Add warning for suspected slow system clocksource setting (#10636)

This PR does 2 main things:
1) Add warning for suspected slow system clocksource setting. This is Linux specific.
2) Add a `--check-system` argument to redis which runs all system checks and prints a report.

## System checks
Add a command line option `--check-system` which runs all known system checks and provides
a report to stdout of which systems checks have failed with details on how to reconfigure the
system for optimized redis performance.
The `--system-check` mode exists with an appropriate error code after running all the checks.

## Slow clocksource details
We check the system's clocksource performance by running `clock_gettime()` in a loop and then
checking how much time was spent in a system call (via `getrusage()`). If we spend more than
10% of the time in the kernel then we print a warning. I verified that using the slow clock sources:
`acpi_pm` (~90% in the kernel on my laptop) and `xen` (~30% in the kernel on an ec2 `m4.large`)
we get this warning.

The check runs 5 system ticks so we can detect time spent in kernel at 20% jumps (0%,20%,40%...).
Anything more accurate will require the test to run longer. Typically 5 ticks are 50ms. This means
running the test on startup will delay startup by 50ms. To avoid this we make sure the test is only
executed in the `--check-system` mode.

For a quick startup check, we specifically warn if the we see the system is using the `xen` clocksource
which we know has bad performance and isn't recommended (at least on ec2). In such a case the
user should manually rung redis with `--check-system` to force the slower clocksource test described
above.

## Other changes in the PR

* All the system checks are now implemented as functions in _syscheck.c_.
  They are implemented using a standard interface (see details in _syscheck.c_).
  To do this I moved the checking functions `linuxOvercommitMemoryValue()`,
  `THPIsEnabled()`, `linuxMadvFreeForkBugCheck()` out of _server.c_ and _latency.c_
  and into the new _syscheck.c_. When moving these functions I made sure they don't
  depend on other functionality provided in _server.c_ and made them use a standard
  "check functions" interface. Specifically:
  * I removed all logging out of `linuxMadvFreeForkBugCheck()`. In case there's some
    unexpected error during the check aborts as before, but without any logging.
    It returns an error code 0 meaning the check didn't not complete.
  * All these functions now return 1 on success, -1 on failure, 0 in case the check itself
    cannot be completed.
  * The `linuxMadvFreeForkBugCheck()` function now internally calls `exit()` and not
    `exitFromChild()` because the latter is only available in _server.c_ and I wanted to
    remove that dependency. This isn't an because we don't need to worry about the
    child process created by the test doing anything related to the rdb/aof files which
    is why `exitFromChild()` was created.

* This also fixes parsing of other /proc/\<pid\>/stat fields to correctly handle spaces
  in the process name and be more robust in general. Not that before this fix the rss
  info in `INFO memory` was corrupt in case of spaces in the process name. To
  recreate just rename `redis-server` to `redis server`, start it, and run `INFO memory`.

1 files changed, 41 insertions, 17 deletions

diff --git a/src/zmalloc.c b/src/zmalloc.c
index d19d87b7d..857baa8ac 100644
--- a/src/zmalloc.c
+++ b/src/zmalloc.c
@@ -55,6 +55,8 @@ void zlibc_free(void *ptr) {
 #include "zmalloc.h"
 #include "atomicvar.h"
 
+#define UNUSED(x) ((void)(x))
+
 #ifdef HAVE_MALLOC_SIZE
 #define PREFIX_SIZE (0)
 #define ASSERT_NO_SIZE_OVERFLOW(sz)
@@ -395,35 +397,58 @@ void zmadvise_dontneed(void *ptr) {
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#endif
 
-size_t zmalloc_get_rss(void) {
-    int page = sysconf(_SC_PAGESIZE);
-    size_t rss;
+/* Get the i'th field from "/proc/self/stats" note i is 1 based as appears in the 'proc' man page */
+int get_proc_stat_ll(int i, long long *res) {
+#if defined(HAVE_PROC_STAT)
     char buf[4096];
-    char filename[256];
-    int fd, count;
+    int fd, l;
     char *p, *x;
 
-    snprintf(filename,256,"/proc/%ld/stat",(long) getpid());
-    if ((fd = open(filename,O_RDONLY)) == -1) return 0;
-    if (read(fd,buf,4096) <= 0) {
+    if ((fd = open("/proc/self/stat",O_RDONLY)) == -1) return 0;
+    if ((l = read(fd,buf,sizeof(buf)-1)) <= 0) {
         close(fd);
         return 0;
     }
     close(fd);
+    buf[l] = '\0';
+    if (buf[l-1] == '\n') buf[l-1] = '\0';
 
-    p = buf;
-    count = 23; /* RSS is the 24th field in /proc/<pid>/stat */
-    while(p && count--) {
-        p = strchr(p,' ');
+    /* Skip pid and process name (surrounded with parentheses) */
+    p = strrchr(buf, ')');
+    if (!p) return 0;
+    p++;
+    while (*p == ' ') p++;
+    if (*p == '\0') return 0;
+    i -= 3;
+    if (i < 0) return 0;
+
+    while (p && i--) {
+        p = strchr(p, ' ');
         if (p) p++;
+        else return 0;
     }
-    if (!p) return 0;
     x = strchr(p,' ');
-    if (!x) return 0;
-    *x = '\0';
+    if (x) *x = '\0';
 
-    rss = strtoll(p,NULL,10);
+    *res = strtoll(p,&x,10);
+    if (*x != '\0') return 0;
+    return 1;
+#else
+    UNUSED(i);
+    UNUSED(res);
+    return 0;
+#endif
+}
+
+#if defined(HAVE_PROC_STAT)
+size_t zmalloc_get_rss(void) {
+    int page = sysconf(_SC_PAGESIZE);
+    long long rss;
+
+    /* RSS is the 24th field in /proc/<pid>/stat */
+    if (!get_proc_stat_ll(24, &rss)) return 0;
     rss *= page;
     return rss;
 }
@@ -748,7 +773,6 @@ size_t zmalloc_get_memory_size(void) {
 }
 
 #ifdef REDIS_TEST
-#define UNUSED(x) ((void)(x))
 int zmalloc_test(int argc, char **argv, int flags) {
     void *ptr;