0)
... error ...
/* Memory mapping the fd permits _rdpmc calls from userspace */
void *slots_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, slots_fd, 0);
if (!slot_p)
.... error ...
/*
* Open metrics event file descriptor for current task.
* Set slots event as the leader of the group.
@@ -110,6 +116,14 @@ int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0);
if (metrics_fd < 0)
... error ...
/* Memory mapping the fd permits _rdpmc calls from userspace */
void *metrics_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, metrics_fd, 0);
if (!metrics_p)
... error ...
Note: the file descriptors returned by the perf_event_open calls must be memory
mapped to permit calls to the _rdpmd instruction. Permission may also be granted
by writing the /sys/devices/cpu/rdpmc sysfs node.
The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used
to read slots and the topdown metrics at different points of the program:
@@ -141,6 +155,10 @@ as the parallelism and overlap in the CPU program execution will
cause too much measurement inaccuracy. For example instrumenting
individual basic blocks is definitely too fine grained.
_rdpmc calls should not be mixed with reading the metrics and slots counters
through system calls, as the kernel will reset these counters after each system
call.
Decoding metrics values
=======================